Early tetrapods

The evolution of fishes to tetrapods (four-limbed vertebrates) was one of the most important transformations in vertebrate evolution. Hypotheses of tetrapod origins rely heavily on the anatomy of a few tetrapod-like fish fossils from the Middle and Late Devonian period (393-359 million years ago) 1. These taxa-known as elpistostegalians-include Panderichthys 2 , Elpistostege 3,4 and Tiktaalik 1,5 , none of which has yet revealed the complete skeletal anatomy of the pectoral fin. Here we report a 1.57-metre-long articulated specimen of Elpistostege watsoni from the Upper Devonian period of Canada, which represents-to our knowledge-the most complete elpistostegalian yet found. High-energy computed tomography reveals that the skeleton of the pectoral fin has four proximodistal rows of radials (two of which include branched carpals) as well as two distal rows that are organized as digits and putative digits. Despite this skeletal pattern (which represents the most tetrapod-like arrangement of bones found in a pectoral fin to date), the fin retains lepidotrichia (fin rays) distal to the radials. We suggest that the vertebrate hand arose primarily from a skeletal pattern buried within the fairly typical aquatic pectoral fin of elpistostegalians. Elpistostege is potentially the sister taxon of all other tetrapods, and its appendages further blur the line between fish and land vertebrates. The first tetrapods known from skeletal remains date back to the Late Devonian period (about 374 million years ago) 6,7 , while trackway fossils showing digitate impressions of limbs suggest an earlier origin for this clade 8. Over the past decade, fossils that provide information on the fish-to-tetrapod transition have been used to better understand anatomical transformations associated with locomotion 5,9-12 , breathing 13 , hearing 14 and feeding 11,15 , with regard to the change in habitat from water to land. Until now, the terrestrialization of vertebrates has primarily been a matter of comparing six relatively well-known Devonian taxa among stem-group tetrapods 16 : a true piscine sarcopterygian, Eusthe-nopteron foordi; a piscine elpistostegalian, Panderichthys rhombolepis; a near-tetrapod elpistostegalian, Tiktaalik roseae; and three true basal tetrapods, Acanthostega gunnari, Ventastega curonica and Ichthyostega sp. Here we adopt an apomorphy-based definition of tetrapods as 'all organisms derived from the first sarcopterygian to have possessed digits homologous with those in Homo sapiens' 17,18. However, these inferences regarding terrestrialization rely critically on the handful of specimens that have been referred to elpistostega-lians, none of which has been completely described. The postcranial anatomy of Panderichthys is primarily restricted to the morphology of the pectoral fins and girdle 2,19,20 , the vertebrae 2,21 , the scale patterning 22 and very little on the pelvic fin and girdle morphology 9. Although more than 60 specimens 1,10 of Tiktaalik have been found, most of the anatomy of this species has been described from a fairly complete individual for which the skull 1,15 , pectoral and pelvic fins and girdles 5,10,23 , scales 22 and the trunk region 1 anterior to the pelvic region are preserved.

The placoderm fauna of the late Famennian tetrapod-bearing locality of Strud, Belgium, is studied on the basis of historical and newly collected material. It includes the previously described antiarch Grossilepis rikiki, the groenlandaspidid Turrisaspis strudensis sp. nov. and the actinolepidoideid Phyllolepis undulata. P. undulata is thoroughly described and joins the list of the valid Phyllolepis species confidently diagnosed. A morphometrical analysis performed on the centronuchal and anterior ventrolateral plates of the Phyllolepis material demonstrates that there is only one species of Phyllolepis in Belgium (thus, Phyllolepis konincki becomes a junior synonym of P. undulata), that P. rossimontina (Pennsylvania) is a synonym of P. undulata and that the unity of the genus Phyllolepis is strongly supported, although the characterization of several species within this genus is blurred. The strong resemblance between the faunal compositions in Strud and Red Hill (Pennsylvania, USA) suggests important faunal exchanges between these regions of the Euramerica landmass.

Capitosaurs were among the largest amphibians that have ever lived. Their members displayed an amphibious lifestyle. We provide new information on functional morphology data, using finite element analysis (FEA) which has palaeoecological implications for the group. Our analyses included 17 taxa using (2D) plate models to test four loading cases (bilateral, unilateral and lateral bitings and skull raising system simulation). Our results demonstrates that, when feeding, capitosaurs concentrated the stress at the circumorbital region of the capitosaur skull and cranial sutures probably played a key role in dissipating and absorbing the stress generated during biting. Basal members (as Wetlugasaurus) were probably less specialized forms, while during Middle- and Late Triassic the group radiated into different ecomorphotypes with closed otic notch forms (as Cyclotosaurus) resulting in the strongest skulls during biting. Previous interpretations discussed a trend from an open to closed otic notch associated with lateral repositioning of the tabular horns, but the analysis of the skull-raising system reveals that taxa exhibiting posteriorly directed tabular horns display similar results during skull raising to those of closed otic notch taxa. Our results suggest that various constraints besides otic notch morphology, such as the elongation of the tabular horns, snout length, skull width and position, and size of the orbits affect the function of the skull. On the light of our results, capitosaur skull showed a trend to reduce the stresses and deformation during biting. Capitosaurs could be considered crocodilian analogues as they were top-level predators in fluvial and brackish Triassic ecosystems.

