Tuesday, January 19, 2021

New (Extinct) Maniraptors of 2020

The "new year" is not so new anymore (three new maniraptors have been named in 2021 already), but this is a tradition and I'm sticking to it, for now. Having done a sweeping overview of the past year in maniraptoran discoveries, I will now take a more in-depth look at the new extinct taxa that were described last year. In addition to the new genera and species of 2020, this time around I will also make an effort to mention other relevant nomenclatural changes that were proposed.

Last year saw the naming of Trierarchuncus, the youngest known alvarezsaur. Fragmentary alvarezsaur specimens had been previously reported from the latest Cretaceous Hell Creek and Lance Formations of the western United States, but these fossils had been left unnamed. Trierarchuncus itself is not known from a whole lot—it was described based on three thumb claws (belonging to three different individuals) from the Hell Creek Formation, and there are parts of an arm and a foot as well as a previously described partial hip that may also belong to it. Shortly after its initial description, a second paper assigned two more thumb claws to Trierarchuncus.

Even though the specimens don't represent much of the skeleton, Trierarchuncus is potentially quite informative about alvarezsaur biology. I noted last year in a post about alvarezsaurids that completely preserved alvarezsaurid thumb claws are rare. One of the claws assigned to Trierarchuncus, however, is very complete, showing a sharp tip and stronger curvature than is typically assumed for alvarezsaurid thumb claws. To my eye at least, this claw shape is in keeping with use of the claw in hook-and-pull digging, as I had contended in my blog post (and others had contended in scientific papers). Perhaps in reference to the strongly curved claw, the name Trierarchuncus can be translated as "Captain Hook".

The most completely preserved thumb claw referred to Trierarchuncus, from Fowler et al. (2020).

Seeing as the claws assigned to Trierarchuncus come from individuals of different sizes, they may also provide insight into how alvarezsaurid claws changed as they grew. The larger Trierarchuncus claws are widened at their base and the surface of the bone exhibits a roughened texture, which likely developed as a response to considerable stresses. The authors who made these observations noted that this is also consistent with the hypothesis that alvarezsaurids used their thumb claws for digging.

One of the most spectacular new maniraptors from last year, in terms of the fossil material represented, may have been the oviraptorid Oksoko from the Late Cretaceous Nemegt Formation of Mongolia. It is known from the skeletons of several individuals, including nearly complete skeletons. Notably among these specimens is an assemblage of at least three (maybe four) juveniles preserved together, which had been confiscated from fossil poachers in 2006. Some of these juveniles are preserved in lifelike crouched postures, suggesting that they had been buried rapidly and simultaneously. They therefore provide strong evidence of gregarious behavior in oviraptorosaurs. Oksoko was unusual among oviraptorosaurs in that its third finger was highly reduced, giving it functionally two-fingered hands, and the describers note that similar hands may have been present in Heyuannia huangi.

Several juvenile specimens of Oksoko (including the holotype, marked in blue) preserved together, from Funston et al. (2020).

2020 also saw a much-needed taxonomic overhaul of the caenagnathid oviraptorosaurs from the Late Cretaceous Dinosaur Park Formation in Alberta. Many Dinosaur Park caenagnathids were named based on non-overlapping parts of the skeleton, which has naturally raised questions about whether they all really represent distinct species or not. Based on the examination of newly described, more complete specimens, as well as assessment of individual growth stage based on bone microstructure, this latest study concluded that there are probably three distinct caenagnathids known from the Dinosaur Park Formation: the large Caenagnathus, the medium-sized Chirostenotes, and the small, newly-named genus Citipes, which had formerly often been considered a species of either Chirostenotes or Leptorhynchos. Although not the main focus of the study, this paper additionally mentions that the Gobiraptor from the Nemegt Formation likely represents a juvenile of Conchoraptor, as had been hinted in the past.

Also in the world of oviraptorosaur taxonomy, the feather-preserving specimens of Similicaudipteryx were reassigned to Incisivosaurus (previously known only from a skull and partial neck vertebra).

Non-ornithothoracean Paravians
In contrast to 2019, which was conspicuously lacking in new dromaeosaurids, at least two new dromaeosaurids were described in 2020. One of these was the microraptorian Wulong from the Early Cretaceous Jiufotang Formation of China, known from the exquisitely-preserved complete skeleton of an immature individual. Like Microraptor, Wulong had large feathers on its hindlimbs, and the specimen also preserves a single pair of elongated tail feathers at the tip of its tail.

Holotype of Wulong, from Poust et al. (2020).

The other new dromaeosaurid was Dineobellator, a mid-sized (Velociraptor-sized) dromaeosaurid from the Ojo Alamo Formation of the United States. It is known from a partial skeleton, and as in Velociraptor and Dakotaraptor, its ulna exhibits a series of bumps for the attachment of wing feathers along the forearm.

One new paravian that might be a dromaeosaurid was Overoraptor, known from a couple of partial skeletons from the Late Cretaceous Huincul Formation of Argentina. The phylogenetic analysis in the description of Overoraptor recovered it as a close relative of Rahonavis, a smaller, likely flight-capable paravian from the Late Cretaceous of Madagascar. The phylogenetic position of Rahonavis is controversial; it has been found as either an unenlagiine dromaeosaurid or an avialan in recent studies. This particular study found Overoraptor and Rahonavis as avialans, though the phylogenetic dataset used tends to find paravian relationships that are not strongly supported by other analyses, with microraptorians and unenlagiines outside of dromaeosaurids and instead being more closely related to modern birds. It would be interesting to see where Overoraptor turns up when included in other phylogenetic datasets.

Rounding out the long-tailed paravians of 2020 is Kompsornis, a Jeholornis-like avialan from the Jiufotang Formation. The taxonomy of the various Jeholornis-like avialans is debated, but the describers of Kompsornis argue that Shenzhouraptor, Jixiangornis, Jeholornis prima, Jeholornis palmapenis, and Jeholornis curvipes all represent distinct, valid taxa. This paper is also notably one of the few to address "Dalianraptor", a supposedly short-armed, long-tailed avialan. The authors confirm that the original specimen of "Dalianraptor" is likely a composite, and find no definitive features that distinguish it from other Jeholornis-like avialans.

