Development of the Synarcual in the Elephant Sharks (Holocephali; Chondrichthyes): Implications for Vertebral Formation and Fusion

The synarcual is a structure incorporating multiple elements of two or more anterior vertebrae of the axial skeleton, forming immediately posterior to the cranium. It has been convergently acquired in the fossil group ‘Placodermi’, in Chondrichthyes (Holocephali, Batoidea), within the teleost group Syngnathiformes, and to varying degrees in a range of mammalian taxa. In addition, cervical vertebral fusion presents as an abnormal pathology in a variety of human disorders. Vertebrae develop from axially arranged somites, so that fusion could result from a failure of somite segmentation early in development, or from later heterotopic development of intervertebral bone or cartilage. Examination of early developmental stages indicates that in the Batoidea and the ‘Placodermi’, individual vertebrae developed normally and only later become incorporated into the synarcual, implying regular somite segmenta- tion and vertebral development. Here we show that in the holocephalan Callorhinchus milii, uniform and regular vertebral segmentation also occurs, with anterior individual vertebra developing separately with subsequent fusion into a synarcual. Vertebral elements forming directly behind the synarcual continue to be incorporated into the synarcual through growth. This appears to be a common pattern through the Vertebrata. Research into human disor- ders, presenting as cervical fusion at birth, focuses on gene misexpression studies in humans and other mammals such as the mouse. However, in chondrichthyans, vertebral fusion represents the normal morphology, moreover, taxa such Leucoraja (Batoidea) and Callorhinchus (Holocephali) are increasingly used as laboratory animals, and the Callor- hinchus genome has been sequenced and is available for study. Our observations on synarcual development in three major groups of early jawed vertebrates indicate that fusion involves heterotopic cartilage and perichondral bone/mineralised cartilage developing outside the regular skeleton. We suggest that chondrichthyans have potential as ideal extant models for identifying the genes involved in these processes, for application to human skeletal heterotopic disorders

Global Priorities for Conserving the Evolutionary History of Sharks, Rays, and Chimaeras

In an era of accelerated biodiversity loss and limited conservation resources, systematic prioritization of species and places is essential. In terrestrial vertebrates, evolutionary distinctness has been used to identify species and locations that embody the greatest share of evolutionary history. We estimate evolutionary distinctness for a large marine vertebrate radiation on a dated taxon-complete tree for all 1,192 chondrichthyan fishes (sharks, rays and chimaeras) by augmenting a new 610-species molecular phylogeny using taxonomic constraints. Chondrichthyans are by far the most evolutionarily distinct of all major radiations of jawed vertebrates—the average species embodies 26 million years of unique evolutionary history. With this metric, we identify 21 countries with the highest richness, endemism and evolutionary distinctness of threatened species as targets for conservation prioritization. On average, threatened chondrichthyans are more evolutionarily distinct—further motivating improved conservation, fisheries management and trade regulation to avoid significant pruning of the chondrichthyan tree of life

Early members of ‘living fossil’ lineage imply later origin of modern ray-finned fishes

Modern ray-finned fishes (Actinopterygii) comprise half of extant vertebrate species and are widely thought to have originated before or near the end of the Middle Devonian epoch (around 385 million years ago). Polypterids (bichirs and ropefish) represent the earliest-diverging lineage of living actinopterygians, with almost all Palaeozoic taxa interpreted as more closely related to other extant actinopterygians than to polypterids. By contrast, the earliest material assigned to the polypterid lineage is mid-Cretaceous in age (around 100 million years old), implying a quarter-of-a-billion-year palaeontological gap. Here we show that scanilepiforms, a widely distributed radiation from the Triassic period (around 252–201 million years ago), are stem polypterids. Importantly, these fossils break the long polypterid branch and expose many supposedly primitive features of extant polypterids as reversals. This shifts numerous Palaeozoic ray-fins to the actinopterygian stem, reducing the minimum age for the crown lineage by roughly 45 million years. Recalibration of molecular clocks to exclude phylogenetically reassigned Palaeozoic taxa results in estimates that the actinopterygian crown lineage is about 20–40 million years younger than was indicated by previous molecular analyses. These new dates are broadly consistent with our revised palaeontological timescale and coincident with an interval of conspicuous morphological and taxonomic diversification among ray-fins centred on the Devonian–Carboniferous boundary. A shifting timescale, combined with ambiguity in the relationships of late Palaeozoic actinopterygians, highlights this part of the fossil record as a major frontier in understanding the evolutionary assembly of modern vertebrate diversity

Stratigraphy and vertebrate paleoecology of upper cretaceous-? Lowest paleogene strata on vega island, antarctica

The Upper Cretaceous (Maastrichtian) Sandwich Bluff Member of the López de Bertodano Formation is well exposed on Vega Island in the James Ross Basin off the northeastern coast of the Antarctic Peninsula. Although this unit is one of the richest sources of end-Cretaceous vertebrate fossils in Antarctica, it is also one of the least sedimentologically and stratigraphically characterized units in the basin. New facies and stratigraphic analyses of the Sandwich Bluff Member and the underlying Cape Lamb Member of the Snow Hill Island Formation were performed in tandem with intensive prospecting for fossil vertebrates and stratigraphic assessment of historic paleontological localities on Vega Island. This effort has led to a revised stratigraphy for the Sandwich Bluff Member and the precise stratigraphic placement of important terrestrial and marine vertebrate fossil localities.Facies analysis reveals a fining and shallowing upward trend through the section that culminates in a newly recognized sequence boundary near the top of the Sandwich Bluff Member, followed by the deposition of a previously unrecognized, 6. m-thick, matrix-supported pebble-cobble conglomerate of probable alluvial origin. Immediately overlying this unit, well-developed Thalassinoides burrow networks in fine-grained transgressive sandstones and siltstones indicate a rapid return to marine conditions. A similar stratigraphic pattern is well documented at the top of the López de Bertodano Formation and the base of the overlying (Paleocene) Sobral Formation on Seymour Island in the southern part of the basin. Although no fossils were recovered to constrain the age of the upper 10-15. m of the succession on Vega Island that preserves the newly recognized upper sequence boundary, strata below this level can be confidently placed within the Manumiella bertodano interval zone, which extends to a short distance below the K-Pg boundary on Seymour Island. Hence, based on sequence stratigraphic and lithostratigraphic evidence, the uppermost 10-15. m of the succession on Vega Island may encompass the Cretaceous-Paleogene boundary together with a few meters of the Paleocene Sobral Formation. © 2014