Avian Evolution

For all its faults, 2016 was a good year for new science books, and in the comparatively narrow field of paleornithology a whopping number of two major titles were published. I previously reviewed one, Birds of Stone by Luis Chiappe and Meng Qingjin. As promised, here is my review of the other.

Avian Evolution is authored by Gerald Mayr, and I can think of few who are as qualified to cover the subject. Mayr is an incredibly prolific paleontologist known for his research on both Mesozoic and Cenozoic birds, and though I haven't crunched the numbers, he is singlehandedly responsible for a considerable amount of recent paleornithological literature.

Not even close to the entirety of Mayr's output.

After an introduction to avian osteology and relevant geological settings, the first few chapters of Avian Evolution discuss the Mesozoic evolution and diversity of birds, covering the anatomy of Mesozoic avialans and non-avialan pennaraptors, competing hypotheses on the origins of feathers and avian flight, and evolutionary trends leading to neornithines. Though the overview of the Mesozoic is very welcome, the most valuable part of this tome lies in the chapters following it. Whereas information on Mesozoic birds is no longer as difficult to come by as it used to be, there remain very few comprehensive reviews of the Cenozoic bird fossil record. Barring some lone chapters in larger volumes, previous syntheses of the subject have largely been... idiosyncratic in various ways, not to mention increasingly outdated. Mayr's previous work, Paleogene Fossil Birds, is excellent, but only covers Paleogene birds and is extremely expensive. (Avian Evolution is by no means cheap, but is still far more affordable by comparison.)

Given this dreadful situation, the sections on Cenozoic birds in Avian Evolution fill a much needed gap. These chapters are mostly arranged using a phylogenetic framework, insofar as uncertainties in neornithine phylogenetics allow, but a few clades are discussed alongside one another based on ecological similarities rather than close kinship (e.g.: the major groups of hypercarnivorous diurnal birds—accipitrimorphs, falconiforms, and cariamiforms—are covered in one chapter). The final chapter deviates from this structure and instead focuses on the unusual evolutionary trends and unique morphologies of island birds. The book is not confined to fossil taxa clearly closely related to modern forms, nor to the most charismatic extinct clades such as phorusrhacids and teratornithids, but, being a truly comprehensive review, also includes many enigmatic and obscure fossil groups like the eogruids (cursorial birds possibly closely related to cranes), diomedeoidids (procellariiforms convergently similar to oceanitid storm petrels), plotopterids (flightless diving birds of the northern Pacific), and zygodactylids (abundant stem-passerines with zygodactyl feet). For each avian group, the overall anatomy, known fossil record, biogeographic history, and possible paleoecology are all described, with areas in need of future research noted where necessary. One take-home message of this book is that much more work needs to be done on the phylogeny of Cenozoic fossil birds (which will no doubt be facilitated once we have a more stable topology for the extant representatives).

The book is well illustrated with phylogenetic trees, skeletal diagrams, and comparative photographs of specimens. A number of colored plates in the middle of the book display photographs of various spectacular specimens of fossil birds (and some non-avialan dinosaurs). Unlike Birds of Stone, Avian Evolution is not meant to be a pictorial guide and its images are accordingly much smaller, but they are nonetheless of high quality and helpfully supplement the text. Interestingly, a skeletal restoration of Jeholornis by Scott Hartman lacks a retractable second toe, unlike other (and presumably older) versions of this image that I've seen.

In addition to reviewing the primary literature, Mayr provides some observations that are to my knowledge novel or undescribed.

• Like Chiappe and Meng in Birds of Stone, Mayr supports the proposed synonymy between Iteravis and Gansus zheni. (Unlike Birds of Stone, Mayr directly cites Mortimer for this suggestion.)
• Alamitornis, originally described as a possible close relative of Patagopteryx, is suggested to be a squamate rather than a bird.
• In light of the reinterpretation of "Genyornis" eggs as belonging to the megapode Progura, Mayr argues that purported castaway Aepyornis eggs found in Australia should be considered possible candidates for true Genyornis eggs.
• Neogaeornis, a supposed Cretaceous loon, is considered to share more similarities with grebes.
• Mayr points out that Paracrax, generally assumed to be a member of the carnivorous Cariamiformes, has a hoatzin-like sternum and may have had a large crop and herbivorous diet similar to that bizarre South American bird.
• An undescribed London Clay stem-owl specimen, currently held in a private collection, has pieces of the skull indicating that it may have had smaller eyes than extant owls.