Only one new enantiornithean was named last year, but it was a doozy. Falcatakely from the Late Cretaceous Maevarano Formation of Madagascar is known from a nearly complete skull, and it's a skull unlike that of any other known theropod. Its snout was long and tall, giving it a superficial resemblance to a toucan or a hornbill. However, most of the snout was composed of enlarged maxilla bones, contrasting with the condition in modern birds, in which the snout is primarily composed of the premaxillae. The maxillae of Falcatakely were toothless and probably covered in a keratinous sheath, but the specimen preserves at least one small premaxillary tooth.

Holotype of Falcatakely, from O'Connor et al. (2020).

The describers of Falcatakely ran several different phylogenetic analyses and consistently recovered it as an enantiornithean each time. Even so, it's probably fair to wonder whether this bizarre creature really was an enantiornithean, especially considering that very few other Late Cretaceous enantiornithean skulls are known for comparison. As Mickey Mortimer has pointed out, a highly modified Late Cretaceous theropod with no obvious close relatives and known from limited remains is likely going to be difficult to place no matter what. (It's perhaps worthy of note that little to no skull material has been found for the two other paravians that have been named from the Maevarano Formation, Rahonavis and Vorona.) Regardless of what it turns out to be, the unusual and unexpected morphology of Falcatakely easily qualifies it as one of the highlights among last year's new dinosaurs.

In other news, a review of the Mesozoic and Paleocene pennaraptoran fossil record considered the poorly-studied Jiufotang enantiornitheans Largirostrornis and Longchengornis to be synonyms of Cathayornis, though this was not elaborated upon.

Non-neornithean Euornitheans
The taxonomy of yanornithid euornitheans was revised in 2020, resulting in the recognition of two new genera, Abitusavis and Similiyanornis. Similiyanornis from the Jiufotang Formation had a distinctive tooth arrangement in which the frontmost tooth in its lower jaw was enlarged. Abitusavis was larger than Similiyanornis and was discovered in the older Yixian Formation. Of note is that the first yanornithid specimen reported with fish remains preserved in its digestive tract (originally assigned to Yanornis) is now considered a specimen of Abitusavis. Several other previously described specimens (including the poorly-studied "Aberratiodontus") were deemed indeterminate yanornithids in this study, whereas Yanornis guozhangi was sunk into the type species of Yanornis, Y. martini.

Holotype of Similiyanornis, from Wang et al. (2020).

Another new Early Cretaceous euornithean is Khinganornis from the Longjiang Formation of China. It is the first dinosaur to be described from this formation, and is known from a nearly complete skeleton, but its anatomical details are not well preserved. It appears to have been generally similar to the semi-aquatic euornitheans Gansus and Iteravis, and may have led a similar lifestyle.

No new paleognaths were named last year (at least, none based on skeletal material), but the old genus Palaeostruthio was brought back and applied to "Struthio" karatheodoris, a large ostrich from the Miocene of Eurasia.

Alright, let's do this one more time... Asteriornis from the Late Cretaceous Maastricht Formation of Western Europe is one of the few well-established examples of a Mesozoic crown bird. It is known from several bones, most notably a nearly complete skull. The skull exhibits features characteristic of both anseriforms and galliforms, and Asterornis may thus be a close approximation of what the ancestral galloanseran looked like. There's plenty more that could be said, but I already wrote about Asteriornis in more detail here.

Skull of Asteriornis, from Field et al. (2020).

The Cenozoic contributed its fair share of new galloanserans as well. From the Eocene of Kazakhstan came the anseriform Cousteauvia, known from a tarsometatarsus that displays features suggestive of diving behavior (hence the name honoring Jacques Cousteau). Its full binomial, Cousteauvia kustovia, is a bit of fun with homophones. There were also a couple of new pheasants from Bulgaria, the Miocene Phasianus bulgaricus and the Pleistocene Chauvireria bulgarica.

It's not often that we get a new gastornithid, but last year gave us Gastornis laurenti from the Eocene of France. It was named based on a distinctive lower jaw, but its describers mention that it is known from postcranial bones as well, which will be described in a later publication.

What's this? A fossil columbimorph older than the Pleistocene and represented by decently complete material?! Linxiavis was a sandgrouse from the Miocene Liushu Formation of China, and is known from a partial skeleton including most of the forelimbs. Sandgrouse today live in arid habitats (they famously use their belly feathers to transport water for their young), so the discovery of Linxiavis is consistent with other lines of evidence suggesting that the Liushu Formation was deposited in a relatively dry environment.

Holotype of Linxiavis, from Li et al. (2020).

A new columbimorph that lived in more recent times is the Pleistocene–Holocene pigeon Tongoenas, remains of which are known from several islands in the Kingdom of Tonga. Its hindlimb anatomy suggests that it was a primarily tree-dwelling form, similar to the fruit doves and imperial pigeons that are still extant today. Tongoenas was one of the largest known flying pigeons, surpassed in that department only by the crowned pigeons, which can reach the size of a small turkey.

The one new gruiform named last year was the coot Fulica montanei, known from a few tarsometatarsi found in the Pleistocene–Holocene Laguna de Tagua Tagua Formation of Chile. It was large for a coot, comparable in size to the extant horned coot (F. cornuta), and potentially had limited flight capabilities due to its size.

Meanwhile, to maintain consistency with the current taxonomy of extant cranes, the Pleistocene Cuban flightless crane "Grus" cubensis was transferred to the genus Antigone, which includes its probable close relatives such as the sarus (A. antigone) and sandhill (A. canadensis) cranes.

The origins of charadriiforms are murky, but 2020 saw the publication of a new taxon that may bear on the subject. That new taxon was Nahmavis from the Eocene Green River Formation of the United States, and it is known from a partial skeleton (missing the forelimbs and shoulder girdle) with preserved feathers. This specimen had previously been reported in popular books and websites as a potential close relative of the seriema-like bird Salmila from the Eocene of Germany, but the describers of Nahmavis instead find it to be more similar to possible stem-charadriiform Scandiavis from the Eocene of Denmark.

