Aetosaurs 101: Osteoderm Nomenclature

I think that I will try to stay with aetosaurs for a little bit (and put Julia's law to a test). First I would like to provide an explanation of the osteoderm nomenclature that workers use for these animals. The terminology we use stems from the seminal paper by Long and Ballew in 1985, which was the first comprehensive work to utilize osteoderm morphology in aetosaur taxonomy (I discussed this in more detail in a post last February). Thus, aetosaur plate nomenclature generally differs from that of other armored animals such as crocodiles and alligators and may not even make sense to some of you.

Figure A shows a (an old) reconstruction of Desmatosuchus spurensis in dorsal (top) view. Aetosaurs are characterized by an extensive carapace of generally rectangular osteoderms (plates) that cover the entire back of the animal. Furthermore in most taxa the belly and the underside of the tail are also covered, as well as irregular shaped, small osteoderms that cover the limbs and possibly the flanks. This armor makes aetosaurs interesting to study, because there is lots of armor to look at, but a nightmare to excavate and prepare as you usually find them in a huge, jumbled mass of plates and the other bones.



The dorsal surface armor is arranged in four columns straddling the midline of the animal. The two inner columns are termed the paramedian columns (left and right), whereas the outer two columns are termed lateral columns (left and right). In addition there are numerous transverse rows, each consisting of a total of four plates (one from each column). Because generally each row overlies a vertebra, the rows are distinguished (divided into regions) by what type of vertebra they cover. Thus, we have cervical (neck), dorsal (back), and caudal (tail) rows. Sometimes we even separate out the sacral (or pelvic) rows. The belly plates are called "ventral plates" and in some aetosaurs there are additional rows under the tail, which mirror the ones covering the top of the tail. The nomenclature of the tail plates can get cumbersome as you have dorsal caudal lateral plates and ventral caudal lateral plates. Dorsal and cervical plates are only those that occur on the upper surface, so you do not have dorsal cervicals, ventral cervicals, ventral dorsals, or dorsal dorsals...whew!

Figure B shows a dorsal paramedian (so it is covering a dorsal [thoracic for you mammal workers] vertebra) of the aetosaurine Calyptosuchus wellesi. Anterior is up. All aetosaurs have a wide smooth transverse area on the anterodorsal portion of the plate for articulation under a more anterior plate. If this area is raised in relation to the rest of the plate it is termed an "anterior bar" (ab); if it is depressed then it is an "anterior lamina" (al). In aetosaurine aetosaurs there are several projections off of the anterior bar, an "anterolateral projection" (alp), which projects laterally (Fig. B), and often an "anteromedial projection", which projects anteriorly (not shown). The medial edge (me) is straight for articulation with the corresponding paramedian from the other side, whereas the lateral edge (le) is usually sigmoidal for articulation with the lateral plate. Finally there is almost always a boss or eminence on the dorsal surface of the plate and this is usually termed the "dorsal eminence" (de). Some taxa possess a strong transverse thickening on the ventral plate surface termed the "ventral strut" (not shown).
Figure C is a dorsal lateral plate of the desmatosuchine Desmatosuchus spurensis. Lateral plates have a medial edge that is often sigmoidal for articulation for the paramedian plate. There is no articulation on the lateral surface. There is still the anterior, transverse articular surface and in Desmatosuchus it is depressed so it is an "anterior lamina". The dorsal eminence is often very pronounced (e.g., spikes) in the lateral plates, and this eminence marks an area of strong flexion, which divides the plate into dorsal (df) and lateral flanges (lf). In addition Desmatosuchus possesses a rounded projection of the lateral flange termed the "ventrolateral projection". Some other taxa have simple straight edges.
The upper surfaces of all aetosaur plates possess some kind of ornamentation of grooves, pits, and/or ridges. This ornamentation is usually diagnostic of clades and heavily used in aetosaur taxonomy; however, I argue for dividing aetosaurs into clades based on the overall morphology of the lateral armor (Parker, 2007). I will discuss this and the taxonomic utility of the armor ornamentation in future posts. For those of you who followed and were keeping score the paramedian plate in Figure B is from the right side, as is the lateral plate in Figure C.

Figure B is taken from Long and Ballew (1985). Figures A and C are from Parker (2003).

REFERENCES

Long, R. A., and K. L. Ballew. 1985. 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. Museum of Northern Arizona Bulletin 47:45-68.

Parker, W. G. 2003. Description of a new specimen of Desmatosuchus haplocerus from the Late Triassic of Northern Arizona. Unpublished M. S. thesis, Northern Arizona University, Flagstaff, 315 p.

Parker, W. G. 2007. Reassessment of the aetosaur “Desmatosuchus” chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology 5:41–68.