Romer’s ‘gap’ gets filled by Clack et al. 2016

According to Clack et al. 2016
“The term ‘Romer’s Gap’ was coined for a hiatus of approximately 25 million years (Myr) in the fossil record of tetrapods, from the end-Devonian to the mid-Mississippian (Viséan).”

This paper starts to fill Romer’s gap
with five new, incomplete taxa. three stem tetrapods and two stem amphibians, suggesting a deep split among crown tetrapods. That conclusion confirms an earlier one first reported here based on: (1) tetrapod footprints in the Middle Devonian, (2) the first appearance of reptiles in the Viséan and (3) the earlier split of microsaurs + amphibians, evidently before the end of the Devonian or at the very origin of the Carboniferouus following the post-Devonian extinction event.

Figure 1. Above, two trees recovered by Clack et al. 2016 compared to the one tree recovered by the LRT below.

Figure 1. Above, two trees recovered by Clack et al. 2016 compared to the one tree recovered by the LRT below. Left tree: Strict consensus of four equally parsimonious trees obtained from implied weights search with K = 4. Right tree:Bayesian analysis tree. Note the widely varying nesting sites of certain taxa.

Unfortunately the phylogenetic analyses
of Clack et al, (Fig. 1) fail to separate basal reptiles from microsaurs, fail to nest Gephyrostegus and Silvanerpeton as basalmost reptiles and fail to split basal reptiles into Archosauromorpha and Lepidosauromorpha in or before the Viséan. These problems are in addition to their inability to find accord in their own two published topologies (Fig. 1).

Figure 2. A new Romer's Gap taxon, Koilops, nests with basalmost tetrapods. The skull is similar to that of Acanthostega.

Figure 2. A new Romer’s Gap taxon, Koilops, nests with basalmost tetrapods. The skull is similar to that of Acanthostega. Note the interpretive changes between the the color version, which helps one understand the anatomy and the line art Clack et al. 2016 tracing, which still includes some guesswork and perhaps some misinterpretation. Large parts of the squamosal and postorbital are missing here. I’m not saying the colored tracing is 100% correct. This is a difficult fossil. I am saying it is so much easier to understand than the line drawing.

Koilops (Fig. 2) was a basal tetrapod, smaller than most.

Figure 2. Aytonerpeton is a tiny taxon with an inch-long skull. Here CT scans have helped delineate the bones colored for identification.

Figure 3. Aytonerpeton is a tiny taxon with an inch-long skull. Here CT scans have helped delineate the bones colored for identification. The orbit appears to have on odd bean shape, relatively large, but this is a tiny taxon.  CT scan from Clack et al. 2016.

Based on chronological and phylogenetic bracketing
we should expect to find amphibian-like reptiles (with amniote eggs) prior to the Viséan in Romer’s Gap, but they are likely to be a minority component. Utegenia, or similar sister, should be found there, along with other basal seymouriamorphs and reptilomorphs.

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

Figure 4. From August 22, 2016, this graph shows what taxa are likely to be found in the late Devonian and Tournaisian (earliest Carboniferous = Romer’s Gap).

References
Clack JA and 14 other authors 2016. Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature ecology & evolution 1, 0002 (2016) | DOI: 10.1038/s41559-016-0002

 

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3 thoughts on “Romer’s ‘gap’ gets filled by Clack et al. 2016

  1. Great! Now I wish they’d start finding insect fossils from their much larger late-Devonian to end-Mississippian gap. Drives me crazy.

  2. You’ve put the suture between the nasals and the frontals of Koilops in the wrong place. This is clear from the ornamentation: the ossification centers have regular pitted ornamentation, while the recently, quickly grown parts have striated ornamentation (which would be remodeled to regular ornamentation later in ontogeny, after fast growth would have ceased).

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