In the new study, Nahmavis and Scandiavis were indeed sometimes found as stem-charadriiforms. However, other analyses in this paper placed them as gruiforms instead. It is unfortunate that forelimb material is unknown for both Nahmavis and Scandiavis, seeing as the forelimb bones of charadriiforms are often distinctive. For what it's worth, playing around with the phylogenetic dataset used in this study, I've found that Nahmavis and Scandiavis are consistently recovered as stem-charadriiforms when the internal topology of crown charadriiforms is constrained to the results of molecular studies. In any case, the holotype of Nahmavis is a lovely specimen, and it's good to see it described at last.

Holotype of Nahmavis, from Musser and Clarke (2020).

Among crown charadriiforms, a new species described last year was the recently extinct sandpiper Prosobonia sauli from Henderson Island in the Pitcairn Islands. The genus Prosobonia also includes at least three other extinct sandpiper species as well as the extant Tuamotu sandpiper (P. parvirostris). Unlike typical sandpipers, the members of this genus are not migratory, and are confined to various remote Polynesian islands. Compared to the Tuamotu sandpiper, P. sauli had a wider, straighter bill tip and longer hindlimbs.

As yet, it appears that no one has formally suggested in technical literature what name should be used for the clade uniting phaethontimorphs (tropicbirds, sunbitterns, and kagus) and aequornitheans ("core waterbirds", such as penguins, petrels, pelicans, etc.). I have provisionally settled on resurrecting the old name Natatores for this group, as it has been used in relatively recent literature for a very similar assemblage of birds.

Fossil penguins have generally had a good showing in recent years, and 2020 added Eudyptes atatu from the Pliocene Tangahoe Formation of New Zealand to the lineup. It is known from several specimens, and the holotype is a partial skeleton that includes much of the skull and shoulder girdle. E. atatu was closely related to the extant crested penguins, but its beak was shallower in depth compared to the fairly tall beak seen in the living members of this genus.

Two other new natatoreans belonged to a different group of wing-propelled divers, the extinct plotopterids, which were probably close relatives of boobies and cormorants. The new plotopterids Empeirodytes and Stenornis both come from the Oligocene of Japan and are known from isolated coracoids, with Stenornis being the larger of the two.

Whereas no raptorial telluravians were named in 2019, 2020 turned out to be extremely productive in the realm of new raptors. Two New World vultures were named from the Quaternary of Cuba, Coragyps seductus (which was similar to the extant black vulture, Coragyps atratus) and Cathartes emsliei (the smallest member of the genus Cathartes, which also includes the turkey vulture, C. aura, among others).

The Quaternary of Cuba gave us three new hawk species as well, these being Gigantohierax itchei, Buteogallus royi, and Buteo sanfelipensis. G. itchei was the biggest of the three (though it wasn't as big as the type species of Gigantohierax, G. suarezi), whereas B. sanfelipensis was the smallest, being somewhat smaller than a red-tailed hawk (Buteo jamaicensis). From further back in geologic time, we got the large Vinchinavis from the Miocene Toro Negro Formation of Argentina, represented by partial forearm bones. Even older was ?Palaeoplancus dammanni from the Eocene of the United States, known from a tarsometatarsus.

Arguably the most spectacular new accipitrid, however, was Aviraptor from the Oligocene of Poland. It wasn't spectacular due to its size—it was the smallest of the new accipitrids, about the same size as the aptly-named tiny hawk (Accipiter superciliosus). However, it is known from a nearly complete skeleton, a rarity among fossil accipitrids. Its combination of small body size and relatively long hindlimbs is commonly seen in extant accipitrids that hunt other birds, suggesting that it may have had similar habits. It may not be a coincidence that early hummingbirds and passeriforms are known to have lived in Europe at around the same time as Aviraptor.

Holotype of Aviraptor, from Mayr and Hurum (2020).

The surge of new fossil raptors was not limited to vultures and hawks, as fossil owls had a very good year, too. There was the pygmy owl Glaucidium ireneae from the Pliocene–Pleistocene of South Africa, the oldest unambiguous member of its genus from Africa. The Quaternary of Cuba contributed the small barn owl Tyto maniola and the giant Ornimegalonyx ewingi, the latter based on specimens previously assigned to the horned owl Bubo osvaldoi. Although O. ewingi was much smaller than the type species of Ornimegalonyx, O. oteroi, it was still larger than any living owl species. The remaining species of Ornimegalonyx (O. acevedoi, O. gigas, and O. minor), however, have been sunk into O. oteroi.

Another large owl was Asio ecuadoriensis from the Pleistocene Cangagua Formation of Ecuador. Known from a couple of robust hindlimb bones, it was about the same size as a great horned owl (Bubo virginianus). Interestingly, its remains were found in association with the bones of smaller owls (and small mammals) that appear to have been etched by stomach acid, suggesting that it may have preyed on other owl species.

Numerous owls have been named from Paleogene fossil deposits, but most of these species are based on very limited material. Primoptynx from the Eocene Willwood Formation of the United States is known from a partial skeleton, making it one of the most completely known Paleogene owls. Like accipitrids, but unlike extant owls, Primoptynx had enlarged claws on its first and second toes, perhaps implying that it captured prey in a similar manner to accipitrids (maybe making use of the infamous "raptor prey restraint" method that may have also been employed by dromaeosaurids).

Toes of Primoptynx, from Mayr et al. (2020). The claws on the first and second toes are particularly large.

Moving away from raptors for a moment, Jacamatia from the Oligocene of France was a tiny piciform known from a partial wing skeleton. Its describers suggest that it was closely related to the jacamars and puffbirds, which are otherwise poorly represented in the fossil record. It may have belonged to the Sylphornithidae, a group of similarly tiny Paleogene piciforms, though very little overlapping material is available for comparison.

On the australavian side of Telluraves, we meet the small caracara Milvago diazfrancoi, yet another new raptor from the Quaternary of Cuba. The remaining new fossil australavians of 2020 were all songbirds: the corvid Corvus bragai from the Pliocene–Pleistocene of South Africa (the oldest known corvid from Africa) and the New World blackbirds Icterus turmalis and Molothrus resinosus from the Pleistocene Talara tar seeps of Peru.

