Field of Science

An Upclose Look at the Microanatomy of Aetosaur Osteoderms

Aetosaurs are characterized by their elaborate bony carapaces composed of numerous osteoderms.  In fact aetosaur taxonomy is almost based solely on the morphology (especially the surface ornamentation) of osteoderms.  Despite this detailed studies of the microstructure of aetosaur oseoderms are lacking.  In 2008 I published a paper with Michelle Stocker and Randall Irmis that provided the first histological data for aetosaur osteoderms, but we were mostly looking at providing an estimated age at time of death for the holotype of Sierritasuchus macalpini to determine the ontogenetic stage of the specimen.

This new study focuses on aetosaurine osteoderms from Argentina and Brazil, including specimens assigned to Aetosauroides scagliai. One of the very cool things these authors did was not only to look a parasaggital sections of the rectangular osteoderms, they also looked at transverse sections. Some of the key findings are as follows:

- Aetosaur osteoderms lend themselves well to this type of study as secondary remodeling is minimal.

-Unlike all other sampled archosaurs, aetosaur osteoderm ossification was not metaplastic in nature (i.e. pre-existing, fully developed tissue is ossified), instead the osteoderms seemingly underwent intermembraneous ossification where new tissue displaces preformed tissue rather than incorporating it.  This is currently unique among archosaurs.

- Cyclic growth lines (Lines of arrested growth of LAG's) are well developed. Based on this the specimens sampled belonged to a range of subadult animals between two and nine years of age at time of death (minimum ages).

- The center of ossification in aetosaur osteoderms is at the level of the raised dorsal eminence.

- Aetosaur plates probably grew by adding peripheral layers.  Interestingly most faster growth occurred along the medial and lateral margins.  This accounts for the assymetrical placement of the dorsal eminence that is characteristic of aetosaurines.

- Well-developed Sharpey's fibers along the medial and lateral margins of the osteoderms suggest strong lateral and medial attachments along a row of osteoderms.  In contrast the attachments with anterior of posterior plates were poor, presumably allowing for flexion and movement in the carapace.

-Finally, the ornamentation of the osteoderms is formed by local resorption and partitial redeposition of the cortical bone. Acceleration of growth in particular areas enhances the degree of sculpture through time and the pattern is established early and then maintained through future growth.  This is seemingly why the ornamentation in juvenile specimens does not differ significantly from that of adults. This is extremely significant if you are using this patterning to diagnose taxa.

Overall an important study and excellent paper.

Cerda, I. A., and J. B. Desojo. 2010: Dermal armour histology of aetosaurs (Archosauria: Pseudosuchia), from the Upper Triassic of Argentina and Brazil. Lethaia, DOI: 10.1111/j.1502-3931.2010.00252.x.

Abstract - One of the most striking features documented in aetosaurs is the presence of an extensive bony armour composed of several osteoderms. Here, we analyse the bone microstructure of these elements in some South American Aetosaurinae aetosaurs, including Aetosauroides scagliai. In general terms, Aetosaurinae osteoderms are compact structures characterized by the presence of three tissue types: a basal cortex of poorly vascularized parallel-fibred bone tissue, a core of highly vascularized fibro-lamellar bone, and an external cortex of rather avascular lamellar bone tissue. Sharpey’s fibres are more visible at the internal core, toward the lateral margins and aligned parallel to the major axis of the dermal plate. No evidence of metaplastic origin is reported in the osteoderms, and we hypothesize an intramembranous ossification for these elements. The bone tissue distribution reveals that the development of the osteoderm in Aetosaurinae starts in a position located medial to the plate midpoint, and the main sites of active osteogenesis occur towards the lateral and medial edges of the plate. The osteoderm ornamentation is originated and maintained by a process of resorption and redeposition of the external cortex, which also includes preferential bone deposition in some particular sites. Given that no secondary reconstruction occurs in the osteoderms, growth marks are well preserved and they provide very important information regarding the relative age and growth pattern of Aetosaurinae aetosaurs.

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