The Importance in Understanding Historical Context in Solving Taxonomic Problems - A Case Study With Aetosaurs

Parker, W. G. and Martz, J. W. 2010. Using positional homology in aetosaur (Archosauria: Pseudosuchia) osteoderms to evaluate the taxonomic status of Lucasuchus hunti, Journal of Vertebrate Paleontology 30:1100-1108.

Abstract - The Otis Chalk quarries in the Upper Triassic Dockum Group of West Texas have produced aetosaur material that most workers have suggested represents two distinct morphotypes. We use characters from aetosaur specimens with articulated or semi-articulated carapaces in which the anteroposterior placement of osteoderms can be established with certainty to compare homologous osteoderms in the Otis Chalk material. This study confirms that the genera Longosuchus and Lucasuchus are distinct morphotypes, which differ in that the former taxon has paramedian osteoderms with random pitted ornamentation and low pyramidal bosses that contact the posterior margin, and spines on the lateral osteoderms that are posteriorly emarginated, whereas the latter taxon has paramedians with a strongly radial ornamentation and large conical eminences, and spines on the lateral osteoderms that are not posteriorly emarginated. Both taxa also have paramedians that are overlapped anteriorly by the laterals, a character that may be a synapomorphy of desmatosuchine aetosaurs. The arguments that these morphotypes represent ontogenetic stages or sexual dimorphs of a single biological species cannot be corroborated using either comparisons with modern pseudosuchians, other aetosaur taxa, or stratigraphic ranges. Longosuchus is known only from the type area and has no utility as an index taxon of the Otischalkian land-vertebrate faunachron, although Lucasuchus suggests a tentative correlation between part of the Dockum Group of Texas and the Pekin Formation of North Carolina.

Discussion

Study and management of any resource, including vertebrate fossils, requires in-depth understanding of the context under which previous work on these materials was conducted. For example, where, how, and by whom were the specimens collected? What was their original association? How were they prepared? Who identified them and why did they apply the identification that they did? Often in vertebrate paleontology we are faced with taxonomic questions, which to answer properly we need this detailed contextural information. A good example of this in aetosaurs (the group I mainly work with) is the ongoing debate regarding the proposed synonymy of Longosuchus meadei and Lucasuchus hunti. Much confusion exists regarding the specimens assigned to these taxa, much which stems from the original collection, as well as the subsequent curation and description of these materials. Recently I, along with my co-author Jeffrey Martz, tried to tackle this taxonomic problem and our resulting paper was just published in the most recent issue of the Journal of Vertebrate Paleontology. This article necessitated discussion of the history of the collection, study, and curation of these specimens and unfortunately some of the information I now have did not make it into the final article. At the time this article was in press I came across more important information on the collection and study of these specimens and unfortunately it was too late to add to the introductory section. Some of this new information (from quarry reports and correspondence), included here, clarifies or corrects introductory statements made in our paper.

In the late 1930s and early 1940s a paleontology inventory of portions of Texas was conducted under the Works Progress Administration (WPA), a relief program established by the U.S. Government to put millions of unemployed American men to work. Extensive quarrying was conducted at several sites in Texas including a series of Late Triassic age quarries near Otis Chalk in Howard County. These quarries contained thousands of bones of metoposaurs, phytosaurs, aetosaurs, etc., and are especially known for the well preserved skeletons of the archosauromorph Trilophosaurus buettneri (Gregory, 1945; Elder, 1978; Spielmann et al, 2008). Quarries 3 and 3A contained the majority of the aetosaur material including several partial to fairly complete aetosaur skeletons, only some of which has been prepared. The “best” two skeletons; however, were prepared and subject to a full description by Sawin (1947). Specimen lists compiled at the time of collection as well as work reports reveal that all of the aetosaur material was assigned to the genus Desmatosuchus at the time of collection. Many of the specimens still contain field numbers, which include the county name, quarry number, the year collected, and a fourth number that represents the order in which the specimen was removed from the ground and documented. Furthermore, collection records sometimes provide the name of the collector. Unfortunately, this number just reflects the collection order and often can be ambiguous about the association of specimens in the ground. In our paper we state that no excavation records could be found, but specimen lists and reports have since come to light although no detailed quarry maps are still known to exist.

Workers prepared the two associated skeletons, as well as the articulated tail of a third individual, and a variety of isolated elements. In the final publication (Sawin, 1947) the majority of these specimens are assigned to a new species, Typothorax meadei, based mainly on the sigmoidal shape of the femur (Sawin, 1947). Sawin also recognized a second species, T. coccinarum, as present in the quarry, but restricted this assignment to several osteoderms with large conical dorsal eminences, which he thought resembled the conical eminences in T. coccinarum osteoderms figured and described by von Huene (1915) and originally studied by Cope in 1887. Sawin (1947) designated the two main skeletons (Typothorax meadei) as syntypes and one of the specimens (TMM 31185-84a) was partially incorporated into a museum mount (some of the armor as well as the limbs) along with the articulated tail of the third individual. Much of the remaining material was left identified and catalogued in the collections as Desmatosuchus (contra Parker and Martz, 2010), including many lateral plates which were superficially similar to those of T. meadei. What is important is that none of this material is actually referable to Desmatosuchus (Parker and Martz, 2010).

In an unpublished M.S. thesis Elder (1978) noted this similarity and argued that Desmatosuchus and Typothorax were therefore synonymous. This was later refuted by Small (1989), but what is important to note is that like Sawin, Elder recognized two distinct morphologies in the Otis Chalk material.

