New publication: Postcranial anatomy and histology of Seymouria, and the terrestriality of seymouriamorphs (Bazzana et al.; PeerJ)
Title: Postcranial anatomy and histology of Seymouria, and the terrestriality of seymouriamorphs
Authors: K.D. Bazzana; B.M. Gee; J.J. Bevitt; R.R. Reisz
Journal: PeerJ vol. 8, article #8698
DOI to paper: 10.7717/peerj.8698
General summary: Back again with some more non-temnospondyl things (those are basically the rest of the papers I have coming out in the near future) on the amniote stem with Seymouria. The last paper led by Kayla looked at braincase anatomy based on some super neutron-CT datasets for Richards Spur material. This one is again looking at Seymouria from Richards Spur, but in the postcranial skeleton this time. Somewhat unusual for early Permian tetrapods, there is a substantial record of the postcranial skeleton for Seymouria based on fully articulated specimens from Germany and the redbeds of the southwest. However, most of the postcrania hasn't been described in great detail, and there's been very few records of postcrania from Richards Spur. This paper rectifies both of those to a degree, with description of really well-preserved isolated postcrania from the site that expands the osteology and knowledge of ontogenetic changes.
We also have some nice new femoral material; the illustration below is from Sullivan & Reisz (1999), who reported the first material of Seymouria from Richards Spur, and the photographs and digital segmentation is from our study. Seymouria has a really short and stocky femur that is also very distinct from a lot of other more gracile, long-legged animals at Richards Spur (on either side of the phylogeny). This is a really high-resolution scan by the way if anybody wants to use the data (on Morphobank; see the link in the paper).
And here's a nice humerus...
This paper also contains some of the first histology of Seymouria (that's my contribution to this paper). Previous workers (Danto et al., 2016) have sectioned single vertebrae, but we intentionally went for two very different sized specimens here in order to get a better idea of what ontogenetic changes occur at the microanatomical and histological scales. Part A in each figure is a sagittal section (from front to back), and Part D is a half-transverse section (side-to-side after making that front-to-back cut). Note that the larger specimen in the figure on the left had the centrum still attached, whereas it fell off in the smaller specimen on the left (not well-attached early in development = falls off). The neural arch (the chunky thing at the top that carries the spine) is always pretty hollow on the inside, but the posterior end really thickens in the larger animal, which probably captures some difference in biomechanical load along each axial position.
We also did some femoral histology, again on a small specimen (left) and a large specimen (right). Both still show a lot of evidence for rapid growth, with a lot of vascularization, and neither appears to be super old even though the largest one is pretty big already. Rather than distinct LAGs like we find in some of the other Richards Spur tetrapods (Gee et al., 2020; Huttenlocker et al., 2020), we see zones of growth and zones of slow growth (annuli) that are punctuated by rest lines. There are only two years' worth of zones in both specimens, but remodeling is already apparent at the margin of the medullary cavity (the big opening in the middle), so it's likely that some growth marks have already been lost.
The presence of rest lines versus LAGs is interesting because they suggest slightly different responses to climatic changes. Usually all animals will be affected by seasons, whether because of temperatures or water availability or sunlight, but some animals totally stop growth (LAGs) and other only slow down (rest lines). As such, you rarely see more than two LAGs in a year (some modern animals like teiids and sea turtles can form three per year (e.g., Snover et al., 2004), but you can get many more rest lines in a given year. There's plenty more work to be done examining the different skeletochronological patterns from the Richards Spur tetrapods, which presumably were all exposed to the same conditions but reacted differently, but it's interesting to note that a stem amniote like Seymouria may not have slowed down as much as an amphibian like Doleserpeton.
One other feature that we noted was the presence of a well-developed medullary trabecular network (infilling of the central cavity). This is often associated with more aquatic animals, and it's taken to the extreme in obligately aquatic temnospondyls like metoposaurids and other stereospondyls in which there basically is no discernible medullary cavity. Seymouria doesn't have nearly as much as those taxa, but it's got more than any of the very terrestrial temnospondyls from Richards Spur. This might suggest that it's either less terrestrial than previously thought or that there might be some hold-over from an earlier stage of development. More work needed for sure!
TLDR: a lot of postcrania of Seymouria and some pretty pictures of some postcrania that we destroyed ~in the name of science.~
About the blog
A blog on all things temnospondyl written by someone who spends too much time thinking about them. Covers all aspects of temnospondyl paleobiology and ongoing research (not just mine).