Some additional revisions of note concerning fossil songbirds: the holotype of "Pliocalcarius" from the Pliocene of Central Asia, originally described as a longspur, was reinterpreted as a lark and transferred to the horned lark genus Eremophila, whereas "Eremophila" prealpestris from the Pleistocene of Bulgaria was argued to have been more similar to a different lark genus, Ammomanes, and removed from Eremophila.

Friday, January 1, 2021

Review of 2020

Wow, a record low in annual post count for this blog. It hasn't been for a lack of topics to write about; I had wanted to blog about estimating neoavian divergence times, the fossil record of shorebirds, the many origins of flightlessness in waterfowl, and more, but I never got around to doing so. Time will tell if I ever do. In part, I've been inactive here because I've been busy. The pandemic hasn't slowed down my academic activities much (for which I'm thankful), so I've still had my hands full working on research projects, reviewing manuscripts, and attending conferences. Those conferences went well for most part, but the switch to online formats meant that I felt less motivated to blog about them, further contributing to the lack of activity here. (Although come to think of it, a post about how online and in-person conference experiences differ might have been interesting...) I also wasn't able to make any new trips to zoos or museums, so there weren't any posts about those either.

My neglect of the blog notwithstanding, I did find the opportunity to work on a few other non-academic projects in 2020. In June, I helped put together the Dino Nerds for Black Lives livestream, and even now I'm flabbergasted that we managed to pull it off. For those who missed or would like to re-experience the livestream, most of the segments have been made available online. In addition to organizing and hosting parts of the event, I also presented a segment on the stream with my longtime friend and collaborator Joan Turmelle, in which we did a one-time revival of our old radio show Incidents and Reflections

Joan and I enjoyed doing the radio show again, so we later started a new podcast, Through Time and Clades. Strictly speaking, our show also includes visual aids, so it's not quite a conventional podcast. We currently operate on a rotating schedule. For the first episode of every month, we discuss a selection of notable new studies on natural history that came out during the previous month. Following that, we switch back and forth each week between two different series until the next month rolls around. One series, led by Joan, discusses human origins, whereas the other series, led by me, discusses the diversity and evolution of crown-group birds (and thus might be of particular interest to readers of this blog).

The title slide for the first episode of "Dinosaurs, the Second Chapter", my podcast series on crown bird evolution.

Possibly the most exciting personal achievement for me in 2020, however, was the fact that I helped name a new dinosaur in Nature! That dinosaur was Asteriornis maastrichtensis, one of the few clear examples of a Cretaceous crown bird, and the holotype includes one of the best-preserved fossil bird skulls yet found. As I wrote about previously, the discovery of this new species was highly serendipitous, and I don't expect that I'll have the chance to take part in such a high-profile study again anytime soon.

Skull of the holotype of Asteriornis maastrichtensis, from Field et al. (2020).

The description of Asteriornis might have been my favorite study from last year (if I may say so myself), but it was far from alone. Despite the state of the world, plenty of new maniraptoran research continued to be published throughout 2020, so let's take a look. As always, my coverage of papers about modern birds is necessarily going to be incomplete, so I put more focus on those that have more direct connections to paleontology, such as studies on anatomy, ontogeny, and higher-order phylogeny.

In January, African gray parrots were reported to voluntarily help members of their own species obtain food rewards. Eggshells from the Late Cretaceous of Europe thought to belong to geckos were reinterpreted as maniraptoran in origin. Avian paleoneurology was reviewed. Seabirds from the Pleistocene of Japan, birds from the Eocene of Antarctica, a partial skeleton of a Paleocene penguin, preserved skin in Palaeeudyptes, and the vocal tract anatomy of king penguins were described. Reversible outer toes were reported in gray-headed and lesser fish eagles. Red-breasted nuthatches were shown to vary their alarm calls according to eavesdropped signals from other birds. New studies came out on how flight feathers stick to one another, the brain structure of extinct avialans, convergent evolution in birds, pelvic ontogeny in greater rheas, life history changes in birds from the La Brea tar pits, the evolution of melanosomes in hummingbirds, evolutionary rates in passeriforms, evolutionary drivers in Acanthiza thornbills, and the phylogeny of Turdus thrushes. Newly-named maniraptors included the microraptorian dromaeosaurid Wulong bohaiensis, the Taliabu myzomela (Myzomela wahe), the Peleng fantail (Rhipidura habibiei), the Taliabu grasshopper warbler (Locustella portenta), the Peleng leaf warbler (Phylloscopus suaramerdu), and the Taliabu leaf warbler (Phylloscopus emilsalimi).

Holotype of Wulong bohaiensis, from Poust et al. (2020).

In February, a horned lark preserved in permafrost was identified. A series of isolated feathers and an avialan wing were reported from Burmese amber. (As I'll mention under March, Burmese amber would become a particularly hot topic later in the year.) A new specimen of Chirostenotes and a paraortygid from the Uinta Formation were described. Evidence of dietary plasticity in passenger pigeons was presented. Convergent evolution between the syrinxes of hummingbirds and songbirds was documented. The vocal sequences of African penguins were suggested to conform to linguistic laws. The nomenclature of Lophorina birds-of-paradise was reexamined. New studies came out on the taphonomy of feathers, inferring lifestyle from paravian claw curvature, locomotion in dromaeosaurids, the diversity of island birds, the origin of complex sociality in birds, introgression in the origin of the domestic chicken, the phylogeny of rails and waxbills, the shape of great auk eggs, the evolution of ornamentation in gulls, hearing in great cormorants, the aerodynamic role of raptor tails, the wing and tail musculature of barn owls, the evolution of plumage coloration in lorikeets and Australasian robins, hybridization among American crow lineages, the processing of encounters with dead conspecifics in American crows, social learning in Eurasian blue tits, and craniofacial integration in Hawaiian honeycreepers and Darwin's finches. Newly-named maniraptors included the Cretaceous euornithean Khinganornis hulunbuirensis, the white-winged tapaculo (Scytalopus krabbei), the jalca tapaculo (Scytalopus frankeae), and the Ampay tapaculo (Scytalopus whitneyi).

Pleistocene horned lark recovered from permafrost, from Dussex et al. (2020).

