Summary if you’re in a rush:
Mann et al. 2020 mistakenly reassessed their ‘microsaur’, Asaphestera platyris (Fig. 1), as at ‘the earliest synapsid’. The LRT nests this taxon as a microsaur after demonstrating interpretation and reconstruction errors.
Mann et al. 2020 bring us
their view of ‘microsaurs’ from Joggins, Nova Scotia (Westphalian, Late Carboniferous) with the recognition that “Asaphestera platyris as a synapsid provides the earliest unambiguous evidence of ‘mammal-like reptiles’ in the fossil record.”
No. Just because they say so, does not mean it is true.
By contrast (1):
In the large reptile tree (LRT, 1685+ taxa) the earliest amniote (determined by the last common ancestor method) is Silvanerpeton, from the Viséan (Early Carboniferous) at least 15 million years earlier. Gephyrostegus is more primitive in the LRT, but appears as a late survivor in the Westphalian (Late Carboniferous, coeval with Asaphestera platyris) of an earlier radiation. Archaeothyris, another slightly younger Westphalian taxon, was widely considered the earliest known synapsid, and remains so in the LRT. These three taxa are not mentioned in the Mann et al. text.
By contrast (2):
When added to the LRT, ‘Asaphestera platyris’ (RM 2.1192, Steen 1934) nests with and is not much different from the microsaur, Kirktonecta (Fig. 2), far from any amniotes or synapsids. Kirktonecta is mentioned only once in the Mann et al text as part of a list that “do not fit clearly into this [microbrachomorph] framework.”
From the abstract:
‘‘Microsaurs’ are traditionally considered to be lepospondyl non-amniotes, but recent analyses have recovered a subset of ‘microsaurs’, the fossorially adapted Recumbirostra, within Amniota.”
The LRT does not support this nesting.
Recumbirostra = pantylids, gymnarthrids, brachystelechids, ostodolepids, and rhynchonkids. In the LRT all these taxa are in the clade Microsauria, a sister clade to the Reptilomorpha. Kirktonecta is basal to the microsaur clade that ultimately produced the extant caecilian, Dermophis.
From the Mann et al. description:
“Most of the right maxilla and portions of both temporal regions are known only from impressions of the bones that have weathered away; nevertheless, valuable information is present in what remains. Parts of the dorsal margins of both temporal fenestrae are preserved on either side of the cranium, but the morphology is more completely represented on the right side.”
A reconstruction (Fig. 1) based on the same specimen does not support this description. There is no tall dorsal process to the maxilla, contra Mann et al. The ‘palbebral’ (PB) is below several loose dentary teeth, so it is a palate or mandible element. The ‘dorsal process’ of the maxilla is not represented by bone. The reconstruction nearly matches Kirktonecta (Fig. 2).
From the text:
“As a result, we tentatively attribute RM 2.1192 (Fig. 1) to the Eothyrididae. If this identification is correct, RM 2.1192 would extend the record of eothyridids substantially.”
Co-author, B Gee,
writing on his blogpost (link below) reported, “Among synapsids, this specimen most closely resembles the eothyridids, although it shares a number of features with acleistorhinid parareptiles, which were often confused for eothyridids in their earlier history of study (perhaps they still are eothyridids?).”
Mann et al. 2020 mistakenly reassessed their microsaur, Asaphestera platyris, as a synapsid. The LRT nests it as a microsaur close to Kirktonecta, a taxon essentially overlooked by the authors. Nearly coeval Archaeothyris remains the earliest known synapsid, but several synapsids are more primitive, indicating an earlier radiation. So, they’re out there somewhere! Mann et al. did not find them…yet.
A reader (J) wondered how I was able to reconstruct Kirktonecta if, given the limitations provided by another reader (DM) that only the inside of the skull bones were visible. Here (Fig. 3) I show the method and the data, a crushed skull in which the bones are slightly separated along their sutures and sometimes split during taphonomic crushing. I traced the skull bones of Kirktonecta, then reassembled them using the DGS method (color tracing using Photoshop). The first step was to invert the colors (creating a negative) of the original image, something a paleontologist with firsthand access to the specimen would be unable to do without repeating this method. The original image had higher resolution, reduced here for online publication. Apparently the insides were little different from the outsides given the two-dimensional, plate-like shapes of the skull bones with few-to-no complex curves in the bones of this taxon. I leave it to the reader to decide whether or not the DGS method was successful in this case, whether inside or not.
Mann A et al. (7 co-authors) 2020. Reassessment of historic ‘microsaurs’ from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem. Papers in Palaeontology 2020:1–17.
Steen MC 1934. The amphibian fauna from the South Joggins. Nova Scotia. Journal of Zoology, 104, 465–504.