Saturday, June 22, 2019

What's the Deal with Enantiornithean Tails?

You are drawing a scene set in the Cretaceous and you need some birds in the picture, maybe as a way to show off the size of some much larger, more charismatic dinosaur. The setting is not oceanic enough to use something vaguely Ichthyornis-like, so what do you do? You draw in a few enantiornitheans, of course.

That's a good choice. Enantiornitheans, or "opposite birds", were probably the most diverse and abundant bird-like dinosaurs during the Cretaceous. They would have looked quite similar to modern birds and were probably doing many of the same things that modern birds do. However, recent findings have also given us insights into how enantiornitheans differed from modern birds, not only in the details of their skeletons but also in their external appearance and behavior. This means that scientifically rigorous depictions of enantiornitheans should not show them simply as modern birds transplanted into a different time period. And probably the most noticeable difference between most enantiornitheans and modern birds would have been the feathering on their tail.

(You might have noticed that I've switched to using "enantiornitheans" instead of the more commonly used "enantiornithines". As has been noted by Harris et al., 2006 and Matt Martyniuk, the suffix "-ine" is more typically used for the vernacular names of clades that end in "-inae", and Enantiornithes is not one of those clades. Seeing as Harris et al. suggested using "enantiornitheans", that's what I'm going with for now.)

Restoration of the enantiornithean Dapingfangornis, by Jack Wood (used with permission).

Unlike most other dinosaurs, the bony part of a modern bird's tail is very short, even in birds that have long tail feathers. Despite this, the tail serves important functions in most birds. The last few tail vertebrae are fused together into a structure called the pygostyle, which supports a pair of muscular bulbs in which the flight feathers on the tail are embedded. These bulbs are known as rectricial bulbs, and they are responsible for the ability to spread the tail feathers out like a fan. This allows the tail feathers to act as an additional lift-generating surface, which among other things grants finer control during low-speed flight, such as while landing.

A white-necked jacobin showing off its spread tail fan, photographed by Paul Tavares, under CC BY-NC 4.0.

Like modern birds, enantiornitheans had short tails with a pygostyle. As a result, they have traditionally been depicted with tail fans similar to those of modern birds. In fact, several early papers about enantiornitheans outright suggested that they likely had tail fans based on their possession of a pygostyle. However, by the mid-2000s, a good number of enantiornithean fossils had been described from the Jehol Group in China, famous for its preservation of soft tissue structures, including feathers.

How many of these enantiornitheans have been found with tail fans? None.

Julia Clarke and colleagues pointed this out in a 2006 paper describing the anatomy of Yixianornis, an Early Cretaceous avialan from the Jehol that was more closely related to modern birds than enantiornitheans were. Whereas the holotype of Yixianornis was preserved with a tail fan similar to those of modern birds, all enantiornitheans preserved with tail feathering that had been described up to that point either lacked large tail feathers entirely (leaving only a fuzzy stump of a tail) or had just a single pair of long tail feathers that wouldn't have provided much of a lift-generating surface.

Clarke et al. also highlighted the fact that (as had been noted by previous researchers) the pygostyle of enantiornitheans differed from those of Yixianornis and modern birds. Enantiornitheans had a longer, more rod-like pygostyle, whereas Yixianornis and modern birds have a shorter, more tab-like one. Thus, Clarke et al. suggested that the presence of a pygostyle alone does not indicate tail feathering similar to that of modern birds. Instead, it is specifically the short, tab-like pygostyle that correlates with a tail fan.

Comparison of the last few tail vertebrae among different maniraptor groups, mapped onto their phylogenetic relationships, from Wang and O'Connor (2017). In most long-tailed maniraptors (e.g.: Archaeopteryx, dromaeosaurids), the vertebrae are not fused into a pygostyle, but some oviraptorosaurs and therizinosaurs have evolved fused tail vertebrae independently of short-tailed avialans. Note that members of Ornithuromorpha (a group that includes modern birds) have a short, tab-shaped pygostyle.

If we had only a few fossils to base these conclusions on, these observations might not mean much. Even the Jehol Group rarely preserves a complete record of plumage in any individual specimen. However, dozens if not hundreds of feather-preserving Mesozoic avialan fossils have now been studied, and for most part, the pattern observed by Clarke et al. has held up under subsequent discoveries. So far, all Mesozoic avialans confirmed to have had a modern-style tail fan are species more closely related to modern birds than enantiornitheans were, with short, stumpy pygostyles.

Among enantiornitheans, on the other hand, the most commonly seen condition is the absence of any large tail feathers, which has been documented in Eoenantiornis, Longipteryx, Cruralispennia, and Avimaia. Enantiornithean genera that have been found with a single pair of long tail feathers include Protopteryx, Dapingfangornis, Bohaiornis, Parapengornis, Eopengornis, Cratoavis, Junornis, and Orienantius. Longirostravis was described as preserving these long tail feathers as well, though Mesozoic avialan specialist Jingmai O'Connor is reportedly doubtful of this interpretation. There are even very small juvenile enantiornithean specimens that apparently show a pair of tail feathers that had begun to grow, as seen in one of the amber-preserved enantiornitheans. (Yeah, we have amber-preserved enantiornitheans.)

Two enantiornithean specimens preserved with feathers, from O'Connor et al. (2012). Protopteryx (A) preserves a single pair of streamer-like tail feathers, whereas Eoenantiornis (B) preserves no large tail feathers at all. (C) shows isolated body feathers of Protopteryx, which are not especially relevant to this post.

These forms of tail plumage are similar to that of confuciusornithiforms, which were more distant relatives of modern birds compared to enantiornitheans. It's perhaps no accident that, like enantiornitheans, confuciusornithiforms had rod-shaped pygostyles. Currently, we don't have enough specimens described of any enantiornithean to determine whether long tail feathers were only present in some individuals of the same species, as has been documented in Confuciusornis and Eoconfuciusornis. At least in the case of confuciusornithiforms, it appears likely that the individuals with the long tail feathers were males.

The long tail feathers in enantiornitheans and confuciusornithiforms have been described as being ribbon-like or streamer-like. They are known by the technical names Rachis-Dominated Feathers (RDFs) or Proximally Ribbon-like Pennaceous Feathers (PRPFs). Recent findings of such feathers preserved in amber have been very informative regarding their detailed structure, which had been difficult to interpret from flattened Jehol-style fossils. It turns out that the central shaft of RDFs is an extremely thin sheet that is less than 50 micrometers thick. The underside of the shaft is open such that it forms an arch, unlike the enclosed tube seen in typical feathers.

Examples of RDFs preserved in amber, from Xing et al. (2018). (D) is a schematic showing the structure of RDFs, as inferred from these specimens.

Several groups of modern birds, from hummingbirds to parrots, have evolved superficially similar, streamer-like feathers, but none of these plumes exhibits the same type of structure seen in RDFs. It appears that RDFs are an entirely extinct form of feather that modern birds have "forgotten" how to make. Like most other types of feathers, RDFs have barbs coming off the central shaft. In some enantiornitheans, such as Eopengornis and Parapengornis, the barbs are present along almost the entire length of the tail feather, but in many others the barbs are restricted towards the feather tip.