In March, a special volume on the evolution of feathers was published. A large oviraptorosaur from the Hell Creek Formation was described. The sensory systems of birds were reviewed. The Vaurie's nightjar was reevaluated as possibly belonging to the European nightjar. Evidence for statistical inference in kea was presented. A new classification for fluvicoline tyrant flycatchers was proposed. The supposed Pliocene longspur "Pliocalcarius" was reinterpreted as a species of the lark genus Eremophila. Red-winged blackbirds were shown to eavesdrop on the alarm calls of yellow warblers. New studies came out on snout morphology in dromaeosaurids, the modularity of the avian neck, the phylogeny of junglefowl, trochilin hummingbirds, and Scytalopus tapaculos, lung morphometrics in high-altitude waterfowl, the anatomy of rictal bristles in strisoreans, bill disparity in penguins, the evolution of migratory behavior in tyrant flycatchers, and phylogenetic signal in cowbird skulls. Newly-named maniraptors included the dromaeosaurid Dineobellator notohesperus, the Miocene pheasant Phasianus bulgaricus, the Miocene sandgrouse Linxiavis inaquosus, the Pleistocene owl Asio ecuadoriensis, and... yes, the Cretaceous pan-galloanseran Asteriornis maastrichtensis. Then there was the maniraptor that wasn't: Oculudentavis khaungraae, originally described as an avialan based on a skull preserved in Burmese amber, was quickly met with skepticism regarding its phylogenetic affinities. The paper was retracted in July, and a peer-reviewed study reassessing the specimen as a lizard was later released in October. In addition to the taxonomic controversy, the publicity that Oculudentavis received also sparked discussions about the human rights violations that often underlie the procurement of Burmese amber, resulting in official statements on the subject from scientific journals and organizations like the Society of Vertebrate Paleontology. Having been previously guilty myself of excitedly discussing Burmese amber discoveries without acknowledging the associated ethical issues, it's a topic that I've tried to treat with appropriate circumspection going forward.

Red-winged blackbird, photographed by Walter Siegmund, under CC BY-SA 3.0.

In April, red-billed oxpeckers were shown to warn black rhinos of approaching humans. A new specimen of Longusunguis and an owl from the Jebel Qatrani Formation were described. An enantiornithean foot was reported from, well... Burmese amber. Peramorphic features were identified in avian skulls. New studies came out on the ultramicrostructure of paravian teeth, the evolution of avian brain size, humeral disparity in birds, the genetic basis of avian foot feathering, the development of spinal nerves in avian tails and the olfactory system in chickens, bone histology in elephant birds, the adaptive radiation of neoavians, the phylogeny of turacos and chlorophonias and euphonias, the diversification of babblers, adaptations to migration in the flight feathers of European robins, and speciation rates in tanagers.

Diagram showing black rhinos reacting to approaching humans when warned by red-billed oxpeckers, from Plotz and Linklater (2020).

In May, giant petrels were documented attacking sperm whales. An oviraptorosaur specimen preserved with associated eggs was reported. Evidence of ontogenetic dietary shifts in Deinonychus and iridescent plumage in Calciavis were presented. Avian wing shape was found to correlate with a variety of environmental and ecological factors. A possible case of a common cuckoo being killed by mobbing from a great reed warbler was recorded. New specimens of Anhinga pannonica were described. Avian scavengers were suggested to use auditory cues to help locate food. The classification of xolmiin tyrant flycatchers was revised. Black-capped chickadees were shown to be able to identify female conspecifics by call. New studies came out on pelvic morphology of caenagnathid oviraptorosaurs, the cranial osteology of Sapeornis, the anatomy of Parahesperornis, scaling trends in avian alular feathers, the evolution of avian developmental durations, the development of the avian vertebral column, webbed feet in waterbirds, and flight feather positioning, the genetic basis of reduced lifespans in flightless birds, the phylogeny of kiwi, lorikeets, bowerbirds, and nuthatches, introgression within bean geese, the diversification of frogmouths, and biomechanical diversity in kingfishers. Newly-named maniraptors included the the basal paravian Overoraptor chimentoi, the basal avialan Kompsornis longicaudus, the recently extinct New World vulture Coragyps seductus, the recently extinct accipitrids Gigantohierax itchei, Buteogallus royi, and Buteo sanfelipensis, the recently extinct caracara Milvago diazfrancoi, the Oligocene piciform Jacamatia luberonensis, the Pleistocene troupial Icterus turmalis, and the Pleistocene cowbird Molothrus resinosus.

Northern giant petrel attacking sperm whale, from Towers and Gasco (2020).

In June, the osteology of penguins and plotopterids was compared. The osteology of Rahonavis was described in detail. An ornithuran from the Dinosaur Park Formation, a new specimen of Struthio karatheodoris, and a tyrannidan passeriform from the Oligocene of France were reported. The development of avian flight behaviors and the role of extended parenting in the evolution of corvid cognition were reviewed. Broad-tailed hummingbirds were shown to be able to disciminate non-spectral colors. The endocast of the night parrot was described. New studies came out on chemical preservation in the tail feathers of Anchiornis, the scaling of the avian middle ear, the genetic basis of sexual dichromatism in birds, casque ontogeny in southern cassowaries, the phylogeny of hill partridges, the origin of the domestic chicken, convergent responses to flightlessness in rails, flight efficiency in auks, the mandibulosphenoidal joint in penguins and procellariiforms, inbreeding avoidance in long-tailed tits, and the evolution of Aimophila and Peucaea New World sparrows.

Ontogeny of the casque in the southern cassowary, from Green and Gignac (2020).