The origin of tetrapods is one of the key events in vertebrate history. The oldest tetrapod body fossils are Late Devonian (Frasnian–Famennian) in age, most of them consisting of rare isolated bone elements. Here we describe tetrapod remains from two Famennian localities from Belgium: Strud, in the Province of Namur, and Becco, in the Province of Liège. The newly collected material consists of an isolated complete postorbital, fragments of two maxillae, and one putative partial cleithrum, all from Strud, and an almost complete maxilla from Becco. The two incomplete maxillae and cleithrum from Strud, together with the lower jaw previously recorded from this site, closely resemble the genus Ichthyostega, initially described from East Greenland. The postorbital from Strud and the maxilla from Becco do not resemble the genus Ichthyostega. They show several derived anatomical characters allowing their tentative assignment to a whatcheeriid-grade group. The new tetrapod records show that there are at least two tetrapod taxa in Belgium and almost certainly two different tetrapod taxa at Strud. This locality joins the group of Devonian tetrapod-bearing localities yielding more than one tetrapod taxon, confirming that environments favourable to early tetrapod life were often colonized by several tetrapod taxa.

Biting is an integral feature of the feeding mechanism for aquatic and terrestrial salamanders
to capture, fix or immobilize elusive or struggling prey. However, little information is
available on how it works and the functional implications of this biting system in amphibians
although such approaches might be essential to understand feeding systems performed by
early tetrapods. Herein, the skull biomechanics of the Chinese giant salamander, Andrias
davidianus is investigated using 3D finite element analysis. The results reveal that the prey
contact position is crucial for the structural performance of the skull, which is probably related
to the lack of a bony bridge between the posterior end of the maxilla and the anterior
quadrato-squamosal region. Giant salamanders perform asymmetrical strikes. These
strikes are unusual and specialized behavior but might indeed be beneficial in such sit-andwait
or ambush-predators to capture laterally approaching prey. However, once captured
by an asymmetrical strike, large, elusive and struggling prey have to be brought to the anterior
jaw region to be subdued by a strong bite. Given their basal position within extant salamanders
and their “conservative” morphology, cryptobranchids may be useful models to
reconstruct the feeding ecology and biomechanics of different members of early tetrapods
and amphibians, with similar osteological and myological constraints.

Examining the geological past of our planet allows us to study periods of severe climatic and biological crises and recoveries, biotic and abiotic ecosystem fluctuations, and faunal and floral turnovers through time. Furthermore, the recovery dynamics of large predators provide a key for evaluation of the pattern and tempo of ecosystem recovery because predators are interpreted to react most sensitively to environmental turbulences. The end-Permian mass extinction was the most severe crisis experienced by life on Earth, and the common paradigm persists that the biotic recovery from the extinction event was unusually slow and occurred in a step-wise manner, lasting up to eight to nine million years well into the early Middle Triassic (Anisian) in the oceans, and even longer in the terrestrial realm. Here we survey the global distribution and size spectra of Early Triassic and Anisian marine predatory vertebrates (fishes, amphibians and reptiles) to elucidate the height of trophic pyramids in the aftermath of the end-Permian event. The survey of body size was done by compiling maximum standard lengths for the bony fishes and some cartilaginous fishes, and total size (estimates) for the tetrapods. The distribution and size spectra of the latter are difficult to assess because of preservation artifacts and are thus mostly discussed qualitatively. The data nevertheless demonstrate that no significant size increase of predators is observable from the Early Triassic to the Anisian, as would be expected from the prolonged and stepwise trophic recovery model. The data further indicate that marine ecosystems characterized by multiple trophic levels existed from the earliest Early Triassic onwards. However, a major change in the taxonomic composition of predatory guilds occurred less than two million years after the end-Permian extinction event, in which a transition from fish/amphibian to fish/reptile-dominated higher trophic levels within ecosystems became apparent.

Temnospondyls were a successful group of early tetrapods that lived during the Palaeozoic and Mesozoic periods. Different ecomorphotypes were present (terrestrial, amphibious and fully aquatic) with a wide range of lifestyles. Herein, we analysed several clades of temnospondyls using geometric morphometrics, Finite Element Analysis, and comparative phylogenetic analysis. Some temnospondyli clades were ‘crocodilomorph’ feeding analogues. The skull analysis reveals a concordance between form and feeding function, in amphibious and fully aquatic feeders. The form of terrestrial feeders could be consequences of adaptative or phylogenetical constraints. Basal temnospondyls, as edopoids, were able to leave the water and feed on land. Eryopids continued as terrestrial feeders, although some members showed a shift to increased aquatic feeding. The aquatic environment was especially occupied by archegosaurs during the Permian. After the Permo-Triassic extinction, trematosaurs and capitosaurs returned to the aquatic environment and their members were amphibious and fully aquatic feeders until their disappearance.