The Step-wise Bird: Andrea Cau on Bird Evolution

Above: WIP reconstrcution of one potential Connecticut River Valley trackmaker, the bird-like reptile Anchisaurus polyzelus. By M. Martyniuk, all rights reserved.

This morning saw the publication of a new paper by Andrea Cau, titled Assembly of the Avian Body Plan, and what a mammoth (dinosaurian?) work it is! Cau does an amazing job of synthesizing the step-wise nature of bird evolution that is so often hidden behind imprecise or muddy nomenclature. Far from a dichotomy between "non-avian" and "avian" dinosaurs, the important features we associate with modern birds gradually accumulated in a particular lineage of stem-birds ever since the early Triassic period. I should have a lot more to say on the nitty-gritty of this paper this weekend after I've had a chance to fully digest this important work on avian origins. In the mean time, I wanted to share a brief excerpt from (one of) my upcoming book(s), this one dealing with the struggles to interpret some of the earliest known dinosaur remains in an era before the nature of dinosaurs as weird transitional members of the bird lineage was fully understood. The chapter this comes from is discussing Edward Hitchcock's work in the early-mid 1800s on bird-like footprints found in the Connecticut River Valley. The footprints date to the early Jurassic (Cau's "Huxleyan stage" of bird evolution).

Several, more prominent, scientists of the time criticized Hitchcock’s interpretation of the footprints as having been made by birds. He was ridiculed for imagining huge birds that must have been many times the size of the largest living bird, the ostrich. Soon, the rediscovery of giant extinct birds like the moa granted him some level of vindication. But more serious criticisms followed. The sandstone of the Connecticut River Valley was simply too old, other scientists argued. Birds, being “higher” life forms in the ranked scheme of life most believed in at the time, must have also been newer, having developed fully only after the so-called “age of reptiles”. Some scientists went so far as to argue that the three-toed tracks belonged to giant frogs, and that only the large, strong hind limbs left impressions while the lighter forelimbs often did not. And, indeed, one fact which was very inconvenient to Hitchcock’s explanation was that some of the tracks preserved light forelimb impressions, and some were found along with tail drag marks.

What could Hitchcock do to save his bird hypothesis from the facts? By 1861, the discovery of an archaic proto-bird named Archaeopteryx lithographica provided the answer. Here was an example of a “bird” with primitive, reptilian features and a long tail. Perhaps, Hitchcock suggested, his sandstone prints were not made by giant moa-like birds, but giant Archaeopteryx-like birds. And what of the occasional forelimb impressions? Hitchcock actually suggested that, along with its primitive skeletal anatomy, the Archaeopteryx may have been a facultative quadruped! In his view, the Archaeopteryx was halfway between birds and reptiles in both anatomy and gait. Hitchcock had, rather unscientifically, crafted his hypothesis to be immune to all criticism. His peers weren’t buying it.

By the time of Hitchcock’s death in 1864, the bipedal, bird-like nature of many Mesozoic reptiles like Hadrosaurus and Compsognathus had been discovered. For most scientists, these creatures provided a more plausible explanation for Hitchcock’s “sandstone bird” tracks than actual birds. By the late 1800s, the tracks were universally accepted as having been made by prehistoric reptiles, though intriguingly bird-like ones. Today, we know that these ancestrally bipedal reptiles, the dinosaurs and their kin, did indeed have more in common with modern birds than with any of the modern reptile groups, and in fact included the evolutionary ancestors of true birds.

In the end, it turns out that Hitchcock was half-right. His sandstone bird tracks were made by creatures in many ways more like Archaeopteryx than any modern bird or reptile, some of which were partly or fully quadrupedal, with great sweeping tails and enormous body sizes compared to any birds alive today. Many of them even had feathers and feather-like filaments covering parts of their bodies. What Hitchcock had actually discovered were the bird-like reptiles, creatures descended from the same ancestors as crocodiles and turtles, but which had evolved a wide array of uniquely avian features. At a time when most mainstream scientists envisioned dinosaurs as huge, quadrupedal, mammal-like reptiles (in appearance and gait if not lineage), Hitchcock was able to use the traces they made in life to arrive at a conclusion that was actually much closer to the truth in many ways. The Mesozoic was not an “age of reptiles”, at least not on land. It was an age dominated by the bizarre, archaic relatives of birds.

I think the above is a good example of Cau's thesis that a false, dichotomous paradigm, like "bird" vs. "reptile", or "non-avian dinosaur" vs. "bird", and focusing mainly on "key" specimens like Archaeopteryx, can actively mask the reality behind fossil evidence. What do you think?