In July, high-frequency hearing was documented in Ecuadorian hillstars. Evidence of sequential molting in Microraptor was presented. A juvenile dromaeosaurid from the Prince Creek Formation, a new specimen of Protopteryx, and a crane from the Miocene of Germany were described. Soft tissue analyses were used to support the identification of ovarian follicle preservation in Mesozoic avialans. A partial enantiornithean specimen was reported from (one more time...) Burmese amber. The syrinx of the black jacobin was visualized. A sense of numerical ordinality was found in rufous hummingbirds. Visual adaptations in raptors were reviewed. The classification of lorikeets was revised. The songs of thrush nightingales were found to share categorical rhythms with human music. Birdsong learning was shown to be beneficial to both tutees and tutors in song sparrows. New studies came out on variation in Mesozoic feathers (note: based on Burmese amber specimens), facial pneumaticity in dromaeosaurids, the endocranial anatomy of Velociraptor, sub-surface foot kinematics in birds, the morphology of avian flight feathers, the evolution of ostriches, eggshell coloration in tinamous, flight performance in Andean condors, the phylogeny of Dendrocolaptes ovenbirds, host mimicry in viduids, and cultural evolution in the song of white-throated sparrows. Newly-named maniraptors included the alvarezsaur Trierarchuncus prairiensis, the oviraptorosaur Citipes (a new genus for "Leptorhynchos" elegans), the Pleistocene pheasant Chauvireria bulgarica, the recently extinct pigeon Tongoenas burleyi, the Eocene owl Primoptynx poliotauros, the Ayacucho antpitta (Grallaria ayacuchensis), the Oxapampa antpitta (Grallaria centralis), the Puno antpitta (Grallaria sinaensis), the Chamí antpitta (Grallaria alvarezi), the Graves's antpitta (Grallaria gravesi), and the O'Neill's antpitta (Grallaria oneilli).

Preserved gap in wing feathers of Microraptor, interpreted as indicative of molting pattern, from Kiat et al. (2020).

In August, a special volume on pennaraptoran paleontology was published. The potential for powered flight was assessed in Mesozoic maniraptors. Expanded sternal ribs were documented in Jeholornis. The birds from the Eocene of the Geiseltal and avian macroevolution were reviewed. Large brains were shown to be linked to increased lifespan in birds. Fork-tailed flycatchers were found to produce sounds using their wing feathers. Red-billed queleas were reported to be able to tolerate higher temperatures than any other bird. New studies came out on rates of skull evolution in birds (and other dinosaurs), the correlation of down feather morphology with habitat and temperature, squamosal morphology in birds, the phylogeny of crown birds, pheasants, and elanine kites, the evolution of host use in avian brood parasites and breeding plumages in New World warblers, the flight behavior of Anna's hummingbirds through waterfalls, the diversification of penguins, the genetic basis of nocturnal adaptations in owls, the cranial anatomy of Otus murivorus, and ecosystem engineering by superb lyrebirds. Newly-named maniraptors included the Pleistocene–Holocene coot Fulica montanei, the Pliocene penguin Eudyptes atatu, the Eocene accipitrid Palaeoplancus dammanni, and the Pleistocene barn owl Tyto maniola.

Phylogeny of paravians, with lineages potentially capable of powered flight highlighted, from Pei et al. (2020).

In September, the isolated holotype feather of Archaeopteryx was argued to indeed belong to Archaeopteryx, contrary to a previous study from 2019. A new specimen of Piscivorenantiornis was described. Avian endocasts were shown to be reliable proxies for the sizes of corresponding brain regions. A cortex-like canonical circuit in the avian forebrain was identified and a neural correlate of sensory consciousness was reported in carrion crows. Avian plumage patterns were reviewed. Extreme torpor was documented in Andean hummingbirds. Breeding success was shown to correlate with divorce in plovers. Azure-winged magpies were found to share food with conspecifics depending on availability of food to the recipient. New studies came out on skull modularity in birds (and other archosaurs), hindlimb morphometrics in avialans, beak morphology in Confuciusornis, phylogenetic signal in the lacrimal/ectethmoid of waterfowl, ossification sequences in black-headed gulls and Eurasian reed warblers, vertebral pneumaticity and serial variation in storks, the evolution of growth patterns in passeriforms, and the phylogeny of stipplethroats. Newly-named maniraptors included the Pleistocene–Holocene New World vulture Cathartes emsliei, the Pliocene–Pleistocene owl Glaucidium ireneae, and the Pliocene–Pleistocene corvid Corvus bragai.

Black metaltail, which during torpor attains the lowest body temperature recorded of any bird, photographed by Mickaël Villemagne, under CC BY-NC 4.0.

In October, the aerodynamics of scansoriopterygids were assessed. A shark-bitten hesperornithiform, giant pelagornithids from the Eocene of Antarctica, and large New World vultures from the Pleistocene–Holocene of Uruguay were described. Visual adaptations in birds were shown to correlate with foraging niche in the Peruvian Andes. Differences in visual adaptations between predatory and scavenging raptors was reviewed. Cryptic sexual dimorphism was recorded in Sulawesi babblers. New studies came out on vertebral pneumaticity in Nothronychus, contact incubation in troodontids, plantar adaptations of bird feet, the gust-rejecting mechanism of bird wings, Quaternary avian extinctions in the Bahamas, group organization in foraging phalaropes, the demographic histories of condors, the evolution of communication signals in woodpeckers, and species delimitation in Alaudala larks. Newly-named maniraptors included the oviraptorosaur Oksoko avarsan, the Eocene waterfowl Cousteauvia kustovia, the Eocene possible stem-charadriiform Nahmavis grandei, the plotopterids Empeirodytes okazakii and Stenornis kanmonensis, and the Oligocene accipitrid Aviraptor longicrus. A paper I co-authored also got published this month, in which we proposed a phylogenetic taxonomy of strisoreans.

Holotype of Aviraptor longicrus, from Mayr and Hurum (2020).

In November, the wing musculature of flightless auks was reconstructed. New specimens of Trierarchuncus were described. Feathers preserved in amber were reported... not from Myanmar, but from the Escucha Formation in Spain. Putative ovarian follicle preservation in Mesozoic avialans was again disputed (in response to the earlier paper from July). A large dataset of avian genomes was published. Migratory birds (and mammals) were found to have faster paces of life than non-migratory species. Ecomorphology was shown to correlate with diversity in corvidean passeriforms. New studies came out on group movement and decision-making in vulturine guineafowl, the evolution of the Galápagos rail, the phylogeny of manakins, species delimitation in Calandrella larks, auditory memory in zebra finches, and introgression in Setophaga New World warblers. Newly-named maniraptors included the enantiornithean Falcatakely forsterae (with a highly unusual skull), the gastornithid Gastornis laurenti, the recently extinct sandpiper Prosobonia sauli, the South Georgia gentoo penguin (Pygoscelis poncetii), and the Miocene accipitrid Vinchinavis paka.