So enantiornitheans either lacked flight feathers on the tail or had a single pair of ribbon-like feathers not found in any modern bird. Simple, right? Yet, as happens so often in science, some new discoveries have arisen that complicate the picture. For starters, we now also know of enantiornitheans that had two pairs of RDFs on the tail, as first discovered in Paraprotopteryx.

Furthermore, a few enantiornitheans described since 2006 have been purported to challenge the notion that enantiornitheans lacked tail fans. The first of these was Shanweiniao, described in 2009 by O'Connor and colleagues. Shanweiniao was found with at least four (possibly six) tail feathers closely aligned with one another, as though forming a tail fan. It was even named for this feature, Shanweiniao meaning "fan-tailed bird" in Chinese.

However, these feathers were not very well preserved, so their structure and arrangement are difficult to confirm. In a 2016 paper describing another enantiornithean, Chiappeavis (which will factor into this story shortly), O'Connor et al. reevaluated the evidence for a tail fan in Shanweiniao. They noted that there were narrow gaps between its tail feathers, unlike what was preserved in avialans with more definite tail fans. Therefore, they concluded, it was more likely that the tail feathering of Shanweiniao consisted of at least two pairs of RDFs, similar to Paraprotopteryx. Shanweiniao may have had more tail feathers than the average enantiornithean, but there is little reason to think that it had a modern-style tail fan.

The tail feathers of Shanweiniao, from O'Connor et al. (2009).

The next new find that seemed to question the dearth of enantiornithean tail fans was Feitianius, described in 2016. Feitianius was found with not just two or even four large tail feathers, but many. It had a single pair of RDFs, but it was also preserved with 5-7 shorter feathers that appear to have been barbed for only about half their length, as well as at least 5 more tail feathers that were shorter still. In addition, an organic mass preserved around the pygostyle was interpreted as the remnants of the soft tissues that include rectricial bulbs.

In 2017, Wang Wei and O'Connor put forth a more detailed study on the relationship between pygostyle form, tail musculature, and tail feathers. They found that it was unlikely for any enantiornithean pygostyle to have supported well-developed rectricial bulbs. However, they did find features of the pygostyle in enantiornitheans (and confuciusornithiforms) that correlate with strong muscles for raising the tail, and suggested that the soft tissues surrounding the pygostyle in Feitianius may instead include these muscles instead of rectricial bulbs. Thus, Feitianius was likely well adapted to raising its elaborate set of tail feathers (perhaps in display), but not for spreading them apart as a true tail fan.

The tail of Feitianius with preserved soft tissues, from O'Connor et al. (2016a).

Only a day after the paper naming Feitianius was posted online, along came the aforementioned description of Chiappeavis. The preserved tail feathers of Chiappeavis really do resemble a tail fan. There are no visible gaps between the feathers as in Shanweiniao, and though their detailed structure is poorly preserved, they appear to have been standard flight feathers instead of RDFs.

When O'Connor et al. presented a more detailed study* on the anatomy of Chiappeavis in 2017, they also estimated the aerodynamic qualities of its supposed tail fan. They found that its lift-generating capabilities were very limited compared to those of tail fans in modern birds and their close relatives. Wang and O'Connor (2017), which was published a few months later, also found no features that made Chiappeavis any more likely to have had rectricial bulbs than other enantiornitheans. So even though the tail feathers of Chiappeavis were arranged in a similar manner to a tail fan, they probably didn't function like the spreading tail fan of modern birds.

*Interestingly, O'Connor et al. did not exclude the possibility that enantiornitheans had rectricial bulbs in this study, claiming that some modern birds also lack the pygostyle features that typically support rectricial bulbs. They cite the Wang and O'Connor study (in-prep at the time) for this, but the final version of Wang and O'Connor (2017) does not repeat the claim, instead concluding that rectricial bulbs were most likely absent in enantiornitheans.

The tail of Chiappeavis with preserved feathers, from O'Connor et al. (2016b).

In the end, Feitianius and Chiappeavis demonstrate that enantiornithean tail feather arrangements came in more varieties than the two that Clarke et al. (2006) originally identified. Given the great diversity of enantiornitheans, it's not unlikely that more will be found in the future. However, after so much back and forth, it appears that Clarke et al.'s central contention that a true tail fan is found only in avialans more closely related to modern birds remains valid.

So if enantiornitheans weren't using their tail feathers like modern birds do, what were their tail feathers for? In many cases, the most obvious possibility is some form of visual signalling. That would certainly be consistent with the evidence for well-developed tail-raising muscles, as previously noted with respect to Feitianius. In addition, modern birds that have superficially similar, streamer-like feathers tend to use them in display.

That being said, a few other possibilities have been put forth. The abundance of isolated RDFs found in amber has been suggested as evidence that these feathers detached easily, which may have allowed them to misdirect attacks from predators (much like the detachable tails of many lizards). A variety of modern birds certainly have tail and rump feathers that shed readily if grabbed by a predator. However, as with the feathers of these modern birds, I suspect that distracting predators was not the primary function of RDFs.

The RDFs of Parapengornis in particular have been proposed to have functioned as props, similar to the tail feathers of some modern climbing birds such as woodpeckers. This idea was purportedly backed up by pygostyle anatomy, but Wang and O'Connor (2017) later determined that the pygostyle of Parapengornis wasn't especially similar to that of woodpeckers after all.

Restoration of Feitianius showing it using its tail feathers for display, by Scott Reid (used with permission).

How would the absence of a lift-generating tail fan impact the flight of enantiornitheans? That a tail fan is not a requirement for avian flight is evident from observations of modern birds. Individual birds that have lost their tail feathers can usually still adapt to flying without them. There are even flying birds today that naturally lack tail fans, namely grebes (though their large, lobed feet perform some of the functions that a tail fan normally would).

Even so, I wonder if enantiornitheans were generally clumsier fliers compared to similarly-sized modern birds, or perhaps needed to expend more energy to perform equivalent aerial maneuvers. Martyniuk has speculated that enantiornitheans may have preferred to land on large surfaces such as tree trunks before climbing to desired perches, instead of trying to precisely target small twigs and branches from the get-go. Given that many enantiornitheans appear to have been small, forest-dwelling animals, they might not have needed much endurance or agility if most of their flight involved flitting from tree to tree in this manner. Some recent research has focused on inferring the likely flight behavior of Mesozoic avialans from their wing shape and body mass. It would be interesting for future studies to compare the effects of different types of tail feathering on flight ability.

Restoration of the enantiornithean Alethoalaornis in flight, by Scott Reid (used with permission).

