Follow the footprints and mind the gaps: a new look at the origin of tetrapods

Abstract

The hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods. Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water. Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral coastal lakes that preserve few, if any, body fossils; contemporary elpistostegids occur in deltas and the lower reaches of permanent rivers where body fossils are preserved. During the Frasnian, elpistostegids disappear and these riverine-deltaic environments are colonised by tetrapods. This replacement has, in the past, been misinterpreted as the origin of tetrapods.

The smallest known Devonian tetrapod shows unexpectedly derived features

“…The mandibular lateral line canal is difficult to visualize in the scan, but it is certainly present and appears to consist of short open grooves crossing the infradentary sutures, alternating with enclosed sections of canal in the centres of these bones. Brittagnathus was thus at least partly aquatic [31].…”

Deconstructing the long‐standing a priori assumption that serial homology generally involves ancestral similarity followed by anatomical divergence

“…Digit‐bearing limbs of early tetrapods evolved in the Devonian period (∼350–420 mya), and had more than five digits (e.g., Saxena, Towers, & Cooper, 2017; Ahlberg, 2019; Figures 9a and 11a). Acanthostega gunnari had eight digits on each forelimb and hindlimb (Coates & Clack, 1990; Coates, 1996; Figure 11b,c).…”

The feeding system of Tiktaalik roseae : an intermediate between suction feeding and biting

“…Late Devonian finned tetrapodomorphs, typified by Eusthenopteron foordi , have expansive, kinetic skulls with open sutures, robust gill covers, large hyomandibulae, tall palatal elements, and a jointed neurocranium all thought to be features that play a role in suction feeding ( 5 , 9 , 20 ). In contrast, the Late Devonian limbed tetrapodomorph Acanthostega gunnari has a flat skull, interdigitating sutures between the bones of the skull roof, absent gill covers, reduced hyomandibulae, horizontal palatal elements, and a consolidated neurocranium that are hypothesized to be derived adaptations for biting ( 5 , 6 , 21 , 22 ). Analyses of tetrapodomorph lower jaws have produced equivocal results, noting few differences between presumed aquatic and terrestrial forms ( 7 , 8 ).…”