Holotype of Falcatakely forsterae, from O'Connor et al. (2020).

In December, evidence of tactile foraging in lithornithids was presented. An oviraptorosaur preserved on top of an embryo-bearing egg clutch, a Saurornitholestes specimen from the Judith River Formation, a new specimen of Macranhinga, and a hornbill from the Miocene of Uganda were described. Interpreting molting patterns from exceptionally-preserved maniraptor fossils appears to be all the rage now, as evidence of sequential molting was also reported in Archaeopteryx. Cartilage preserved on the wishbone of Confuciusornis was documented. Male superb fairywrens were found to maintain vibrant breeding coloration regardless of individual quality. New studies came out on the development of the avian ankle, anthropogenic extinctions of flightless birds, the evolution of paleognath mitogenomes, endocranial ontogeny in ostriches (and alligators), the anatomy of the respiratory system in ostriches (and alligators), the evolutionary history of junglefowl, song frequency in passeriforms, the diversification of suboscines, and the phylogeny of doraditos and white-eyes. Newly-named maniraptors included the Cretaceous euornitheans Abitusavis lii and Similiyanornis brevipectus and the recently extinct owl Ornimegalonyx ewingi.

Oviraptorid preserved on top of egg clutch, from Bi et al. (in press).

Wednesday, August 19, 2020

Not Hawks, Not Owls, and Maybe Not Even Parrots: the Parrot-like Mystery Birds of the Eocene

The rapid diversification of surviving mammal and bird groups in the wake of the Cretaceous–Paleogene (K–Pg) mass extinction has routinely been characterized as a pivotal episode in the origin of modern ecosystems. However, the very rapidity at which it occurred has made it hard for us to gain a fine-scale understanding of the evolutionary transitions that took place during this event. Probably as a result of this, determining how Paleogene mammals were related to modern mammals has been notoriously difficult

We've been slightly more fortunate when it comes to Paleogene birds—though isolated bones are frequently challenging to identify, more complete specimens can often be confidently linked to specific modern groups. Recognizable stem-swifts looked fairly similar to modern swifts, stem-penguins looked fairly similar to modern penguins, stem-rollers looked fairly similar to modern rollers, and so on.

However, there are nonetheless some well-preserved Paleogene bird fossils that have defied easy classification. Among these are a range of small, arboreal birds that have been recently likened to parrots... but read on.

One of these mystery bird groups is the halcyornithids. Fossils of these birds are fairly common in some Eocene deposits of Europe and North America. In the late 1990s and early 2000s, halcyornithids were more commonly known as "pseudasturids", because Halcyornis, represented only by an incomplete skull from the London Clay Formation of England, was not recognized as a member of this group until it was compared to more complete halcyornithid specimens by Gerald Mayr in 2007. Now that Halcyornis has been assigned to the group, Halcyornithidae (named in 1972) takes priority over Pseudasturidae (named in 1998).

Other named halcyornithids are more completely known: Pulchrapollia, also from the London Clay Formation, is known from a partial skeleton, whereas essentially complete skeletons are known for Pseudasturides and Serudaptus from the Messel Formation in Germany and the two species of Cyrilavis (C. olsoni and C. colburnorum) from the Green River Formation in the United States.

Holotype of Cyrilavis colburnorum, from Ksepka et al. (2011).

Halcyornithids were small (thrush-sized) birds with zygodactyl feet (having both the innermost and outermost toe on each foot pointing backwards), a feature found today in cuckoos, woodpeckers, and parrots, among others. The beaks of halcyornithids were quite short and robust, but don't exhibit clear specializations for taking specific foods, so they may have been generalists that fed on both fruits and invertebrates (Mayr, 1998). The structure of the brain cavity in Halcyornis suggests that it had fairly well-developed senses of sight, smell, and hearing, though it may not have been a particularly acrobatic flier (Walsh and Milner, 2011). This is potentially consistent with the short, rounded wings preserved in a specimen of Pseudasturides (Mayr, 1998).

Preserved wing feathering of Pseudasturides, from Mayr (1998).

Halcyornithids have long been known to science. In fact, Halcyornis was the first fossil bird ever to be given a scientific name, having been named in 1825! As might be expected for a fragmentary fossil that was named so long ago, the taxonomic affinities of Halcyornis have been in a state of flux for a long time, and it has been variously considered a coraciiform (thus closely related to kingfishers and rollers) or even a gull-like shorebird.

However, more complete halcyornithid specimens have made them only somewhat less vexing. Cyrilavis olsoni (the type species of Cyrilavis) was originally described in 1976 as a species of Primobucco. Houde and Olson (1989) recognized that it was unlikely to be a member of Primobucco (now considered a stem-roller), and thought it was more likely closely related to jacamars and puffbirds (which are in turn close relatives of woodpeckers and toucans). Mayr (1998) doubted that halcyornithids were particularly close to jacamars and puffbirds, but also found little evidence linking halcyornithids with any specific group of living birds. He later proposed in 2002 that halcyornithids were stem parrots, based on features of their hindlimbs and vertebrae.

Similarities have further been noted between halcyornithids and another group of small Eocene birds, the messelasturids, which are currently known from two genera: Messelastur from the Messel Formation and Tynskya from the Green River Formation. Messelasturids share with halcyornithids, among other features, large "brow ridges" above the eye sockets (also found in many raptorial birds) and the absence of an air sac opening in the humerus (upper arm bone). Their outermost toe on each foot was at least partially reversed, making them semi-zygodactylous if not fully zygodactylous. Unlike halcyornithids, messelasturids were additionally raptor-like in having a sharply hooked beak. Combined with their long, curved talons, it's likely that messelasturids were carnivores that hunted small animals.

The skull of Messelastur, from Mayr (2011).

Messelastur was described on the basis of two isolated skulls with attached neck vertebrae, and given its hooked bill, it was originally considered a type of hawk. Mayr (2005a) reported on a more complete specimen and included both messelasturids and halcyornithids in a phylogenetic analysis. His results supported his previous hypothesis that halcyornithids were stem parrots, but found owls as the closest living relatives of messelasturids. However, following study of another new Messelastur specimen, Mayr (2011) concluded that the features linking messelasturids with owls had been misinterpreted or were poorly substantiated. In an updated phylogenetic analysis, he found messelasturids and halcyornithids to be each other's closest relatives, and both of these groups as stem parrots.

