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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Morphological disparity across the synapsid forelimb: sub-order-level patterns across 80 million years of synapsid evolution
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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- NSF-PAR ID:
- 10088926
- Date Published:
- Journal Name:
- Society of Vertebrate Paleontology 2018 Meeting Program and Abstracts
- Page Range / eLocation ID:
- 172
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Mammals and their closest fossil relatives are unique among tetrapods in expressing a high degree of pectoral girdle and forelimb functional diversity associated with fully pelagic, cursorial, subterranean, volant, and other lifestyles. However, the earliest members of the mammalian stem lineage, the “pelycosaur”-grade synapsids, present a far more limited range of morphologies and inferred functions. The more crownward nonmammaliaform therapsids display novel forelimb morphologies that have been linked to expanded functional diversity, suggesting that the roots of this quintessentially mammalian phenotype can be traced to the pelycosaur–therapsid transition in the Permian period. We quantified morphological disparity of the humerus in pelycosaur-grade synapsids and therapsids using geometric morphometrics. We found that disparity begins to increase concurrently with the emergence of Therapsida, and that it continues to rise until the Permo-Triassic mass extinction. Further, therapsid exploration of new regions of morphospace is correlated with the evolution of novel ecomorphologies, some of which are characterized by changes to overall limb morphology. This evolutionary pattern confirms that nonmammaliaform therapsid forelimbs underwent ecomorphological diversification throughout the Permian, with functional elaboration initially being more strongly expressed in the proximal end of the humerus than the distal end. The role of the forelimbs in the functional diversification of therapsids foreshadows the deployment of forelimb morphofunctional diversity in the evolutionary radiation of mammals.
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‘Small-bodied faunivore’ is the dominant ancestral ecomorphotype early in crown mammalian radiations, but it is unknown how far back this trend extends within Synapsida (the mammalian total group). To examine synapsid macroevolutionary patterns in a phylogenetic context, we built a time-calibrated meta-phylogeny of 2,128 synapsid species from the Carboniferous through Eocene (305–34 Ma), based on 211 published character matrices, each weighted according to their dependence on ‘parent’ matrices. All published character matrices focusing on non-mammalian synapsids were included, making the meta-phylogeny comprehensive for non-mammaliaform synapsids. Further, we used the most comprehensive early mammaliaform matrices. We then collected jaw length measurements (as a proxy for body size) and dietary information for 408 synapsid species (37 non-therapsid pelycosaurs, 134 non-cynodont therapsids, 46 non-mammaliaform cynodonts, 80 non-therian mammaliaforms, and 178 therians). We used the meta-phylogeny in conjunction with jaw length measurements to investigate patterns of body-size and dietary evolution during radiations of synapsid subclades. The results show that faunivory is the typical ancestral diet of each major radiation within Synapsida, but the small-to-large trend in body-size within radiations does not become established until the end-Triassic size bottleneck near the base of Mammaliaformes. Instead, ‘pelycosaur’, ‘therapsid’, and ‘cynodont’ subclades have ancestral jaw lengths that are considerably larger than non-therian mammaliaforms and therians. The shift to small ancestral body sizes is one of several aspects of the mammalian phenotype to emerge in the Late Triassic. Furthermore, by placing ‘mammaliaforms’ and mammals near their likely lower size limit, this change forced most subsequent body-size diversification to consist of trends toward larger sizes, altering macroevolutionary dynamics for the remainder of synapsid history.more » « less
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Evolutionary radiations generate most of Earth’s biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (mammalian total group), ‘small-bodied faunivore’ has been hypothesized as the ancestral state of most major radiating clades. To quantitatively test this hypothesis across multiple radiations, we used a meta-phylogeny (‘metatree’) of Carboniferous through Eocene (305–34 Ma) species in conjunction with jaw lengths (as a proxy for body size) and diet reconstructions for 404 synapsid species. We focus primarily on five major radiations: (i) non-therapsid pelycosaurs, (ii) non-cynodont therapsids, (iii) non-mammaliaform cynodonts, (iv) non-therian mammaliaforms, and (v) therians. Contrary to our expectations, we did not find universal support for the hypothesis that ‘small-bodied faunivore’ is the ancestral state of radiating synapsid groups. Although faunivory was the typical ancestral diet of each major ecological radiation, the radiation forerunners were not relatively small-bodied in many non-mammaliaform synapsid groups. Instead, the small-to-large trend in body-size within radiations does not become common until the end-Triassic size bottleneck near the base of Mammaliaformes. We also find that ecomorphological diversification was often preceded by the extinction of contemporary clades. As a potential causal mechanism for the observed macroevolutionary patterns, it is tempting to assume that the forerunners of major radiations were relatively unspecialized faunivores with reduced extinction risk. However, ‘survival of the unspecialized’ does not fully explain our results. Many of the progenitors of major synapsid radiations may appear to be unspecialized faunivores, but this is likely due to observational bias: the early lineages of each radiation were ‘unspecialized’ relative to many of their later descendant lineages, but, compared to their contemporaries, they exhibit numerous novel characters. These characters were likely important in promoting their long-term survival and diversification, but it appears that mass extinctions and other faunal turnovers were necessary for the lineages that possessed these characters to reach their full evolutionary potential. Therefore, ‘survival of the novel’ appears to be a persistent macroevolutionary pattern throughout synapsid history.more » « less
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null (Ed.)Mammals are noteworthy for their striking ecomorphological diversity in comparison to their non-mammalian synapsid ancestors. However, the lack of a large phenotypic sample coupled with a phylogeny has hindered examination of this characteristic’s acquisition. Did this diversity accumulate at a constant pace, or did evolutionary rate vary between clades and elements? Here I present an analysis of phenotypic evolutionary rate for synapsid forelimbs using 2D geometric morphometric data of 1279 fossil elements, and a time-calibrated composite tree of 160 genera. Rate comparisons were made for five radiations (‘pelycosaurs’, non-cynodont therapsids, non-mammalian cynodonts, Mammaliaformes, Mammalia), and three functional subunits of the forelimb (proximal humerus, distal humerus, ulna). Mammaliaforms were characterized by the highest evolutionary rates for all functional units, followed by therapsids. Both of these groups underwent major ecomorphological diversifications, and the highest rates are found in taxa characterized by specialized forelimb ecologies, such as fossorial dicynodonts (Therapsida) or the semi-aquatic Haldanodon (Mammaliaformes). In all groups the proximal humerus displayed higher rates than the distal humerus, with the highest found in mammaliaforms and therapsids. These groups underwent dramatic morphological change, especially in the gleno-humeral joint. Critically though, the ulna displays the highest evolutionary rates across all groups, highlighting the underappreciated role the ulna played in the morphological and functional transformations of the synapsid forelimb. The simplification of the structure likely increased the possibility for expansion into new morphologies and played a key role in facilitating ecomorphological diversification. Overall, therapsids and mammaliaforms can both be characterized by important functional changes to the forelimb that likely played a role in this dynamic. Phylogenetic signal also varied across the sample, with Pelycosaurs and non-mammalian cynodonts displaying the lowest levels. This in turn reflects these groups’ conservative forelimb morphologies, especially compared with other synapsid clades. Together, these results demonstrate that synapsid forelimb evolution should be characterized as a dynamic and complex accumulation of ‘mammalian’ morphologies. Evolutionary rates varied across taxa and elements as clades adapted their forelimbs in particular ways to accommodate novel ecologies and functions. Funding Sources NSF DEB-1754502, Field Museum Women-in-Science Graduate Fellowship.more » « less
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