I have trampled on a number of cherished childhood memories and sensational claims since I started by career as a science writer. Megarachne was not an enormous spider on the order of Shelob, but a strange sea scorpion. The “terror bird” Titanis was not quite as scary as previously believed. The tiny tyrannosaur Nanotyrannus was probably a juvenile Tyrannosaurus rexa juvenile Tyrannosaurus rex. And the Kraken? Don’t even get me started…
But, despite what you might think given my cheerful deconstruction of sensational paleo scenarios, I do not have a heart of stone. I truly miss some of the terrifying creatures I met during my childhood which have since been debunked, recategorized, or recast into less-ferocious prehistoric characters. The prehistoric bird “Diatryma” is near the top of my list of paleo nostalgia.
If the movies and illustrations I saw as a kid were to be believed, “Diatryma” had an insatiable appetite for early horses. The six-foot-tall avian predator easily ran down small mammals such as Eohippus in the warm, lush forests of prehistoric North America and promptly tore them apart with huge talons and a sharp beak. This was the strange, 55 million year old world of the early Eocene – giant birds ruled the land while the precursors of modern mammals were still small and meek. The imagery got even better after paleontologists confirmed that birds were the direct descendants of small, feather-covered dinosaurs. After the catastrophe which wiped out the non-avian dinosaurs – among other creatures – 65 million years ago, the surviving avian dinosaurs quickly took up the habits of their forebears and snacked upon our cousins and ancestors. For a short slice of Cenozoic time, the world still belonged to the dinosaurs.
But the rapacious bird I used to know has been replaced by a presumably more mild-mannered cousin of ducks with a different name. While “Diatryma” had been described from fossils found in western North America by naturalist E.D. Cope in 1876, the paleontologist E. Hébert had described similar remains from France under the name Gastornis in 1855. Specialists in avian paleontology began to recognize this during the 1990s, and, since the name is older and has priority, Gastornis is often applied to both the North American and European forms. (Whether the application of the name Gastornis to the North American fossils is uncertain, however, and a thorough comparison of the closely-related birds from Europe and North America has yet to be undertaken. The two are closely related, but do they truly belong to the same genus?)
The name change was just the start of the alterations. For decades the hypothesized, highly-carnivorous lifestyle of Gastornis caused paleontologists to think of it as a North American “terror bird.” Recent analyses of family relationships have shown that this was not the case. In the big picture of avian evolution, Gastornis was actually much more closely related to ducks within a broad group called anseriforms. Exactly what the bird was eating also came into question. A 1991 paper by Lawrence Witmer and Kenneth Rose suggested that the skull of North American Gastornis was overbuilt for herbivory and therefore hypothesized that the bird was a carnivorous bone-crusher. Allison Andors countered that the lack of a prominent hook on the beak tip and the reduced development of parts of the toe claws pointed to an herbivorous or omnivorous diet, and Gerald Mayr later pointed out that hard foods such as seeds, nuts, and twigs might require the strong skull development Witmer and Rose analyzed. Even though no one is entirely sure what this bird consumed, the general picture was that the terror of the Eocene had been transformed into a nut-crushing duck relative.
That’s why a presentation given by Estelle Bourdon of London’s Natural History Museum at last week’s Society of Vertebrate Paleontology conference caught my attention. The title was “Gastornis is a terror bird: New insights into the evolution of the Cariamae (Aves, Neornithes).” I knew that results presented at conferences are often preliminary and subject to change, but I was really hoping that Bourdon would restore Gastornis to the image I grew up with.
Bourdon’s presentation, created with Joel Cracraft of the American Museum of Natural History, primarily focused on the family relationships of Gastornis. The big bird has been punted between different groups for a while, Bourdon pointed out, but the analysis she conducted with Cracraft found that Gastornis was nested deep within the group which contains the true terror birds of South America (phorusrhacids). More specifically, Gastornis came out as a close relative of the large, stout terror bird Paraphysornis. On the basis of this revised relationship Bourdon proposed that Gastornis truly was a very specialized terror bird which lacked some of the predatory hallmarks of the group – more likely was that Gastornis was an omnivore or herbivore.
As happy as I would be to see Gastornis as a terror bird, though, I wasn’t convinced of the proposal. Some of the features mentioned by Bourdon as evidence of a close relationship between Gastornis and phorusrhacids – such as the high, compressed upper jaw and features of the stout hindlimbs – could be the result of evolutionary convergence related to similar lifestyles among flightless birds. And then there’s the biogeographic dilemma. The record of Gastornis in the northern hemisphere goes back to about 58 million years ago, making it one of the earliest giant, flightless birds known. If the hypothesis presented by Bourdon and Cracraft is correct, though, Gastornis would have been descended from ancestors which originated in South America, somehow were able to travel from that still-isolated island continent to North America, and then spread to Europe within 10 million years. The connection is even more perplexing because the terror birds Gastornis is hypothesized to have been closest to – such as Paraphysornis – lived around 23 million years ago. If Gastornis is an early terror bird, why didn’t it come out as being more closely related to some of the earliest forms? This discrepancy may indicate how independently-evolved features related to a similar niche or mode of life can confuse attempts to parse evolutionary relationships.
We will have to await the publication of Bourdon and Cracraft’s research before the proposal that Gastornis was a terror bird can be fully examined, but, even if they are correct, the dastardly “Diatryma” I once knew is gone. Terror bird or prehistoric duck cousin, the anatomy of Gastornis appears to be more consistent with the notion that the bird was a relatively slow creature that would have specialized in cracking open tough food items, be they bones or nuts. I suppose that I should not despair, though. Even if Gastornis was not the nimble carnivore which revved my young imagination, the fact that such a strange and confusing creature existed at all is a wonderful facet of prehistory.
Top Image: The reconstructed skeleton of Gastornis. Photo by Flickr user Ryan Somma.
Andors, A. 1992. Reappraisal of the Eocene groundbird Diatryma (Aves: Anserimorphae). Papers in avian paleontology honoring Pierce Brodkorb–Natural History Museum of Los Angeles County Science Series. 36: 109-125
Bourdon, E.; Cracraft, J. 2011. Gastornis is a terror bird: New insights into the evolution of the cariamae (Aves, Neornithes). Society of Vertebrate Paleontology 71st Annual Meeting Program and Abstracts, p. 75
Matthew, W.; Granger, W. 1917. The skeleton of Diatryma, a gigantic bird from the Lower Eocene of Wyoming. Bulletin of the AMNH, (37) 11: 307-354
Mayr, G. (2005). The Paleogene fossil record of birds in Europe Biological Reviews, 80 (04) DOI: 10.1017/S1464793105006779
Mayr, G. 2009. Paleogene Fossil Birds. Berlin: Springer-Verlag. p. 47
Witmer, L.; Rose, K. (1991). Biomechanics of the jaw apparatus of the gigantic Eocene bird Diatryma; implications for diet and mode of life Paleobiology, 17 (2), 95-120