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

New U-Pb constraints and geochemistry of the East Kirkton Quarry, Scotland: Implications for early tetrapod evolution in the Carboniferous

The transition of vertebrates from aquatic to terrestrial environments during the late Devonian to early Carboniferous marks a crucial evolutionary milestone. However, this transition remains poorly understood due to a scarcity of early tetrapod fossils during the late Devonian to early Mississippian, creating a gap in the fossil record known as Romer's Gap (~360-345 Ma). Recent discoveries have narrowed this gap, providing critical insights into early tetrapod evolution. The East Kirkton Quarry in Scotland's Midland Valley, has yielded tetrapod fossils considered early stem amphibians and amniotes. They have been proposed to be Mississippian (early Carboniferous) in age, yet data to inform their precise ages remain limited. Here, zircon grains from two tuffaceous clastic limestones and shales were dated using Laser Ablation-Inductively Couple Plasma-Mass Spectrometry (LA-ICP-MS). The study presents detrital zircon U-Pb dates, which refine the current biostratigraphy ages assigned to Westlothiana lizziae, Silvanerpeton miripedes, Balanerpeton woodi, Ophiderpeton kirktonense, Eucritta melanolimnetes, and Kirktonecta milnerae to a maximum depositional age (MDA) of 341 ± 3 Ma (±2σ, n= 7 dates), placing them in the middle-lower Visean (Holkerian-Arundian) rather than the previous assigned upper Visean (Brigantian). This revised maximum depositional age places the East Kirkton Quarry fossils within the older, critical interval of Romer's Gap, bridging a significant evolutionary time interval in the Mississippian fossil record, and allows for refining future tetrapod time trees. X-ray Fluorescence and X-ray Diffraction analyses reveal heterogeneity in the lower East Kirkton Limestone of the East Kirkton Quarry, with variations in elemental and mineralogical compositions, reflecting episodic volcanic and detrital inputs and hydrothermal activity.

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

Among terrestrial tetrapods, the origin of herbivory marked a key evolutionary event that allowed for the evolution of modern terrestrial ecosystems. A 100 Ma gap separates the oldest terrestrial tetrapods and the first undisputed herbivorous tetrapods. While four clades of early tetrapod herbivores are undisputed amniotes, the phylogenetic position of Diadectomorpha with respect to Amniota has long been controversial. Given that the origin of herbivory coincides with the oldest amniotes, and obligate herbivory is unknown within amphibians, this suggests that a key adaptation necessary to evolve obligate herbivory is unique to amniotes. Historically, phylogenetic analyses have found Diadectomorpha as the sister-group to amniotes, but recent analyses recover Diadectomorpha as sister-group to Synapsida, within Amniota. We tested whether diadectomorphs are amniotes by updating the most recent character-taxon matrix. Specifically, we added new characters from the lower jaw and added diadectomorph taxa, resulting in a dataset of 341 characters and 61 operational taxonomic units. We updated the description of five diadectomorph jaws using microcomputed tomography data. Our majority-rule consensus places Diadectomorpha as sister-group to Synapsida; other methods do not recover this relationship. We revise diadectomorph taxonomy, erecting a new species from the early Permian Bromacker locality, Germany, and a new genus to accommodate 'Diadectes' sanmiguelensis.

Tracking 'transitional' diadectomorphs in the earliest Permian of equatorial Pangea

Diadectomorpha was a clade of large-bodied stem-amniotes or possibly early-diverging synapsids that established a successful dynasty of late Carboniferous to late Permian high-fiber herbivores. Aside from their fairly rich record of body fossils, diadectomorphs are also well-known from widely distributed tracks and trackways referred to as Ichniotherium. Here, we provide detailed description of a diadectomorph trackway and a manus-pes couple originating from two different horizons in the Asselian (lowermost Permian) of the Boskovice Basin in the Czech Republic. The specimens represent two distinct ichnotaxa of Ichniotherium, I. cottae and I. sphaerodactylum. Intriguingly, the I. cottae trackway described herein illustrates a 'transitional' stage in the posture evolution of diadectomorphs, showing track morphologies possibly attributable to a Diadectes-like taxon combined with distances between the successive manus and pes imprints similar to those observable in earlier-diverging diadectomorphs, such as Orobates. In addition, this trackway is composed of 14 tracks, including six well-preserved manus-pes couples, and thus represents the most complete record of Ichniotherium cottae described to date from the Asselian strata. In turn, the manus-pes couple, attributed here to I. sphaerodactylum, represents only the second record of this ichnotaxon from the European part of Pangea. Our study adds to the diversity of the ichnological record of Permian tetrapods in the Boskovice Basin which had been essentially unexplored until very recently.