A Devonian tetrapod-like fish reveals substantial parallelism in stem tetrapod evolution

Paleoenvironments of Late Devonian tetrapods in China

The major evolutionary transition from fish to amphibian included Late Devonian tetrapods that were neither fish nor amphibian. They had thick necks and small limbs with many digits on elongate flexuous bodies more suitable for water than land. Habitats of Devonian tetrapods are of interest in assessing selective pressures on their later evolution for land within three proposed habitats: 1, tidal flats, 2, desert ponds, and 3, woodland streams. Here we assess paleoenvironments of the Late Devonian tetrapod Sinostega from paleosols in Shixiagou Canyon near Zhongning, Ningxia, China. Fossil tetrapods, fish, molluscs, and plants of the Zhongning Formation are associated with different kinds of paleosols, representing early successional vegetation, seasonal wetlands, desert shrublands, and riparian woodlands, and paleoclimates ranging from semiarid moderately seasonal to monsoonal subhumid. The tetrapod Sinostega was found in a paleochannel of a meandering stream below a deep-calcic paleosol supporting well drained progymnosperm woodland in a monsoonal subhumid paleoclimate. This habitat is similar to that of the tetrapods Densignathus, Hynerpeton, and an indeterminate watcheeriid from Pennsylvania, USA. Chinese and Pennsylvanian Late Devonian tetrapods lived in productive woodland streams, choked with woody debris as a refuge from large predators. Habitats of other Devonian tetrapods have yet to be assessed from studies of associated paleosols as evidence for their ancient climate and vegetation.

A fresh look at Cladarosymblema narrienense, a tetrapodomorph fish (Sarcopterygii: Megalichthyidae) from the Carboniferous of Australia, illuminated via X-ray tomography

Background

The megalichthyids are one of several clades of extinct tetrapodomorph fish that lived throughout the Devonian-Permian periods. They are advanced "osteolepidid-grade" fishes that lived in freshwater swamp and lake environments, with some taxa growing to very large sizes. They bear cosmine-covered bones and a large premaxillary tusk that lies lingually to a row of small teeth. Diagnosis of the family remains controversial with various authors revising it several times in recent works. There are fewer than 10 genera known globally, and only one member definitively identified from Gondwana. Cladarosymblema narrienense Fox et al. 1995 was described from the Lower Carboniferous Raymond Formation in Queensland, Australia, on the basis of several well-preserved specimens. Despite this detailed work, several aspects of its anatomy remain undescribed.

Methods

Two especially well-preserved 3D fossils of Cladarosymblema narrienense, including the holotype specimen, are scanned using synchrotron or micro-computed tomography (µCT), and 3D modelled using specialist segmentation and visualisation software. New anatomical detail, in particular internal anatomy, is revealed for the first time in this taxon. A novel phylogenetic matrix, adapted from other recent work on tetrapodomorphs, is used to clarify the interrelationships of the megalichthyids and confirm the phylogenetic position of C. narrienense.

Results

Never before seen morphological details of the palate, hyoid arch, basibranchial skeleton, pectoral girdle and axial skeleton are revealed and described. Several additional features are confirmed or updated from the original description. Moreover, the first full, virtual cranial endocast of any tetrapodomorph fish is presented and described, giving insight into the early neural adaptations in this group. Phylogenetic analysis confirms the monophyly of the Megalichthyidae with seven genera included (Askerichthys, Cladarosymblema, Ectosteorhachis, Mahalalepis, Megalichthys, Palatinichthys, and Sengoerichthys). The position of the megalichthyids as sister group to canowindrids, crownward of "osteolepidids" (e.g.,Osteolepis and Gogonasus), but below "tristichopterids" such as Eusthenopteron is confirmed, but our findings suggest further work is required to resolve megalichthyid interrelationships.

Sustained high rates of morphological evolution during the rise of tetrapods

The fish-to-tetrapod transition is one of the most iconic events in vertebrate evolution, yet fundamental questions regarding the dynamics of this transition remain unresolved. Here, we use advances in Bayesian morphological clock modelling to reveal the evolutionary dynamics of early tetrapodomorphs (tetrapods and their closest fish relatives). We show that combining osteological and ichnological calibration data results in major shifts on the time of origin of all major groups of tetrapodomorphs (up to 25 million years) and that low rates of net diversification, not fossilization, explain long ghost lineages in the early tetrapodomorph fossil record. Further, our findings reveal extremely low rates of morphological change for most early tetrapodomorphs, indicating widespread stabilizing selection upon their 'fish' morphotype. This pattern was broken only by elpistostegalians (including early tetrapods), which underwent sustained high rates of morphological evolution for ~30 Myr during the deployment of the tetrapod body plan.

Elpistostege and the origin of the vertebrate hand

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 Panderichthys2, Elpistostege3,4 and Tiktaalik1,5, 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.

Reappraisal of the Silurian placoderm Silurolepis and insights into the dermal neck joint evolution

Silurolepis platydorsalis, a Silurian jawed vertebrate originally identified as an antiarch, is here redescribed as a maxillate placoderm close to Qilinyu and is anteroposteriorly reversed as opposed to the original description. The cuboid trunk shield possesses three longitudinal cristae, obstanic grooves on the trunk shield and three median dorsal plates, all uniquely shared with Qilinyu. Further preparation reveals the morphology of the dermal neck joint, with slot-shaped articular fossae on the trunk shield similar to Qilinyu and antiarchs. However, new tomographic data reveal that Qilinyu uniquely bears a dual articulation between the skull roof and trunk shield, which does not fit into the traditional 'ginglymoid' and 'reverse ginglymoid' categories. An extended comparison in early jawed vertebrates confirms that a sliding-type dermal neck joint is widely distributed and other types are elaborated in different lineages by developing various laminae. Nine new characters related to the dermal neck joint are proposed for a new phylogenetic analysis, in which Silurolepis forms a clade with Qilinyu. The current phylogenetic framework conflicts with the parsimonious evolution of dermal neck joints in suggesting that the shared trunk shield characters between antiarchs and Qilinyu are independently acquired, and the sliding-type joint in Entelognathus is reversely evolved from the dual articulation in Qilinyu.