One final subject I'll remark on here is the evolutionary history of enantiornithean tail feathers. The structure of RDFs suggests that they evolved from more typical shafted feathers. Most early avialans and other long-tailed maniraptors (including troodonts, dromaeosaurids, and oviraptorosaurs) certainly had large shafted feathers along the length of their tail. Such an array of tail feathers has often been called a "tail fan", but there is no evidence that long-tailed maniraptors had structures similar to rectricial bulbs that could fan out their tail feathers. As a result, I prefer to use Stephen Gatesy's term "tail frond" to distinguish this type of tail from the mobile tail fan of modern birds.

On the lineage leading to enantiornitheans (as well as modern birds), the bony part of the tail shortened. What would an immobile tail frond look like when retained on a shortened tail? Well, probably something similar to the immobile fan of Chiappeavis. (That this represents the ancestral state for short-tailed avialans is potentially supported by the fact that a similar fan is also known in Sapeornis, which was more distantly related to modern birds than enantiornitheans were.) A plausible evolutionary scenario may be that most enantiornithean lineages then lost the majority of their tail feathers, modifying the remainder into RDFs. In 2014, Wang Xiaoli and colleagues further suggested that the fully barbed RDFs seen in Eopengornis might represent a precursor to RDFs that had a mostly naked shaft.

Comparison of tail plumage in different Mesozoic avialans, from Wang et al. (2014). (A) shows the "tail fan" of Sapeornis, (B) shows the RDFs of Dapingfangornis (which only have barbs near their tip), (C) shows the true tail fan of Hongshanornis, and (E) shows the fully barbed RDFs of Eopengornis. ("D" featured the forked tail of Schizooura, which I cropped out as it is not directly relevant to this post.)

However, there's a wrinkle to this tale. Phylogenetic analyses that include Chiappeavis find it to be more closely related to Eopengornis and Parapengornis (which had fully barbed RDFs) than to other enantiornitheans. If these relationships are correct, they may imply that the RDFs of Eopengornis and Parapengornis evolved independently from those of other enantiornitheans, instead of representing an ancestral state for RDFs. An alternative possibility is that Chiappeavis evolved its "tail fan" from an ancestor that had RDFs and is uninformative about the ancestral state of tail plumage in enantiornitheans. Or perhaps our current understanding of enantiornithean phylogeny is wrong. Given that enantiornitheans have some of the most poorly resolved interrelationships among all Mesozoic dinosaurs, that wouldn't surprise me. At present, I don't think we have enough information to rule out any of these possibilities.

Long story short, enantiornithean tail plumage came in a wide variety of forms, all of which were different from that of most modern birds. We still have some ways to go when it comes to understanding their function and evolution, and it's likely that they will continue to surprise us in years to come.

References

Tuesday, June 11, 2019

ProgPal 2019

This year's ProgPal was held in the Lapworth Museum at the University of Birmingham. It's a small museum, but there are gems to be found. The most eye-catching item on display, however, is this cast of the Allosaurus specimen "Big Al".


"Big Al" was made particularly famous by a BBC documentary that also highlighted the many injuries preserved in this specimen. Here is an infected toe, which the documentary portrayed as having ultimately led to "Big Al"'s death.


The tree of life depicted in museum specimens. It's an appealing setup, though bats are incorrectly shown as being more closely related to primates and rodents than to pangolins.


In my few years of experience, ProgPal has always been a nice, relaxed conference for early-career researchers, and this year was no exception. It's surreal to me that among the delegates of ProgPal this year, I can probably now be considered relatively far along in terms of my career progression. Several individuals I'd met at previous ProgPals have since graduated or become too entangled in the final stages of their PhD research to come. Nonetheless, there were still a fair few friends and acquaintances around for me to catch up with, and I had a good time meeting many of the new faces, some of whom recognized me from the talk I gave at TetZooCon last year!

As usual, I will list off a personal highlights reel... though to be honest, I've met enough people now to feel somewhat guilty about not including the presentations of everyone I know (even with my colleagues from the same institution already excluded a priori). I suppose it should be clear that these are the presentations I found especially interesting or outstanding, as essentially every presentation I saw at ProgPal was enjoyable.
  • Emily Brown's talk on the endocranial anatomy of Proterosuchus
  • As with PalAss last year, the foraminiferan talks were surprisingly engaging, with special mention to Caitlin Lebel and Bridget Warren's presentations
  • Alessandro Chiarenza's talk on the Late Cretaceous distribution of sauropods
  • Richie Howard's talk on a sessile Cambrian worm
As for me, I gave an updated account of my work on the phylogeny of Strisores, which I've been presenting at conferences for over a year now. I hope that by the next time I mention that research on my blog, I will have submitted it as a manuscript!

Monday, May 20, 2019

What Were Adzebills?

New Zealand is renowned for its unique avifauna, hosting many distinctive bird clades found nowhere else in the world. And prior to the settlement of New Zealand by humans 700-800 years ago, this ensemble of unusual birds would have been even more diverse than it is now. The most famous of these recently lost birds are the moa (a group of large flightless birds) and the Haast's eagle (the largest known raptorial bird) that preyed upon them.

Less well known but no less remarkable were the two species of adzebills. Named after their robust, downcurved bills, these were another group of flightless birds. Although smaller than the largest moa species, adzebills probably weighed around 15-20 kg, making them large by the standards of most birds. The chemical composition of their bones suggests that they ate small animals, which they may have dug out of the ground or decaying wood by using their strong beaks and feet.

Skeleton of a South Island adzebill from the Auckland War Memorial Museum, under CC BY 4.0.

Unlike moa, adzebills clearly weren't paleognaths (a group of birds that also includes ostriches and kiwi), but their relationships to living birds have otherwise been difficult to figure out. Their overall anatomy doesn't obviously resemble any other group of birds; however, it has long been noted that they share similarities with members of the Gruiformes, a group that includes cranes, rails, and their close relatives. An alternative idea that became particularly popular starting in the 1980s is that adzebills were instead more closely related to the kagu, an unusual flightless bird from New Caledonia.

Initially, these ideas probably wouldn't have seemed dramatically discordant, given that the kagu and its closest living relative, the sunbittern of South America, were once thought to be gruiforms. However, with the advent of molecular phylogenetics, current evidence now suggests that they are more closely related to the marine tropicbirds. (In fact, many other ground-dwelling birds, such as bustards and seriemas, were traditionally classified as gruiforms, but are no longer considered members of that group.) Thus, a close relationship with the kagu would put adzebills in a quite different part of the bird family tree compared to potential affinities among gruiforms. Furthermore, a few researchers have even entertained the possibility that adzebills were closely related to yet a different group, the galloanserans (which includes chickens and ducks).

Adzebills went extinct recently enough that some genetic material can be extracted from their remains. Since the 1990s, small snippets of adzebill DNA have been available for study, and analyses that have included these have generally supported the gruiform hypothesis, placing adzebills as close kin of cranes and rails. However, the limited amount of data has prevented a confident assessment of where adzebills belong within Gruiformes.