Later recognition of the utility of osteoderms ornamentation in aetosaur taxonomy by Long and Ballew (1985) led those workers as well as others (e.g., Small, 1989) to recognize that T. meadei represented a distinct genus from T. coccinarum. Hunt and Lucas (1990) accordingly supplied the name Longosuchus meadei. Curiously, in their renaming of the material and designation of a lectotype Hunt and Lucas (1990) also included the material that Sawin (1947) had assigned to T. coccinarum. Unfortunately, no explanation is given for this and from the article it is not clear if Hunt and Lucas (1990) recognized that some of the material had originally been assigned to a different taxon. They also provide no discussion of the material referred to Desmatosuchus, but infer in their biochronology section that all of the Otis Chalk material belongs to a single taxon.

Subsequent re-examination of the material by Long and Murry (1995) led to the separation of Sawin’s (1947) “T. coccinarum” material as well as the “Desmatosuchus” material from the original T. meadei material of Sawin. Long and Murry (1995) assigned these specimens to a new taxon, Lucasuchus hunti (presumably in return for the name Longosuchus, named for Long). This reassigned was criticized, particularly by Heckert and Lucas (1999, 2000; and in numerous subsequent papers) and Lucas and Heckert (1996) who all stated that the diagnosis of Lucasuchus is “based on minor differences in scute morphology, some so subjective they cannot be replicated”. They also stated that Long and Murry (1995) “split” Longosuchus; however, this is incorrect as they only recognized the originally division of the material set forth by Sawin (1947).

In my own initial examination of the material (Parker, 2003) I felt that there actually were pretty clear differences between the holotype osteoderms of Lucasuchus and Longosuchus, especially in the paramedian osteoderm ornamentation and in the position of the dorsal eminence (see Parker and Martz, 2010). Moreover, although superficially similar there were also key differences between the lateral osteoderms of L. meadei and those catalogued as “Desmatosuchus” and assigned to Lucasuchus by Long and Murry (1995). What was ambiguous; however, was the association of these lateral osteoderms with the Lucasuchus paramedian osteoderms. Although Sawin (1947) felt that there were two morphotypes represented by the paramedian osteoderms in the collections, he clearly stated that no lateral osteoderms were associated with his “T. coccinarum” (Lucasuchus) material.

This last puzzle was solved unintentionally by Ron Tykoski, who while working for the Texas Memorial Museum (TMM) made a small collection of osteoderms to place around the base of the mount. One of the osteoderms he selected was a paramedian of Lucasuchus (TMM 31100-66) and nearby was a lateral osteoderm with the same number. Amazingly these two osteoderms fit together perfectly and unambiguously to form a conjoined pair, and thus must have come from the same individual. Even better, this lateral was of the Lucasuchus morphology and not from Longosuchus. Finally we were able to compare homologous series from both morphotypes to strongly (we feel) demonstrate support for Long and Murry’s (1995) reseparation of the material into two distinct taxa. This was the basis for the entire paper by Parker and Martz (2010).

One final puzzle (at least for me) was why Sawin had assigned the material to Typothorax in the first place? This is where knowledge of the historical context is crucial. At the time the material was collected (1939-1940) there were only three aetosaur taxa known from western North America, Typothorax, Episcoposaurus, and Desmatosuchus. By the time Sawin conducted his study it had been hypothesized by many workers that Episcoposaurus and Desmatosuchus were congeneric, something that was finalized later by Gregory (1953). This was based mainly on the material of Episcoposaurus haplocerus; however, another referred species E. horridus, had been collected in the same general area as the holotype and referred material of Typothorax coccinarum. Personal correspondence between Dr. John Wilson of the TMM and Dr. E. C. Case of the University of Michigan written in the 1940s clarify that Sawin travelled to the American Museum of Natural History to examine Cope’s material of E. horridus and T. coccinarum. This material, figured by Lucas et al (2007) contains two distinct femur morphologies. A gigantic straight femur was referred (by Cope) to E. horridus, whereas a much more gracile and strongly sigmoidal femur was assigned to T. coccinarum. The femora of Longosuchus meadei are nearly identical to that of the referred T. coccinarum specimens contra the statement by Heckert et al. (2010:637) that they are “dramatically different”. It was also on this visit that Sawin noted the conical eminences in the caudal osteoderms of T. coccinarum that formed the basis of his assignment of the Lucasuchus material to that taxon. Thus, finding the Wilson-Case correspondence solves the last part of the puzzle.

I hope that this case provides a good example of how essential understanding the context of the specimens on hand are when we try to determine aspects such as original association, curation, and taxonomy. With this information we can now understand why certain taxonomic assignments were made by past researchers. Allowing ourselves the opportunity to “walk in their boots” of our predecessors for awhile is an important tool in solving important scientific problems.

Paramedian osteoderms of Longosuchus meadei

Paramedian osteoderm of Lucasuchus hunti. Note strong radial patterning and prominent, centralized raised eminence.

REFERENCES

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Heckert, A. B., and S. G. Lucas. 1999. A new aetosaur (Reptilia: Archosauria) from the Upper Triassic of Texas and the phylogeny of aetosaurs. Journal of Vertebrate Paleontology 19:50–68.

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Small, B. J. 1989. Aetosaurs from the Upper Triassic Dockum Formation, Post Quarry,West Texas; pp. 301–308 in S. G. Lucas and A. P. Hunt (eds.), Dawn of the Age of Dinosaurs in the American Southwest. New Mexico Museum of Natural History, Albuquerque, New Mexico.

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