Stem parrot affinities have also been hypothesized for Vastanavis from the Eocene Cambay Formation in India, based on similarities to Quercypsitta, an uncontested stem parrot from the Eocene of France (Mayr et al., 2010). Vastanavis is known from numerous disarticulated specimens, which collectively represent all major limb bones, but its skull is currently unknown. It had short, semi-zygodactyl feet with long claws.

Support for placing all these taxa on the parrot stem appeared to be building in 2012, when Dan Ksepka and Julia Clarke described Avolatavis from the Green River Formation. The only known specimen of Avolatavis comprises a partial skeleton including the tail, pelvis, and hindlimbs. Its feet, originally described as zygodactyl but later reinterpreted as semi-zygodactyl by Mayr et al. (2013), were stout and look well-suited for grasping. In their phylogenetic analysis, Ksepka and Clarke recovered halcyornithids, Messelastur, Vastanavis, and Avolatavis all as stem parrots, with halcyornithids and Messelastur being close relatives of one another, as previously suggested.

Holotype of Avolatavis, from Ksepka and Clarke (2012).

However, in the years following, other developments in avian phylogenetics have demanded further reevaluation of these fossils. For starters, molecular analyses have been painting an increasingly clear picture of how parrots are related to other extant birds, which had formerly been a highly controversial subject. These analyses consistently place parrots in a diverse group of mostly arboreal birds called Telluraves, in which the closest living relatives of parrots are passerines (together forming the clade Psittacopasserae). Psittacopasserans are in turn the closest living relatives of falcons (forming the clade Eufalconimorphae). This new understanding has provided a more solid framework for selecting which modern taxa need to be considered while assessing the phylogenetic position of parrot-like fossil birds. Although Ksepka and Clarke (2012) had rightly included both passerines and falcons in their analysis, their sampling of other telluravians had been limited.

Phylogenetic relationships among Telluraves, based on a consensus of recent molecular analyses. Some uncertainty remains regarding the position of Accipitriformes (hawks and kin), Strigiformes (owls), and Coliiformes (mousebirds), but this topology is the most widely recovered by recent studies.

Other advances came from the paleontological front, namely the recognition that a group of extinct zygodactylous birds, the appropriately-named zygodactylids, were likely stem passerines. Modern passerines do not have zygodactyl feet, but if they evolved from ancestors that did, it raises the possibility that many of the hindlimb characteristics used to place fossil taxa as stem parrots were in fact ancestral features of psittacopasserans, not specific to the parrot lineage.

Mayr (2015a) presented the results of several phylogenetic analyses, in which he included a broad range of extant telluravians, along with putative stem parrots and stem passerines. He consistently found that Psittacopes, another zygodactylous bird from the Messel that had long been considered an unequivocal stem parrot, was more likely to have been a stem passerine. Furthermore, placement along the parrot stem lineage was also not supported for halcyornithids, Messelastur, Vastanavis, or Avolatavis when the analyses were constrained to recover Psittacopasserae and Eufalconimorphae. Halcyornithids were instead found as a grade of stem psittacopasserans, Messelastur in an unresolved position at the base of Eufalconimorphae, and Vastanavis and Avolatavis outside of eufalconimorphs entirely. Mayr observed that these supposed stem parrots actually lack several features shared by parrots and stem passerines, notably a deep groove where the outermost toe attaches to the foot.

Mayr (2015a) did not try constraining the telluravian relationships outside of Eufalconimorphae to conform to recent molecular topologies, so I had a go at doing so myself using his dataset, and here are the results. The relationships within Eufalconimorphae are congruent with his findings. Of note is that the clade uniting Avolatavis, Eurofluvioviridavis, and Vastanavis is more closely related to eufalconimorphs than to other telluravians, despite being excluded from crown Eufalconimorphae. Additionally, Eocuculus (a mystery bird outside the scope of this blog post) is recovered as only a distant relative to all the telluravian taxa included.

Interestingly, Mayr's study found Vastanavis and Avolatavis as close relatives, consistent with previously proposed similarities between the two (Mayr et al., 2013). Joining them as well was Eurofluvioviridavis, a Messel bird that also had stout, semi-zygodactyl feet. As its name suggests, Eurofluvioviridavis was initially thought to have been closely related to Fluvioviridavis from the Green River Formation (Mayr, 2005b). However, whereas Fluvioviridavis was later reappraised as a strisorean, telluravian affinities have been considered more probable for Eurofluvioviridavis (Nesbitt et al., 2011), with Mayr (2015b) noting similarities to Avolatavis and Vastanavis in particular.

More recently, Ksepka et al. (2019) included an extensive phylogenetic analysis of both extant and fossil telluravians in their description of the stem passerine Eofringillirostrum. They corroborated several of Mayr's results, including Psittacopes as a stem passerine and halcyornithids as stem psittacopasserans, though messelasturids, Vastanavis, Avolatavis, and Eurofluvioviridavis were not included in their study. As I remarked in my blog post on Eofringillirostrum, I'd be interested in seeing their dataset further expanded with these taxa.

Phylogenetic results from Ksepka et al. (2019). Note that "Afroaves" should be labeled Australaves.

And that is the state of the art. Although the concept that there used to be a great diversity of stem parrots distributed across the Northern Hemisphere was compelling for a time, it currently appears more likely that these parrot-like birds occupied a range of disparate positions within Telluraves. Their fossils may thus offer the tantalizing potential of shedding light on the ancestral anatomy and lifestyle of this extremely diverse group. Telluravians today include raptorial predators, insectivores, piscivores, frugivores, generalist omnivores, and just about every other avian niche in between, but it has been suggested based on the interrelationships among living species that their last common ancestor was raptorial. Might this hypothesis find support in the raptor-like features that have been identified in the parrot-like mystery birds—the hooked bill in messelasturids, brow ridges in halcyornithids and messelasturids, and large foot claws in several of these species? Perhaps further research will tell.