In a recent study, Alexander Boast and colleagues sequenced nearly complete mitochondrial genomes from the two recent adzebill species. When they analyzed these sequences alongside those of a diverse sampling of other gruiforms, they consistently found a surprising result: the closest living relatives of adzebills are the flufftails, a group of small, rail-like birds from Africa.

Phylogeny of gruiform birds plotted against divergence times estimated by Boast et al. (2019), from their study.

Boast et al. made some novel findings about the relationships among living gruiforms as well. They found that the gray-throated rail (Canirallus oculeus), long thought to be closely related to the flufftail genus Mentocrex, is in fact a true rail. Flufftails in general were formerly considered to be a type of rail, and, especially given the absence of genetic samples for some obscure rail-like birds, it is evident that teasing the two groups apart remains an ongoing process.

But at least the adzebill problem is solved, right? Well... a second study on the phylogenetic position of adzebills has come out this year, and it came to a decidedly different conclusion. For this paper, Grace Musser and Joel Cracraft assessed the likely affinities of adzebills by assembling a new dataset of anatomical features in neornithean birds.

I was quite excited about this study when I first learned of it from Musser's presentation at SVP 2017. Our current understanding of neornithean phylogeny has been greatly refined by analysis of molecular data; however, morphological phylogenetic datasets for neornitheans (i.e.: the only way we can evaluate the phylogenetic relationships of most fossil neornitheans) have remained underdeveloped by comparison. Musser and Cracraft's dataset contains information on 368 skeletal characteristics, making it larger than nearly all other available morphological datasets for neornitheans.

The one morphological phylogenetic dataset on neornitheans that is larger than Musser and Cracraft's was published in Livezey and Zusi (2006). In fact, with over 2,900 characters, it is one of the largest morphological phylogenetic datasets of any kind. Although impressive in scope, however, Livezey and Zusi's study has been criticized for containing numerous errors as well as failing to recover many clades that are otherwise well supported by both molecular and morphological studies. I thus appreciated the fact that Musser and Cracraft built much of their dataset by reassessing the characters used in Livezey and Zusi (2006), building upon that previous work while (hopefully) not repeating its mistakes.

So how do the results of Musser and Cracraft's morphological analysis compare to those of recent molecular analyses? They certainly get closer to molecular results than any previous morphological analysis on neornitheans has gotten. Like Jarvis et al. (2014) and Prum et al. (2015) (the two largest recent molecular studies on bird phylogeny), nightjars and their kin were found to be an early-diverging branch among neoavians. Musser and Cracraft's dataset also placed the sunbittern, kagu, and seriemas outside of Gruiformes. Among gruiforms, rails were mostly recovered forming a clade excluding flufftails (though the Nkulengu rail, Himantornis haematopus, was unexpectedly found closer to flufftails), with Canirallus recovered as a true rail like in Boast et al.'s study.

Nonetheless, there are some notable differences. For example, seriemas were not found as close relatives to the other telluravians included in Musser and Cracraft's dataset (in this case vultures and courols), and grebes and loons were recovered as close relatives, a historically popular idea that is now considered outdated. (The convergent diving adaptations shared between grebes and loons have long been recognized as confounding factors in morphological phylogenies of birds.) Analyzing the morphological dataset in combination with a relatively small molecular dataset put some of these relationships (e.g.: the affinities of seriemas) more in line with molecular trees, though even this wasn't enough to pull grebes and loons apart in this study.

Simplified results of the phylogenetic analyses run by Musser and Cracraft (2019) compared to those of recent molecular analyses of modern birds.

Evidently, there is still much work to be done in the field of avian morphological phylogenetics. There are several bird groups that would have been interesting to see included in this study; flamingos, pigeons, and bustards come to mind. Regardless, I consider the assembly of this dataset to be a step in the right direction and the most valuable contribution of Musser and Cracraft's paper, irrespective of what it has to say about adzebills.

Speaking of which, what does it say about adzebills? Musser and Cracraft's analyses are consistent with molecular studies in putting adzebills among gruiforms. However, their results found the closest living relatives of adzebills to be the trumpeters, a group of South American, fruit-eating gruiforms that are more closely related to cranes than to rails. The analyses identified up to 15 similarities between adzebills and trumpeters, particularly in the hip and hindlimb bones. Their results also conflicted with molecular analyses when it came to another extinct New Zealand gruiform: Hawkins's rail (Diaphorapteryx hawkinsi) was found outside of a clade including both rails and flufftails, instead of being a true rail as previous studies suggested. (Note that even though Musser and Cracraft did run a combined molecular-morphological dataset, they did not include any molecular data from adzebills or Diaphorapteryx, as they had used nuclear instead of mitochondrial genes.)

Musser and Cracraft's study was evidently submitted for publication late enough to take into account the findings of Boast et al. (2019), because they explicitly tested the possibility of Boast et al.'s results by running additional analyses in which a close relationship between adzebills and flufftails was enforced. When this was done, the resulting tree was 18 steps longer (i.e.: it implied 18 more evolutionary changes) compared to the phylogenies in which adzebills were closely related to trumpeters. Phylogenetic trees that require so many more steps to explain are often thought to be less likely in phylogenetic studies. For this reason, Musser and Cracraft considered the potential adzebill-flufftail relationship to be inconsistent with their morphological dataset.

However, given that adzebills were aberrant forms that had been isolated from their closest relatives for more than 16 million years, it wouldn't seem farfetched to me if they really had undergone so many evolutionary changes since their last common ancestor with whatever their closest living relatives turn out to be. It's not as though there aren't any anatomical similarities that might indicate an adzebill-flufftail relationship: Musser and Cracraft found 16 shared features between the two groups. Although none of these features are unique to adzebills and flufftails, the same is true of the similarities between adzebills and trumpeters.

What would be particularly helpful for resolving this conundrum would be the discovery of early members of the adzebill lineage that shed light on their ancestral anatomy. Unfortunately, the oldest known fossil adzebills were already fairly similar to recent species, and thus aren't very informative in that regard.

Phylogenetic tree of gruiforms showing the conflicting positions for adzebills found by Boast et al. (2019) and Musser and Cracraft (2019).

Whether adzebills are more closely related to flufftails or trumpeters, either option raises some interesting biogeographic implications, as they imply that the closest living relatives of adzebills are either restricted to Africa (flufftails) or South America (trumpeters). Interestingly, there are parallels for both of these cases among other New Zealand birds: it is now thought that moa were most closely related to the South American tinamous, whereas kiwi are most closely related to the Malagasy elephant birds. What happened in the distant past that led to these unusual distributions in several different groups of birds?

My best guess is that the ancestral groups that gave rise to each of these closely related pairs were once more widespread across the Southern Hemisphere but have since died out, leaving recent descendants only in geographically restricted ranges. Antarctica would be a prime suspect for the place of origin of these groups, with its complete glaciation during the Neogene being a potential cause of extinction for their ancestral populations. Boast et al. (2019) and Musser and Cracraft (2019) both entertain similar possibilities, but we don't yet have the fossils to confirm this scenario. Oh, for a productive deposit of early Paleogene bird fossils from the Southern Hemisphere!

