Over the next six or seven posts a new hypothesis on the origin of the Amniota will be presented. Get ready for several days of heresy.
If the following sounds like an abstract, that’s because it was one.
A large-scale phylogenetic analysis of basal amniotes is long overdue. Smaller, more focused studies typically followed tradition in creating their inclusion sets because an overarching study was not available to draw from. Too often this led to the recovery of dissimilar sister taxa by default. It is axiomatic that additional taxa test prior results by providing more nesting opportunities, so 389 specimen- and genus-based taxa are employed here. Several taxa formerly considered anamniotes; Gephyrostegus, Bruktererpeton and Eldeceeon, now nest as basalmost amniotes arising from the Seymouriamorpha. They confirm an earlier prediction that the first amniotes would have a small adult body size and contradict current analyses that nest large diadectomorphs as proximal sister taxa to the Amniota. The first amniote clade dichotomy produced an expanded Archosauromorpha (taxa closer to archosaurs, including Synapsida and Enaliosauria) and an expanded Lepidosauromorpha (taxa closer to lepidosaurs, including Caseasauria and Diadectomorpha). The present study sheds new light on the genesis and radiation of the Amniota. Phylogenetic miniaturization is present at the base of several clades, including the Amniota. The ancestry of all included taxa can now be traced back to Devonian tetrapods and every lineage documents a gradual accumulation of derived traits.
Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.
So this is part of what has been keeping my busy this year…
I added several taxa (Fig. 1) to the large reptile tree (not updated yet) that nested at or near the base of the Amniota. Their inclusion shed new light on the basalmost amniotes and subtly changed the tree topology of the large reptile tree. Gephyrostegus bohemicus (Fig. 2) moved to the very base of the Amniota while lacking any traditional amniote traits.
Figure 1. A new reconstruction of Gephyrostegus bohemicus. This specimen lived in the Westphalian, some 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs and the presence of a deep pelvis. These traits shared by all basalmost amniotes, may provide additional space for larger eggs in gravid females, but is also shared with males, if there were males back then. Otherwise, this taxon has none of the traditional amniote traits found in current textbooks. Nevertheless, it nested as the last common ancestor of lepidosauromorphs and archosauromorphs, so by phylogenetic bracketing, it laid amniotic eggs.
Traditional amniote traits include:
- loss/fusion of the intertemporal
- absence of the otic notch
- loss/reduction of palatal fangs
- appearance/expansion of the transverse flange of the pterygoid
- loss of labyrinthine infolding of the marginal teeth
- reduction of the intercentra
- addition of a second sacral vertebra
- narrowing and elongation of the humeral shaft
- appearance of the astragalus from fused tarsal elements.
Ironically, many of the above traits are also found in microsaurs and seymouriamorphs, but not in basalmost amniotes. So there is homoplasy at play here.
Only phylogenetic analysis reveals the origin of the Amniota.
The key trait defining the Amniota is the production of amniotic eggs, a trait shared with all archosauromorphs (all taxa closer to archosaurs, including synapsids and mammals) and lepidosauromorphs (all taxa closer to lepidosaurs). Even though no amniotic eggs were found with the fossil Gephyrostegus bohemicus, phylogenetic bracketing (Fig. 1) indicates that G. bohemicus laid amniotic eggs. It nested as the more recent common ancestor of all lepidosauromorphs and all archosauromorphs (all other amniotes).
Note that Silvanerpeton (Clack 1994, Fig. 2, Viséan, 331 mya) is the proximal anamniote outgroup taxon to the Amniota and lived 30 million years earlier than G. bohemicus.
Figure 2. Silvanerpeton from the Upper Viséan (331 mya) is the outgroup taxon for Gephyrostegus and the Amniota.
Traits that appear in the basal amniote, G. bohemicus,
not present in Silvanerpeton:
- prefrontal separate from postfrontal
- premaxilla not transverse
- major axis of naris less than 30º above jawline
- naris lateral
- nasals and frontals subequal
- maxilla ventrally straight
- longest metatarsal is number four
Nothing very ‘sexy’ about this list. Traditional amniote traits appear later. Like Gephyrostegus bohemicus, Silvanerpeton also lacks posterior dorsal ribs and has a deep pelvis. These traits may indicate that it was the most primitive known taxon to lay large amniotic eggs (in the Viséan), but Silvanerpeton doesn’t quite have the phylogenetic bracketing status that G. bohemicus enjoys. Even so, we’ll soon meet more Viséan taxa that were definite amniotic egg layers. yet were either not considered amniotes or paleontologists wondered about them without adequately testing them in phylogenetic analysis.
Traditional and conventional studies
indicate that diadectomorphs (Fig. 3) are the proximal outgroup taxa for the Amniota, despite the readily apparent differences. In the large reptile tree diadectomorphs nest deep within the Amniota, derived from millerettids.
Figure 3. Click to enlarge. Traditional phylogenies nest large diadectomorphs as amniote outgroup taxa. Here, however, small gephyrostegids share more traits with basal amniotes and are more similar in size. A. Diadectes. B. Orobates. C. Tseajaia. D. Limnoscelis. In the box, basal amniotes: E. Gephyrostegus bohemicus. F. Thuringothyris. G. Westlothiana. H. Hylonomus.
Recent phylogenetic analyses
(Gauthier et al., 1988; Laurin and Reisz, 1995, 1997, 1999; Lee and Spencer, 1997; Ruta, Coates and Quicke, 2003; Ruta, Jefferey and Coates, 2003; Laurin, 2004; Klembara et al., 2014) recovered large, lumbering Limnoscelis and Diadectes (Fig. 3) as proximal amniote outgroup taxa. However, Ruta, Coates and Quicke (2003:292) reported, “The morphological gap between diadectomorphs and primitive crown-amniotes is puzzling”. I think everyone can agree on that one. This puzzle was resolved when Ruta, Jefferey and Coates (2003) nested diadectomorphs and Solenodonsaurus within the Amniota with the addition of the synapsid, Ophiacodon, nesting as a basal taxon. Unfortunately, later workers, like the recent Gephyrostegus paper by Klembara et al. (2014) also nest diadectomorphs outside the Amniota. Taxon exclusion was the problem, like it always is.
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84:369–76.
Gauthier, J A, G Kluge and T Rowe 1988. The early evolution of the Amniota; pp. 103–155 in M. J. Benton (ed.), The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds: Oxford: Clarendon Press.
Klembara J, J Clack, AR Milner and M Ruta 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Laurin M 2004. The evolution of body size, Cope’s rule and the origin of amniotes. Systematic Biologiy 53:594–622.
Laurin M and R R Reisz 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165–223.
Laurin M and R R Reisz 1997. A new perspective on tetrapod phylogeny; pp. 9–59 in S. S. Sumida and K. L. M. Martin (eds.), Amniote Origins: Completing the Transition to Land, Elsevier.
Lee MSY and PS Spencer 1997. Crown-clades, key characters and taxonomic stability: When is an amniote not an amniote?; pp. 6–84 in S. S. Sumida and K. L. M. Martin (eds.), Amniote Origins: Completing the Transition to Land, Elsevier.
Ruta M, MI Coates and DLJ Quicke 2003. Early tetrapod relationships revisited. Biological Reviews 78:251–345.
Ruta M, JE Jefferey and MI Coates 2003. A supertree of early tetrapods. Proceedings of the Royal Society, London B 270:2507–2516.