A Mississippian (early Carboniferous) tetrapod showing early diversification of the hindlimbs

Abstract

The taxonomically diverse terrestrial tetrapod fauna from the late Mississippian East Kirkton Limestone includes the earliest known members of stem Amphibia and stem Amniota. Here we name and describe a new East Kirkton tetrapod with an unusual hindlimb morphology reminiscent of that of several stem- and primitive crown amniotes. It displays a unique ilium with two slender and elongate processes and a 5-digit pes with a long, stout metatarsal IV and a greatly elongate digit IV. The new taxon broadens our knowledge of East Kirkton tetrapods, adding to the remarkable diversity of their hindlimb constructions, functional specializations, locomotory modes, and adaptations to a wide variety of substrates. An unweighted character parsimony analysis places the new taxon in a polytomy alongside some other Carboniferous groups. Conversely, weighted parsimony and Bayesian analyses retrieve it among the earliest diverging stem amniotes, either as the basalmost anthracosaur or within a clade that includes also Eldeceeon and Silvanerpeton, crownward of an array of chroniosaurs plus anthracosaurs.

A comprehensive phylogeny and revised taxonomy of Diadectomorpha with a discussion on the origin of tetrapod herbivory

“…Both our majority-rule consensus tree and Adams consensus place the Seymouria species as the earliest branching seymouriamorphs, followed by Karpinskiosaurus secundus, while Utegenia shpinari is deeply nested (figure 12a,b). By contrast, U. shpinari is consistently found as the earliest branching seymouriamorph in the other studies [37,39,61]. Adams consensus tree and (c) strict consensus tree.…”

“…We used the character-taxon matrix from Clack et al [37] as a basis for the updated and expanded matrix used in this study. The subsequent compound characters from Clack et al [37] were split following Brazeau [63]: 7,22,62,82,103,111,112,113,123,135,136,146,156,160,180,195,196,210,236,238,240,284,285. Three new characters were added or modified from Kissel [40].…”

“…Notably, A. pusillus is placed far outside of Diadectomorpha in the Adams consensus, in a polytomy with a clade of Embolomeri and all taxa closer to Amniota. Lastly, Termonerpeton makrydactylus [37] is removed from a polytomy with Silvanerpeton miripedes and Eldeceeon rolfei into a far more basal position; at the base of Tetrapoda in a polytomy with Temnospondyli and total-group Amniota. Bootstrap values over 50 are reported in the majority-rule tree, Bremer decay values are reported in the strict consensus tree.…”

The origin of Amniota in phylogenetic context

“…Some such questions have indeed risen to prominence recently, for example:  Is Diadectomorpha the closest known relative of Amniota (e.g. Laurin et al, 1997;Pardo et al, 2017, through Pardo, 2023-see Table 1 for the complete lineage), or are its members panmammalian amniotes (Berman, 2013;Marjanović and Laurin, 2019;Klembara et al, 2019Klembara et al, , 2021Ruta et al, 2020;Clack et al, 2022)?  Are Amniota and Diadectomorpha closer to Seymouriamorpha, as very often implied in amniote analyses by the use of seymouriamorphs as outgroups while omitting "microsaurs" completely, or to a clade of mostly "microsaurs" (Marjanović and Laurin, 2019, and references therein)?…”

Tracking ‘transitional’ diadectomorphs in the earliest Permian of equatorial Pangea

“…Diadectomorpha was a widely distributed clade of large-bodied stem-amniotes ( e.g ., Panchen & Smithson, 1988 ; Laurin & Reisz, 1997 , 1999 ; Pardo et al, 2017 ; Ford & Benson, 2020 ; Brocklehurst, Ford & Benson, 2022 ) or perhaps early-diverging synapsids ( Berman, 2000 , 2013 ; Klembara et al, 2019 ; Klembara et al, 2021 ; Clack, Smithson & Ruta, 2022 ; see also the phylogenetic assessment of Marjanović & Laurin (2019) ) that originated in the Carboniferous (see, e.g ., Voigt & Ganzelewski, 2010 ), flourished in late Pennsylvanian and Cisuralian (late Carboniferous and early Permian; e.g ., Berman & Sumida, 1990 ; Berman, Sumida & Lombard, 1992 ; Berman, Sumida & Martens, 1998 ; Berman et al, 2004 ), and died out in or shortly after the Wuchiapingian (late Permian; Liu & Bever, 2015 ). Owing to their phylogenetic placement and recognition as one of the earliest tetrapod lineages to evolve high-fiber herbivory ( e.g ., Beerbower, Olson & Hotton, 1992 ; Hotton, Olson & Beerbower, 1997 ; Sues, 2000 ), diadectomorphs are significant contributors to our understanding of the amniote origins and the structure of land ecosystems in the late Paleozoic.…”