The two recent studies on the affinities of adzebills may not exactly agree with one another, but it's exciting that we've now received two new phylogenetic datasets devoted to resolving this problem in quick succession. If nothing else, we can probably be pretty confident in identifying adzebills as gruiforms now, and we can expect that their closest living relatives are likely one of the other Southern Hemisphere gruiform groups. Furthermore, these new datasets have paved the way for future studies to build upon them (or even combine them together). I certainly foresee Musser and Cracraft's dataset playing a big role in my future research...

References

Monday, April 1, 2019

The Walking with Beasts Evolution Game

Following my withdrawal from writing fake science articles for April Fools', I've been coming up with new ways to celebrate the occasion. In the end, I think Jamie Revell of Synapsida and Meig Dickson of A Dinosaur A Day had the right idea all along: instead of writing fake content for April Fools', they blog about subjects that don't fall under the typical purview of their blogs.

In that spirit, I could write about a recent discovery about some kind of non-maniraptoran animal. There is certainly no shortage of studies I could select from, and I expect that to be the direction I take in the future. However, for this first "rebranded April Fools'" post, I have decided to take the chance to reminisce about a small piece of paleo community internet history that appears to have been largely forgotten.


Fans of Walking with Dinosaurs who explored the series website in the 2000s likely remember the Big Al Game. This game allowed players to take on the perspective of a male Allosaurus, living out its life from hatchling to adulthood. (As far as I could tell, the final, adulthood level could go on indefinitely as long as you weren't killed.) It was a text-based game with point-and-click mechanics; the player could click the arrows on a compass to decide which direction the Allosaurus would travel, and click buttons to determine how to interact with the other animals they encountered.

Unfortunately, the BBC took the Big Al game offline in 2011. Despite the straightforward premise and simple layout, it had probably provided hours of entertainment to numerous paleontology enthusiasts. It is fondly remembered by many in the online communities that I'm a part of and has been cited as a source of inspiration for the ambitious Saurian video game project. The fanmade Walking with Wiki does a decent job at describing the contents and gameplay of the Big Al Game, enough so that a game developer who had never played the original has managed to recreate it with reasonable accuracy.

Far less well known is that Walking with Beasts, the sequel series to Walking with Dinosaurs, also had an accompanying online game with a similar gameplay style, known as the Evolution Game. Unlike the Big Al Game, which played out the life of an individual animal, the Evolution Game took players over the course of primate evolution, starting out the game as the Eocene primate Teilhardina. After playing for some time, the game would do a timeskip of several million years, upon which the player would assume the perspective of a descendant primate species, with different habitats and organisms to encounter. This would occur repeatedly throughout the course of the game.

Compared to the Big Al Game, few details of the Evolution Game have been publicly documented. Among the same circles in which the Big Al Game sparks immediate recognition, I've been hard-pressed to find anyone who even remembers it existed, let alone has memories of playing it. Its Walking with Wiki article contains little information that can't be gathered by Wayback Machine. As a result, this post reflecting on what I remember about the game might be of some interest to a certain portion of the online paleo-community. However, keep in mind that this is based on memories that are over a decade old, so I can't guarantee that it is entirely accurate.

Image showing the overall layout of the Evolution Game, screencapped from Wayback Machine. The archive doesn't appear to have captured the panel below "Hear", which contained information on what your character was perceiving through their sense of smell.

Overall gameplay mechanics were similar between the Evolution Game and the Big Al Game, but there were a few logistical differences reflecting the fact that the player characters were primates instead of an Allosaurus. For one, in addition to traveling through horizontal space using compass directions, players also had the option to climb and travel through the trees. For another, plant life was given a greater focus in the Evolution Game, and players could choose to feed on the leaves and fruits they came across. On the faunal side of things, very small animals such as insects and frogs could also be "eaten", whereas larger ones could be "attacked". At least in the beginning of the game, the largest animals that the player would be capable of killing included leptictids and opossums. Some animals in the game were identified only in general terms (e.g.: "arboreal hyaenodont"), but others were identified as members of specific genera (e.g.: Arctocyon, Kopidodon).

Making this post still somewhat relevant to maniraptors is that some birds could be encountered in the game (the few I recall being Eocene species referred to as "roller-like bird" or "woodpecker-like bird"). Most of the time, they were said to have flown off before you'd even get the chance to interact with them, but they were much too agile for you to catch even on the rare occasion that you did.

One characteristic of the game that I personally found memorable (and sometimes darkly humorous) were the "game over" messages that were shown if the player character died. Whereas the game over messages of the Big Al Game tended to be straightforward and repetitive (e.g.: "You attacked the [insert animal], but it was too strong and killed you"), the Evolution Game would go to the trouble of providing a somewhat detailed description of how you died. Miacids would "pounce on you" as you "tried to grab" them. Pangolins were surprisingly dangerous and could slash you fatally with their claws. Even a Propalaeotherium was too much for your early primate to handle, as it could kick you to death with its "tiny hooflets". And if you ever made the foolish decision to mess with a Pristichampsus, its long jaws would "close on you as you became history".

When the player came across a member of the same species in the game, the conspecific could be attacked as with any other animal, but one or two additional interactive options would appear. One was along the lines of "invite to group". I never fully grasped how group living altered your gameplay, though it required you to share food with other members of your group. It may have also allowed you to kill slightly larger prey than you normally could, but I am less certain about that. Every now and then, members of your group would leave of their own volition. Another option that occasionally showed up was "mate", evidently indicating that you'd encountered a member of the opposite sex. When this option was selected, the "species population" bar on the side panel would go up.

A fossil of the pantolestan Kopidodon, photographed by "Daderot", public domain. Kopidodon was one of the animals that could be encountered in the Evolution Game, and one of many species that could easily kill your Teilhardina if provoked.

Probably the biggest difference the Evolution Game had from the Big Al Game, however, was that it was hard. The Big Al Game had its own challenges, but a few rounds of trial and error were often enough for players to become familiar with in-game hazards and come up with a working strategy to get through the game. The Evolution Game, on the other hand, was much more difficult to figure out. I personally never made it past the Miocene (I remember my character being a Proconsul at that point, though my memory of the primate species in the game is hazy).

The main issue appeared to come to down to a lack of suitable food sources available to the player. Eating most types of leaves diminished the player character's energy instead of replenishing it, probably as a way to indicate that the player could not digest them properly. More baffling was the fact that larger prey items that the player could kill had the same result (even though one would expect meat to be fairly easy to digest). As a consequence, the only viable food for the player at the beginning of the game were small prey such as insects. However, as the game continued, there would come a point where the small animals would become too agile for the player to catch (likely reflecting the larger body size of the later primates), and yet foliage remained largely indigestible. The only substantial foods at that point were fruits and palm leaves, which were few and far between, leaving the player character doomed to starvation.

