S is for Segisaurus

Segisaurus halli is an underappreciated little dinosaur. Described in 1936 by paleontologist Charles Camp, the three-foot-long theropod held telling clues about just how bird-like dinosaurs truly were, yet the significance of Segisaurus largely went unnoticed until other finds more prominently underscored the same lessons.

The only known remains of Segisaurus are a paltry lot of vertebrae, limb bones, and parts of the shoulders and hips. But the fact that we possess anything of Segisaurus at all is a minor miracle. The partial skeleton – discovered on July 27, 1933 by Robert F. Thomas and Max Littlesalt in Arizona’s Segi Canyon – was found entombed in a stack of Early Jurassic sediment called the Navajo Sandstone. These beds record a time when sprawling dune fields covered the American west, and tracks are more commonly found in this formation than skeletons. To find any skeletal material in the Navajo Sandstone is truly exceptional. To have a partial skeleton is even better.

In overall form, Segisaurus was quite similar to the earlier Coelophysis, as well other small, svelte theropods that were strutting around western deserts at the same time as Segisaurus, around 180 million years ago. The general shape of the dinosaur itself was not especially remarkable. Yet another small, bipedal carnivore. But there were two aspects of the Segisaurus skeleton that should have sparked a renewed interest into the idea that birds and dinosaurs shared a close connection – a hypothesis that had gone dormant by the time of Camp’s description.

Segisaurus had a furcula, or, breaking that down, fused clavicles. This fact seems a little mundane, but, by the 1930s, whether or not dinosaurs had these shoulder girdle bones was a critical point of contention in the ongoing debate about how birds evolved.

In 1926, the Danish artist and amateur paleontologist Gerhard Heilmann published The Origin of Birds. The book was an anthologized English version of articles he had previously written in his native language. Heilmann’s argument for bird ancestry, laid out within, was highly influential during the 20th century, and spent a good deal of space considering critical clavicles.

Even though many dinosaurs were exceptionally bird-like in form, Heilmann pointed out, dinosaurs lacked the precursors of a wishbone – that is, clavicles – that would have pinned them as bird ancestors. Dinosaurs appeared to have lost the clavicles over evolutionary time, and, citing work on evolutionary irreversibility by Belgian paleontologist Louis Dollo, the artist argued that dinosaurs could not have re-evolved the essential skeletal features. To solve the problem, Heilmann favored older crocodile cousins – then called “thecodonts” – as ancestors of both birds and dinosaurs. If both lineages arose from the same rootstock, then shared ancestry and close convergence could explain the similarities between birds and dinosaurs.

Heilmann’s hypothesis, further underscored by paleontologists such as Robert Broom, remained the favored view of bird origins until the “Dinosaur Renaissance” of the 1970s. Segisaurus, as well as an Oviraptor that had been found with an intact (but initially misidentified) wishbone, should have reinvigorated the idea that dinosaurs could have been bird forerunners. That never happened. The importance of Segisaurus only became apparent in hindsight, after a major dinosaur image shift and new discoveries that finally connected the avian and non-avian dinosaurs in one of the grandest macroevolutionary episodes of all time.

But the importance of the lone Segisaurus skeleton didn’t just lie within its furcula (which Matthew Carrano and colleagues confirmed truly are present in the specimen as part of their 2005 reexamination of the dinosaur). Though fragmentary, the bones of the dinosaur were laid out in a roosting position. When the gracile dinosaur died and was buried, it was crouching in a way reminiscent of the way modern birds rest. Segisaurus wasn’t the only dinosaur of its time to do so. Early Jurassic imprints found in New England, southern Utah, and Arizona record the resting behavior of other theropods – dinosaurs that sat to leave traces of their legs, hips, tails, and hands in the sediment.

Segisaurus and the ichnological signposts its contemporaries left behind record how remarkably bird-like theropod dinosaurs were from a very early era. Sadly, though, these dinosaurs are sometimes still ignored. While a PLoS One paper published last year mentioned the bird-like sleeping position of the feathery Early Cretaceous dinosaur Mei, the study didn’t mention Segisaurus or the Early Jurassic resting traces – fossils that draw back the record of avian-style relaxation tens of millions of years.

Important, but often forgotten, Segisaurus is a reminder of how much of the dinosaurian reign that we have yet to uncover. The first dinosaurs evolved around 245 million years ago, but it wasn’t until the Early Jurassic that our favorite Mesozoic critters truly became disparate and dominant. In the Navajo Sandstone, threads of the dinosaur story come to us in the form of elusive bones and fleeting glimpses of prehistoric behavior, offering tempting visions of what the world was like when dinosaurs had only just begun to rule.

Previous entries in the Dinosaur Alphabet series:

R is for Rapetosaurus

Q is for Qiaowanlong

P is for Pelecanimimus

O is for Ojoceratops

N is for Nqwebasaurus

M is for Montanoceratops

L is for Leaellynasaura

K is for Kileskus

J is for Juravenator

A-I at Dinosaur Tracking.

References:

Camp, C. 1936. A new type of small bipedal dinosaur from the Navajo sandstone of Arizona. University of California Publications, Bulletin of the Department of Geological Sciences, 24: 39-56.

Carrano, M., Hutchinson, J., Sampson, S. 2005. New information on Segisaurus halli, a small theropod dinosaur from the Early Jurassic of ArizonaNew information on Segisaurus halli, a small theropod dinosaur from the Early Jurassic of ArizonaNew information on Segisaurus halli, a small theropod dinosaur from the Early Jurassic of Arizona. Journal of Vertebrate Paleontology. 25, 4: 835-849

Gao, C., Morschhauser, E., Varricchio, D., Liu, J., Zhao, B. 2012. A second soundly sleeping dragon: New anatomical details of the Chinese troodontid Mei long with implications for phylogeny and taphonomyA second soundly sleeping dragon: New anatomical details of the Chinese troodontid Mei long with implications for phylogeny and taphonomyA second soundly sleeping dragon: New anatomical details of the Chinese troodontid Mei long with implications for phylogeny and taphonomy. PLoS ONE 7,9: e45203. doi:10.1371/journal.pone.0045203

Milàn, J., Loope, D., Bromley, R. 2008. Crouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USACrouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USACrouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USA. Acta Palaeontologica Polonica, 53, 2: 197–205

Milner, A., Harris, J., Lockley, M., Kirkland, J., Matthews, N., 2009. Bird-like anatomy, posture, and behavior revealed by an Early Jurassic theropod dinosaur resting trace. PLoS ONE 4, 3: e4591. doi:10.1371/journal.pone.0004591