Particularly controversial will be Mayr's contention that pennaceous feathers most likely evolved for aerodynamic purposes. This main basis for his argument is that if pennaceous feathers were adaptations for non-aerodynamic functions (such as signaling), it would have been evolutionarily easier to widen the entire shaft rather than forming the complex branching structure of actual pennaceous feathers. Given current knowledge of feather growth, I'm not convinced that it would be developmentally easier to grow a flat sheet rather than adding branches to a feather, despite the structural complexity of pennaceous feathers. Additionally, the argument appears to disregard potential non-aerodynamic benefits of producing branching structures, such as more efficient distribution of material compared to equally-sized continuous sheets. Mayr also suggests that ancestral pennaceous feathers used for display purposes would more likely be sexually dimorphic, which appears to ignore the possible role of mutual sexual selection in ornithodirans (including modern birds).

Mayr points out that pennaceous feathers are generally reduced in flightless birds, attributing the presence of fully formed pennaceous feathers in flightless oviraptorosaurs and dromaeosaurids to potential secondary flightlessness. In particular, he levels the possibility of the small, possibly arboreal scansoriopterygids being close relatives of oviraptorosaurs in support of a volant ancestry for the latter. However, given the many aberrant characteristics of scansoriopterygids, not least of which is the fact that at least some of them appear to have flown or glided using membranes rather than feathers as their primary lift-generating surfaces, I am skeptical that they are particularly informative to oviraptorosaur ancestry even if this phylogenetic hypothesis was correct. In any case, the possibly unusual wing structure of the scansoriopterygids likely makes their relevance to the origin of pennaceous feathers questionable. Though I find it plausible that early pennaceous feathers were used in some form of locomotory behavior (such as increasing maneuverability while running and leaping), I would await more conclusive evidence of secondary flightlessness in non-avialan pennaraptors before declaring aerodynamics to be the most likely explanation for the origin of pennaceous feathers.

I defer to Mayr's knowledge of Cenozoic birds, but I did spot a few minor errors or debatable claims in the Mesozoic chapters besides the discussion of pennaceous feather origins.

• Avimimus is presented as being of uncertain phylogenetic position, even though it is almost invariably recovered as an oviraptorosaur by recent analyses. (Its position within Oviraptorosauria, on the other hand, is less secure.)
• A specimen of Baptornis is said to preserve coprolites, but this specimen has been given the new name Fumicollis. (Inexplicably, Fumicollis is mentioned elsewhere in the book.)
• On a taxonomic note, it is said that affirmation of a close relationship between oviraptorosaurs and scansoriopterygids would make oviraptorosaurs paravians, yet all common definitions of Paraves explicitly exclude oviraptorosaurs. Instead, scansoriopterygids would no longer be considered paravians under such circumstances.
• Caudipteryx is put forth in support of rectrices being restricted to the tail tip in ancestral pygostylians. Though I concur that it's likely that this was the case for Pygostylia, the presence of rectrices down most of the tail's length in the oviraptorosaur Similicaudipteryx, the dromaeosaurid Zhenyuanlong, the troodont Jinfengopteryx, and the basal avialan or troodont Anchiornis suggests that Caudipteryx may not represent the ancestral condition further down the tree.
• Lastly and almost inevitably, there are a few typos throughout the text. Most glaringly, a strict consensus tree of neornithine relationships incorrectly shows secretary birds as being more closely related to New World vultures than to accipitrids, likely due to mislabeling of the branches.

Despite these lapses, Avian Evolution is impressively up to date and even cites references from early 2016, including the description of Dingavis and the aforementioned reidentification of "Genyornis" eggs. (However, Chiappeavis is strangely not mentioned as an example of an enantiornithine with a fan-shaped arrangement of tail feathers, even though Feitianius, which was described at around the same time, is.)

This book is part of the Topics in Paleobiology series, a collection of volumes intended to serve as reviews of the primary literature on specific topics in paleobiology for researchers and advanced students. This installment succeeds admirably in my view, being sufficiently technical and comprehensive to synthesize its main topic but helpfully defining specialist terms for those unfamiliar with the specific subject. Think The Complete Dinosaur rather than The Dinosauria in terms of style and level of detail. An extensive bibilography is provided for readers who wish to pursue the discussed topics even further. Avian Evolution is an indispensable review and index to the current literature on fossil birds and I would strongly encourage all academics and well-read laypeople interested in the subject to obtain a copy. Its synthesis of Cenozoic paleornithology alone should secure its place in any paleontological library.

I'm certainly glad that Topics in Paleobiology decided to produce such an essential title. I wonder what else- hold on, what!?

Thanks to this review taking up most of my time, I have not been able to complete my annual retrospective of the previous year's events punctually, but it will be coming up next...