The game repeatedly hinted that the player's decisions could influence their evolutionary trajectory, and during some of my playthroughs I tried to get over the starvation hurdle in a Lamarckian way by having my early primate eat a diet with a higher proportion of plant material. Although this did alter the species that I ended up evolving into, I still did not gain specializations for folivory quickly enough to avoid going hungry. On one particular occasion, I died while desperately stuffing my face with acacia leaves, as though I could suddenly gain such specializations from doing so. According to the game over message, I lost my remaining strength before I could take another bite of the leaves, and plummeted from the tree.

I never got the chance to think of a better strategy than that, as the Evolution Game went offline in 2007, several years earlier than its predecessor. Naturally, I also never found out how the game depicted the Quaternary Period, or whether the game had any "end goal". On that note, the game over screen I keep mentioning provided tantalizing clues. Following the description of the player's cause of death, it also included a long list of extant primates, with the implication that the player could potentially evolve into any of them if they'd survived to present day. Given the lack of documentation available for the game, we may never know for sure.

A variety of extant primates, composited by "Miguelrangeljr", under CC BY-SA 3.0. Possible "end points" of the Evolution Game?

Friday, February 8, 2019

Finches Before There Were Finches: Eofringillirostrum and the Diversity of Stem-Passerines

Many types of modern birds eat seeds from time to time. It's a concept so familiar to us that the idea of "bird food" is likely to conjure up imagery of seeds, and indeed seeds probably comprise the majority of food that we offer to both pet and wild birds. It has even been suggested that seed-eating helped the ancestors of modern birds survive the end-Cretaceous mass extinction. However, living on a diet composed primarily of seeds is something that only a relatively small number of bird groups do.

Many of these seed-eating birds are passerines. Though most modern birds can perch, passerines are often called "perching birds" because their feet are particularly specialized for this task. As a whole, passerines account for about 60% of modern bird diversity, but most seed-eating specialists belong specifically to a group of passerines called Passeroidea. Seed specialist passeroids include finches, sparrows, buntings, cardinals, weaverbirds, estrildids (such as the colorful Gouldian finch of Australia), some tanagers (including Darwin's "finches", which are not really finches), and more. All of these birds have heavy-duty, cone-shaped beaks that they use for cracking open seeds. It's perhaps not surprising that bites from seed-eating passeroids are among those most dreaded by bird banders.

Based on the latest estimates, seed specialist passeroids evolved fairly recently during the Miocene, roughly 15 million years ago. Thanks to a new discovery, however, we now know that other birds led similar lifestyles to finches and sparrows long before these groups had even appeared. In a new study, Daniel Ksepka and colleagues named two new species of Eocene birds that exhibit adaptations for seed eating similar to those of seed-eating passeroids.

One of these new species, Eofringillirostrum boudreauxi, came from the early Eocene Green River Formation in North America, making it about 52 million years old. It was a small bird, about the size of a red-breasted nuthatch, and is known from an excellent specimen, a nearly complete skeleton preserved with feathers. Its most notable feature, however, is its stout, cone-shaped bill, which bears a strong resemblance to that of finches.

The holotype of Eofringillirostrum boudreauxi, from Ksepka et al. (in press).

The other new species was also assigned to the genus Eofringillirostrum, and was named Eofringillirostrum parvulum. This species came from the other side of the globe, the Messel Shale in Germany (which dates to about 47 million years ago). It was even smaller than E. boudreauxi, though its head was proportionately larger. The type specimen of E. parvulum is not quite as well preserved as that of E. boudreauxi, but the finch-like skull is evident.

The Green River and Messel are two of the richest fossil sites when it comes to preserving Eocene bird fossils, and Eofringillirostrum is not the only Eocene bird genus that has been found at both localities. Some other birds that are known to have had similar distributions include the stem-roller Primobucco and the rail relative Messelornis.

The holotype of Eofringillirostrum parvulum, from Ksepka et al. (in press).

Though their similarity to finches is striking, the skull of both Eofringillirostrum species is notably different from those of finches in having a prominent projection at the back of the lower jaw. This is a feature typically found in birds that can open their jaws widely. The describers of Eofringillirostrum speculate that this ability allowed it to swallow large seeds and deposit them in its crop (a pouch for temporary food storage at the base of the throat in birds), or helped it gulp down fruits as an alternative food source.

The skull of Eofringillirostrum (B), compared to that of a speckled mousebird (A), which has a similar projection behind the lower jaw, and an American goldfinch (C), which has a similar cone-shaped bill, from Ksepka et al. (in press).

To find out how Eofringillirostrum was related to modern birds, the describers included it in a phylogenetic dataset along with many other species of telluravians, a diverse group of mainly tree-dwelling birds including passerines, parrots, birds of prey, woodpeckers, and more. When this dataset was analyzed, Eofringillirostrum turned out to be a stem-passerine. In other words, passerines as a whole are its closest living relatives, but it was not a member of the group exclusive to extant passerine lineages. It certainly was not particularly closely related to finches or any of the other seed-eating passerines today.

Furthermore, Eofringillirostrum was found to be a member of a specific group of stem-passerines, the psittacopedids. This group includes several other Eocene birds, including Psittacopes and Pumiliornis from the Messel and Morsoravis from the Fur Formation in Denmark. Psittacopedids have not always been recognized as stem-passerines, partly because they had a fourth (outermost) toe that was at least partially reversed. This feature (known as zygodactyly) is not found in modern passerines, in which only the first or innermost toe points backwards (as is typical of most modern birds). However, genetic data have consistently shown that the closest living relatives of passerines are parrots, which do have zygodactyl feet. In light of this, it is not so surprising that passerines appear to have evolved from zygodactyl ancestors.

There are other noteworthy aspects of the phylogeny recovered by this study. One of the oldest known true passerines, Wieslochia from the early Oligocene of Germany, was found to be a suboscine, one of the two main passerine lineages. This makes sense given that the other main passerine lineage, the oscines or songbirds, is thought to have been confined to Australia during the early Oligocene. In addition, the halcyornithids, a group of Eocene birds once thought to be most closely related to parrots specifically, were found to be stem-members of Psittacopasserae, the group uniting both parrots and passerines.

The results of the phylogenetic analysis run by Ksepka et al. (in press), from their study. Note that "Afroaves" should be labeled Australaves.

In fact, the phylogeny of psittacopasserans found by this study is strikingly consistent with the results of Mayr (2015), despite the latter having used a much smaller dataset. However, the analysis from the description of Eofringillirostrum still lacks a few more early telluravians that might be interesting to include (such as the possible stem-falcon Masillaraptor and the parrot-like, apparently raptorial Messelastur). I am curious to see this dataset expanded further in the future.

