The Aetosaur Paper That Changed Everything

Recent events have gained the aetosaurs, those Late Triassic armored pseudosuchians, as bit of notoriety lately (see here, here, here, and here), but all of this aside the truth is that these critters are important index fossils for non-marine Upper Triassic strata worldwide.

Unbenownst to many is the impressive pedigree of paleontologists who have worked on aetosaurs. Louis Agassiz named the first aetosaur taxon (Stagonolepis robertsoni) in 1844 (admittedly he thought he was describing large ganoid fish scales from the Devonian, hence the name); Thomas Huxley (yeah, that Thomas Huxley) wrote two monographs on crocodian nature of Stagonolepis, and indeed it was this work that finally convinced poor old Roderick Murchison and his colleagues that portions of the Old Red Sandstone where actually "new" and Triassic in age. The famous German paleontologist H. von Meyer described aetosaur material from Germany in 1861, although until fairly recently these osteoderms were thought to belong to the phytosaur Phytosaurus. Aetosaurus was also described from Germany by O. Fraas in 1887.

E.D. Cope described the first North American aetosaur material, Typothorax and Episcoposaurus in the late 1800s, and not to be outdone, his bitter rival O.C. Marsh also described an aetosaur (Stegomus), but the first two taxa were thought to be phytosaurs at the time. Other European and North American paleontologists to collect and or publish on aetosaurs include Freidrich von Huene, E.C. Case, Barnum Brown, Charles Camp, Ned Colbert, Glen Jepsen, John Wilson, Joe Gregory, and Don Baird. But again to many of the early workers the majority of these remains were considered to be phytosaurs. Aetosaurus was considered an "aetosaur" obviously, but in reality few workers really understood what an aetosaur was. Work in the 1940s through 1960s by Howard Sawin, Joe Gregory, Glen Jepsen, and Don Baird started differentiating aetosaurs from phytosaurs. In 1962, Gregory even suggested that the Phytosaurus material from Germany actually belonged to aetosaurs, however, he retracted this in 1969. By this time aetosaurs has also been found in South America (e.g., Casimiquela, 1961) and Alick Walker had published his influential monograph on Stagonolepis in 1961.

By the earlier 1980s aetosaurs were relegated to a role of being relatively uninteresting "armored thecodonts" and in western North America only two taxa were recognized, Typothorax and Desmatosuchus, but this was all about to change.

In the 1920s Charles Camp of the University of California Museum of Paleontology (UCMP) at Berkeley had conducted extensive research in the Chinle Formation of Arizona, including in the area of Petrified Forest National Park (it was a monument at the time and much smaller in size). Camp collected numerous specimens, took copious field notes and also photographed some of his sites. Although he only published on the phytosaurs (Camp, 1930) and the dicynodonts (Camp and Welles, 1957) he had collected the remains of many different groups including aetosaurs.

In 1981 Robert Long and Kevin Padian of the UCMP reinstituted Camp's field program at the Petrified Forest. Literally following in Camp's footsteps through the detailed notes and photos they relocated the majority of Camp's old collecting sites and discovered many more new ones. Through 1985, numerous specimens were collected including lots of aetosaur material. Camp (1930) had suggested that the phytosaurs of the Chinle Formation were of biostratigraphic significance and Long and his colleagues discovered that the same thing was true for the aetosaurs.

The results of this work culminated in a large monograph on Upper Triassic non-marine tetrapods of the American Southwest (Long and Murry, 1995); however, in 1985 a shorter article was published that would have far reaching affects for the studies of phytosaurs and aetosaurs. The paper titled "Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park" by Robert Long and Karen Ballew was published in a symposium volume (Museum of Northern Arizona Bulletin 54). The paper described and figured new and existing aetosaur material from the Chinle Formation and erected several new taxa. Groundbreaking were the following hypotheses:


1) The dorsal ornamentation pattern on aetosaur osteoderms was diagnostic to taxa and even small pieces of osteoderms could be accurately identified.

2) Instead of only two aetosaur taxa in the American Southwest, there were five, each dignosable by a unique combination of osteoderm characters. Recognozed taxa were Typothorax coccinarum, Desmatosuchus haplocerus, Calyptosuchus wellesi (new taxon), Paratypothorax sp., and "Typothorax" meadei.

3) Based on osteoderm characters "Typothorax" meadei was not referable to Typothorax but instead represented a new genus. This was also later stated by Murry and Long (1989) and Small (1989); however, the new genus was named by Hunt and Lucas in 1990 (who coincidently had edited the 1989 volume containing the first two papers).

4) The purported wide osteoderms of the phytosaur Phytosaurus actually belonged to an aetosaur, which Long and Ballew named Paratypothorax andressi.

5) Following Camp (1930) and Gregory (1957) the aetosaurs and phytosaurs of the American southwest had biochronological significance, demonstrating two distinct assemblages in the Chinle Formation (the type faunas of the Adamanian and Revueltian land vertebrate faunachrons of Lucas and Hunt, 1993).

These five hypotheses revolutionized aetosaur (and phytosaur) research by demonstrating a wide diversity of forms through distinguishable character diagnoses, whereas prior to this spiked forms were assigned to Desmatosuchus and forms with a radial plate ornamentation were assigned to Typothorax despite the fact that the type species of Typothorax, T. coccinarum, does not possess this type of armor ornamantation.

Although recent work (e.g., Martz and Small, 2006, Parker, 2003, 2007, 2008a, b; Parker et al. 2008) has demonstrated that paramedian osteoderm ornamentation is shared within more inclusive clades rather than species and that the shoehorning of taxa into Long and Ballew's five recognized taxa was masking aetosaur diversity, this 1985 paper forms the foundation of all current work on aetosaurs.



Artwork from top to bottom: paramedian plates of Desmatosuchus spurensis, Calyptosuchus (Stagonolepis) wellesi, and Typothorax coccinarum based on material from the southwest USA. All are from Long and Ballew (1985).

REFERENCES

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