Ventastega curonica and the origin of tetrapod morphology

Abstract

The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaalik roseae. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and partial pelvis of Ventastega curonica from the Late Devonian of Latvia, a transitional intermediate form between the 'elpistostegids' Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega. Ventastega is the most primitive Devonian tetrapod represented by extensive remains, and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms.The fossil record of Devonian tetrapods, the earliest and most primitive limb-bearing members of the tetrapod stem group, was for many decades restricted to the iconic 'four-legged fish' Ichthyostega from the Famennian (latest Devonian) of Greenland 1-5 and the fragmentary genus Acanthostega from the same strata 2 . During the last 20 years, intense collecting and research has produced complete skeletal material of Acanthostega 6-8 and a series of new taxa, greatly expanding the temporal and geographical range of these animals. Devonian tetrapods are now known from as early as the late Frasnian, the earlier part of the Late Devonian period, and have been recorded from Gondwana and north China as well as Laurussia 9-18 . However, most of these new forms remain very poorly known, typically represented by no more than lower jaw rami or isolated postcranial bones; Acanthostega and Ichthyostega are still the only Devonian tetrapods known from near-complete skeletons. We know less about the fishtetrapod transition than the taxic diversity suggests.Among the more fragmentary forms are five (Metaxygnathus, Densignathus, Elginerpeton, Obruchevichthys and Ventastega) that combine a characteristically tetrapod lower-jaw morphology with the retention of coronoid fangs and other 'fish' characters absent in Acanthostega, Ichthyostega and more crownward limbed members of the tetrapod stem group 19,20 . These genera seem to fall into the morphological gap between Acanthostega and Ichthyostega and the (paraphyletic) elpistostegids, but all except Ventastega are very incomplete. Ventastega was originally described in 1994 from the Pavāri locality in the late Famennian Ketleri Formation of Kurzeme, western Latvia 21 (Supplementary Information 1). Further excavations at this site up to 2001 have yielded an extensive body of material, including previously unknown or incompletely known elements such as a near-complete skull roof plus braincase and associated cheek (Fig. 1), scapulocoracoid, anocleithrum, interclavicle and ilium (Fig. 2). All come from a single horizon, and the occurrence of multiple identical examples of several elements (jaws, cheek plates, maxillae, clavicles, cleithra, nasals) indicates that only ...

Dynamical, biological and anthropic consequences of equal lunar and solar angular radii

“…The Baltic Devonian Delta thus preserves explicit evidence for the environmental influences of modulated tidal forcing. The formation existed for some 30 Myr, and for much of this time provided habitats for tetrapods and near-tetrapods [11,12].…”

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition

“…An alternative phylogeny based on Ahlberg et al (2008) was also tested, but this affected the results only slightly, as described below.…”

Feeding biomechanics inAcanthostegaand across the fish–tetrapod transition

“…Stem-and crowngroup attributions differ in recent analyses of early tetrapod interrelationships. Here, we follow broadly the scheme of relationships in [22], with amendments and additions from [23][24][25][26]. Jaw orientation and descriptive terminology conform to [23,27].…”