As far as we know, Eofringillirostrum was unique among psittacopedids for its seed-eating adaptations. Other psittacopedids had quite different skulls. Morsoravis had a generalized, thrush-like beak, suggesting a generalist diet of invertebrates and fruit. Psittacopes had a short, slightly downcurved beak, which is found in birds that mainly feed on insects but also eat seeds. Pumiliornis had a long beak and has been found with pollen as gut contents, indicating that it likely fed on nectar. It's often easy to imagine stem-groups as little more than intermediates "on their way" to becoming modern species, but Eofringillirostrum and other psittacopedids show that stem-passerines had their own independent burst of diversification, taking on ecological niches that true passerines wouldn't occupy until millions of years later.

The skulls of stem-passerines (left) compared to those of extant passerines (right) that exhibit similar adaptations, from Ksepka et al. (in press). Morsoravis (A-B) is compared to a hermit thrush (I-J), Eofringillirostrum (C-D) is compared to an American goldfinch (K-L), Pumiliornis (E-F) is compared to a black-throated sunbird (M-N), and Psittacopes (G-H) is compared to a bearded reedling (O-P).

Between Eofringillirostrum, fellow stem-passerine Zygodactylus ochlurus, the stem-hoopoe Laurillardia smoleni, the recently extinct penguin Eudyptes warhami, and the early waterfowl Conflicto, neornithine birds have so far had a strong showing among the new paleontological discoveries of this year. I can only hope that the rest of the year is just as good!

Reference: Ksepka, D.T., L. Grande, and G. Mayr. In press. Oldest finch-beaked birds reveal parallel ecological radiations in the earliest evolution of passerines. Current Biology in press. doi: 10.1016/j.cub.2018.12.040

Monday, February 4, 2019

Conflicto and the Evolution of Waterfowl

Estimating when a specific group of organisms appeared in Earth history is never a simple task. Fossils provide the most direct evidence of when specific organisms were around, but the fossil record is far from complete. As a result, we can't assume that the oldest fossils known from a given clade were the oldest members of that clade to have existed. Fossil taxa can only provide a minimum constraint, telling us that a clade must be at least of a certain age.

If the group we're interested in is still extant (or lived recently enough for genetic material to be recovered), then molecular clocks can help. Molecular clock analyses compare the differences between the molecular sequences of different organisms and use estimated mutation rates to approximate the amount of time that has passed since their lineages diverged. However, when dealing with extremely long timescales (such as tens of millions of years), we usually cannot assume that rates of genetic mutation have remained constant for all that time. As such, most divergence time studies make use of fossils to provide minimum constraints for when specific lineages must have diverged, setting calibrations for their molecular clock. For the results of these studies to be considered reliable though, the fossils used in molecular clock studies need to be well supported as members of the respective lineages they calibrate. After all, using a fossil species to calibrate the age of a certain group does little good if the species is not actually a member of that group.

Neornithine birds are one clade that has been at the center of controversies about the timing of their origin and diversification. However, one conclusion that all divergence time studies on neornithines agree on is that by the end of the Cretaceous, they had diverged into their three major lineages: paleognaths (ostriches, emus, etc.), galloanserans (land- and waterfowl), and neoavians (all other modern birds). This post will focus specifically on the origins of waterfowl.

Modern waterfowl can in turn be split into three main lineages: screamers (an unusual South American group), the magpie goose (a single extant species from Australia), and anatids (the most diverse group, including the ducks and geese we are most familiar with). Compared to other neornithines, the fossil record appears to have been kinder to waterfowl when it comes to preserving traces of their early evolutionary history. Whereas the only potential fossils of Cretaceous paleognaths, landfowl, and neoavians consist of fragmentary specimens of ambiguous affinities, several decently complete skeletons have been posited as strong evidence of waterfowl antiquity.

The phylogenetic relationships among living waterfowl.

The most famous of these ancient purported waterfowl is probably Vegavis, which lived in Antarctica at the very end of the Cretaceous. It is known from two partial skeletons (including one that preserves a syrinx, the vocal organ of modern birds). The original description of Vegavis found it to be more closely related to anatids than to the magpie goose or screamers, which would imply that waterfowl had already diverged into their three modern lineages by the end of the Cretaceous. However, this result has not been replicated by many recent analyses, with some researchers arguing that even galloanseran affinities for Vegavis are not strongly based. A few other Southern Hemisphere birds from around the Cretaceous-Paleogene (K-Pg) boundary have been suggested to be closely related to Vegavis. These include Polarornis from the Late Cretaceous of Antarctica, Neogaeornis from the Late Cretaceous of Chile, and Australornis from the Paleocene of New Zealand. This supposed close relationship has also been questioned though, and in any case these birds are known from far less complete material than Vegavis, limiting their potential in elucidating waterfowl evolution.

Anatalavis is another fossil bird that might provide evidence for an early radiation of modern waterfowl. The type species, A. rex, comes from the North American Hornerstown Formation, which appears to straddle the K-Pg boundary. A. rex is only known from incomplete arm bones, but a second species (A. oxfordi) from the early Eocene of the United Kingdom is known from a partial skeleton. The broad, flattened bill of A. oxfordi is certainly quite duck-like, and its original description suggested that it was a close relative of the magpie goose.

Then there are the presbyornithids, a group of extinct, long-legged waterfowl. Presbyornithids are best known from Paleogene fossils, but a few possible Cretaceous records have been reported, including Teviornis from the Late Cretaceous of Mongolia (known from a partial forelimb). The phylogenetic position of presbyornithids is disputed, but they are often found to be crown-waterfowl (i.e.: nested among the extant waterfowl lineages), usually as close relatives to anatids.

If all of these aforementioned ancient birds were crown-waterfowl as has been suggested, that would indicate that all three extant lineages of waterfowl originated in the Cretaceous and made it through the K-Pg extinction, along with a few extinct waterfowl groups. But were there really modern-type ducks paddling around at the same time that Tyrannosaurus rex was alive? The oldest fossils of unambiguously anatid-like waterfowl are much younger, hailing from the late Eocene, and the oldest unambiguous magpie goose fossil comes from the Oligocene. This by itself does not falsify an early origin of crown-waterfowl; after all, we are familiar with the concept that the fossil record contains many gaps. However, without a better understanding of how the ancient waterfowl relate to extant ones, it is difficult to determine when the lack of fossils reflects true absence and when we're simply looking at a missing record.

A new fossil described by Claudia Tambussi and colleagues might shed light on this question. The fossil comes from the early Paleocene of Antarctica, about 64.5 million years ago, putting it shortly after (by the standards of geologic time) the K-Pg mass extinction of 66 million years ago. Assigned to a new genus and species, Conflicto antarcticus, the specimen is extremely well preserved for a bird fossil. The bones are preserved in three dimensions (instead of being flattened) and represent much of the skeleton, including the skull and most of the major limb bones other than the feet.

The skull of Conflicto, from Tambussi et al. (in press).

Conflicto was about the same size as an extant magpie goose (and thus larger than typical ducks). Its overall anatomy, most prominently its flattened bill, makes it clear that it was a waterfowl, but which modern waterfowl was it most closely related to? To find out, its describers entered it into the phylogenetic dataset used by Worthy et al. (2017), probably the most comprehensive morphological dataset focused on galloanserans so far. When they did so, they found that Conflicto was equally closely related to all extant waterfowl; in other words, it fell outside of the group exclusive to the extant waterfowl lineages. Conflicto was a stem-waterfowl, not a crown-waterfowl.

