Synapsid evolution can be characterized by three successive radiations: the Permo–
Carboniferous pelycosaurs, the Permo–Triassic Therapsida, and the Triassic Eucynodontia.
Previous geometric morphometric research at the clade level revealed a continuous
increase in humeral morphological disparity in Therapsida, in contrast to their pelycosaur
forebearers. Here we present associated data on ulnar morphological disparity, as well an
an overall taxonomic expansion of the analyses. This increase in sample size brings the
dataset to 765 specimens from which functional units across the forelimb were analyzed.
Further, it allows for a more detailed discussion of variance within nearly every major
group of early Synapsida, as well as across 80 million years of geologic history.
Groups were analyzed for Procrustes variance in 5 million year time bins from 305–225
Mya (Carboniferous–Triassic). In all analyzed functional units—the proximal humerus,
distal humerus, and proximal elbow—within group disparity is higher in therapsid families
than in pelycosaur families. In addition, therapsid family level disparity is much more
variable between groups and across time. Ulnar variance values are higher than humeral
values for the entire study period. Procrustes variance for the forelimb decreases across the
End Permian Mass Extinction Event in the major therapsid groups that survived it—
Anomodontia and Cynodontia.
Macroevolutionary changes observed in Synapsida have historically been associated with
ecological diversification. Cynodontia and Anomodontia have the highest variance in
Therapsida, while Gorgonopsia has the lowest. The high values in Anomodontia, as one of
the most taxonomically and ecological diverse clades of Therapsida, suggests that forelimb
variance is linked to aspects of ecological diversification. Further, within pelycosaurs
Sphenacodontidae has the lowest variance through time, while Ophiacodontidae has the
highest. The finding of uniquely high variance levels in Ophiacodontidae, hypothesized by
some to be semi-acquatic, is suggestive of a potentially unique forelimb ecomorphology.
This research provides evidence that along with major shifts in forelimb morphology,
within-family disparity dynamics may have been critical to the evolutionary success of
individual synapsid sub-orders.
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Incongruence of morphological disparity and evolutionary rate in the forelimbs of Paleozoic synapsids
Previous work has shown increased morphological variance within the forelimbs of the Permian synapsid group known as Therapsida over that of their Carboniferous and early Permian forerunners (“pelycosaurs”). Considering that disparity trends have been known to point to underlying macroevolutionary transitions, here we analyzed morphological variance alongside several additional macroevolutionary metrics to better isolate possible evolutionary mechanisms. Shape data was collected on a sample of 119 humeri and 99 ulnae comprising three major synapsid radiations with a temporal
range from the Carboniferous into the Triassic. Taxonomic sample included all major groups of pelycosaur-grade synapsids, all five recognized non-cynodontian therapsid clades, and a sample of pre-prozostrodontian cynodonts. Procrustes variance - a multivariate quantification of morphospace occupation - was the chosen disparity metric for the study. Rate of phenotypic change, which considers the amount of shape change that would be necessary to achieve observed morphologies given the shape of the closely related taxa, was analyzed as the metric for evolutionary rate. Both metrics were considered through-time upon genera present in sequential 5 million year time bins.
Our results expand upon previous findings that disparity increases throughout the earliest stages of the Permian, coincident with the diversification of pelycosaurs and the emergence of Therapsida. This expanded dataset further shows that disparity approaches an asymptote around 270 million years ago and only increases marginally through the late Permian, remaining between 0.018–0.021 from 275-245
mya. In contrast, evolutionary rate does not appear to asymptote during this same interval, starting at a low of 6.17e-6 (300 mya) and increasing to a peak of 1.78e-5 right before the End Permian Mass Extinction Event (252 mya). The continuing increase of evolutionary rate shows that morphological change continues across taxa, but the plateauing of morphological disparity suggests that morphospace is not expanding concurrent with this. The incongruence between these two metrics suggests a critical
change in evolutionary mode, wherein morphological change continues rapidly but does not result in the evolution of novel morphologies. These results provide some of the strongest quantitative data yet of an evolutionary constraint acting upon the morphology of the synapsid forelimb through deep time.
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- Award ID(s):
- 1754502
- NSF-PAR ID:
- 10405931
- Date Published:
- Journal Name:
- Journal of vertebrate paleontology
- Issue:
- Program and Abstracts, 2022
- ISSN:
- 0272-4634
- Page Range / eLocation ID:
- 224
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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