Possibly even more interesting, however, was what happened to other early waterfowl in this analysis. Anatalavis (also included in this dataset for the first time, as far as I'm aware) was recovered not as a close relative of the magpie goose, but as another stem-waterfowl. In fact, it was found to be the closest known relative to Conflicto, though the authors of the study point out that statistical support for this result is weak. The presbyornithids (formerly recovered as crown-waterfowl by Worthy et al.) turned out to be stem-waterfowl as well. Vegavis was found to be yet another stem-waterfowl, but most curiously it was found to be a close relative of gastornithiforms, which were giant flightless galloanserans so far only known from the Cenozoic Era. It should be noted that the relationship between Vegavis and gastornithiforms is not entirely new, as it was also found by some of the analyses run by Worthy et al. In addition, it does not have strong statistical support.

In any case, the analysis including Conflicto excludes all of these ancient birds from the radiation of modern waterfowl (and that's not to mention the aforementioned skepticism of Vegavis being a galloanseran at all). Thus, they probably should not be used as calibrations for the age of modern waterfowl in future molecular clock studies. If this phylogeny is correct, we have no fossil evidence of modern-type waterfowl from before the K-Pg boundary. Modern waterfowl could well have originated later during the Paleogene, which would be consistent with their known fossil record.

The results of the phylogenetic analysis run by Tambussi et al. (in press), from their study.

In addition to the timing of waterfowl evolution, Conflicto also has implications for the evolution of notable waterfowl features. Among modern waterfowl, the screamers stand out in lacking a flattened beak (along with other strange characteristics), instead having one superficially similar to that of landfowl such as chickens. Given that the magpie goose and anatids are more closely related to each other than to screamers, it might be reasonable to assume that the characteristic flattened bill of these waterfowl originated relatively recently, after their lineage had diverged from that of screamers. However, it has long been noted that screamers have vestigial versions of the filter-feeding plates found inside the mouths of other waterfowl, raising the possibility that the ancestral waterfowl was a flat-beaked filter feeder and that these features were later lost by the screamer lineage.

Conflicto supports this second possibility. With the finding that the flat-billed Conflicto, Anatalavis, and presbyornithids might have been stem- rather than crown-waterfowl, it appears likely that all modern waterfowl (including screamers) descended from a filter-feeding bird with a duck-like bill. Furthermore, Conflicto and presbyornithids both had long hindlimbs, suggesting that this might have been another feature present on the line leading to modern waterfowl.

Skeletal of Conflicto, from Tambussi et al. (in press). Preserved bones are shown in white.

The describers of Conflicto named it for their prediction that its phylogenetic position and evolutionary implications are likely to become the subject of heated debate. They are probably correct. However, for the time being I personally find the results of this study to be very appealing in terms of its congruence with the known fossil record and previous observations regarding extant waterfowl. Let's see what the future brings.

Reference: Tambussi, C.P., F.J. Degrange, R.S. De Mendoza, E. Sferco, and S. Santillana. In press. A stem anseriform from the early Palaeocene of Antarctica provides new key evidence in the early evolution of waterfowl. Zoological Journal of the Linnean Society in press. doi: 10.1093/zoolinnean/zly085

Monday, January 21, 2019

The Raptormaniacs List of Extinct Cenozoic Birds

For almost as long as there has been an online paleontology community, there have been online Lists of Dinosaurs. An entire article could probably be written on the "List of Dinosaurs" phenomenon. To be fair, one can find taxonomic lists of nearly any sufficiently charismatic group of organisms in existence, but the dinosaur enthusiast community appears to be exceptional for the number of times it has independently compiled the "List of Dinosaurs". Examples of some that are still around include George Olshevsky's Dinosaur Genera List, Thomas Holtz's supplement to his dinosaur encyclopedia (last updated in early 2012), and The Compact Thescelosaurus (successor to Thescelosaurus.com). Of course, these lists don't include every dinosaur. They generally only cover non-avialan dinosaurs or, at most, Mesozoic dinosaurs.

A few months after I started this blog in 2010, I wanted in on the fun. I wasn't quite as ambitious as try and make my own version of the List of Every Mesozoic Dinosaur, but I did want to make a list of Mesozoic maniraptors. With all the Lists of Dinosaurs that were already in existence for me to reference, it didn't take long for me to set one up. My list likely doesn't do anything that the other Lists of Dinosaurs don't. It's written for most part in very non-technical language and doesn't cite any technical sources. Its main function over the years has probably been to help myself keep track of Mesozoic maniraptors more than anything else.

However, I am not only interested in Mesozoic maniraptors. Ever since its inception, the heading to my list included the sentence: "I hope to include the Cenozoic taxa someday, but for now I'm focusing on the Mesozoic ones." A comprehensive, up to date list of extinct Cenozoic birds remained an empty niche, but that also made starting one up at all much more challenging. Even Wikipedia doesn't have articles on all Cenozoic extinct birds at the time of writing.

Well, I have now taken the plunge. I have started a separate page on the blog where I've compiled a list of Cenozoic birds.

Before anyone gets too excited, I'd like to make clear that my list does not completely fill the void that is there. At present, the list only includes entirely extinct bird genera, so it does not include extinct species that are classified in the same genera as still-living species. It is possible that I will change that one day. (Will it take another eight years? We'll find out.)

It is very unlikely though that I will ever compile my own list of bird genera known only from extant members. This is due to the immense amount of work that would be required, as well as the fact that several lists of extant birds already exist (a couple of my favorite examples being Taxonomy in Flux and the IOC World Bird List). I suppose if Mesozoic dinosaur fans and Cenozoic dinosaur fans have anything in common beyond liking dinosaurs, it's an inordinate fondness for lists!

My Cenozoic extinct bird list is written in the same non-technical style as my Mesozoic maniraptor list. However, I did have to consult numerous technical publications to compile it. I'd like to highlight in particular Gerald Mayr's Avian Evolution and Paleogene Fossil Birds, Jirí Mlíkovsky's Cenozoic Birds of the World (yes, I am aware of the caveats associated with this work), and Pierce Brodkorb's Catalogue of Fossil Birds. If it weren't for these books, my task would likely have been almost impossible to complete. Some other (less technical) sources of importance were Julian Hume's Extinct Birds and Meig Dickson's A Dinosaur A Day (which, given its coverage of extant birds, is almost certainly the most ambitious List of Dinosaurs of all!).

Given the state of affairs, I would be surprised if I really had managed to include every extinct Cenozoic bird taxon on my first try, so if you spot any errors or omissions, please leave a comment! Keep in mind though that I have excluded certain taxa if they are now considered congeneric with extant taxa or if their genus name is preoccupied and a replacement name has not yet been coined.