A new rhynchocephalian from the late jurassic of Germany with a dentition that is unique amongst tetrapods.

Rhynchocephalians, the sister group of squamates (lizards and snakes), are only represented by the single genus Sphenodon today. This taxon is often considered to represent a very conservative lineage. However, rhynchocephalians were common during the late Triassic to latest Jurassic periods, but rapidly declined afterwards, which is generally attributed to their supposedly adaptive inferiority to squamates and/or Mesozoic mammals, which radiated at that time. New finds of Mesozoic rhynchocephalians can thus provide important new information on the evolutionary history of the group. A new fossil relative of Sphenodon from the latest Jurassic of southern Germany, Oenosaurus muehlheimensis gen. et sp. nov., presents a dentition that is unique amongst tetrapods. The dentition of this taxon consists of massive, continuously growing tooth plates, probably indicating a crushing dentition, thus representing a previously unknown trophic adaptation in rhynchocephalians. The evolution of the extraordinary dentition of Oenosaurus from the already highly specialized Zahnanlage generally present in derived rhynchocephalians demonstrates an unexpected evolutionary plasticity of these animals. Together with other lines of evidence, this seriously casts doubts on the assumption that rhynchocephalians are a conservative and adaptively inferior lineage. Furthermore, the new taxon underlines the high morphological and ecological diversity of rhynchocephalians in the latest Jurassic of Europe, just before the decline of this lineage on this continent. Thus, selection pressure by radiating squamates or Mesozoic mammals alone might not be sufficient to explain the demise of the clade in the Late Mesozoic, and climate change in the course of the fragmentation of the supercontinent of Pangaea might have played a major role

The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins.

Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic acanthodians are possibly the earliest gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418-412 Myr bp) acanthodian, Ptomacanthus anglicus, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters

First shark from the late Devonian (Frasnian) gogo formation, Western Australia sheds new light on the development of tessellated calcified cartilage

Background: Living gnathostomes (jawed vertebrates) comprise two divisions, Chondrichthyes (cartilaginous fishes, including euchondrichthyans with prismatic calcified cartilage, and extinct stem chondrichthyans) and Osteichthyes (bony fishes including tetrapods). Most of the early chondrichthyan (‘shark’) record is based upon isolated teeth, spines, and scales, with the oldest articulated sharks that exhibit major diagnostic characters of the group—prismatic calcified cartilage and pelvic claspers in males—being from the latest Devonian, c. 360 Mya. This paucity of information about early chondrichthyan anatomy is mainly due to their lack of endoskeletal bone and consequent low preservation potential. Methodology/Principal Findings: Here we present new data from the first well-preserved chondrichthyan fossil from the early Late Devonian (ca. 380–384 Mya) Gogo Formation Lägerstatte of Western Australia. The specimen is the first Devonian shark body fossil to be acid-prepared, revealing the endoskeletal elements as three-dimensional undistorted units: Meckel’s cartilages, nasal, ceratohyal, basibranchial and possible epibranchial cartilages, plus left and right scapulocoracoids, as well as teeth and scales. This unique specimen is assigned to Gogoselachus lynnbeazleyae n. gen. n. sp.Conclusions/Significance: The Meckel’s cartilages show a jaw articulation surface dominated by an expansive cotylus, and a small mandibular knob, an unusual condition for chondrichthyans. The scapulocoracoid of the new specimen shows evidence of two pectoral fin basal articulation facets, differing from the standard condition for early gnathostomes which have either one or three articulations. The tooth structure is intermediate between the ‘primitive’ ctenacanthiform and symmoriiform condition, and more derived forms with a euselachian-type base. Of special interest is the highly distinctive type of calcified cartilage forming the endoskeleton, comprising multiple layers of nonprismatic subpolygonal tesserae separated by a cellular matrix, interpreted as a transitional step toward the tessellated prismatic calcified cartilage that is recognized as the main diagnostic character of the chondrichthyans

Growth and mineralogy in dental plates of the holocephalan Harriotta raleighana (Chondrichthyes): novel dentine and conserved patterning combine to create a unique chondrichthyan dentition

The dentition in extant holocephalans (Chondrichthyes) comprises three pairs of continuously growing dental plates, rather than the separate teeth characterizing elasmobranchs. We investigated how different types of dentine in these plates, including hypermineralized dentine, are arranged, and how this is renewed aborally, in adult and juvenile dentitions of Harriotta raleighana (Rhinochimeridae). Individual plates were analysed using x-ray computed tomography (µCT), SEM in back scattered mode with EDX and EDR analysis, and optical microscopy on hard tissue sections

The phylogenetic origin and evolution of acellular bone in teleost fishes: insights into osteocyte function in bone metabolism

Vertebrate bone is composed of three main cell types: osteoblasts, osteoclasts and osteocytes, the latter being by far the most numerous. Osteocytes are thought to play a fundamental role in bone physiology and homeostasis, however they are entirely absent in most extant species of teleosts, a group that comprises the vast majority of bony ‘fishes’, and approximately half of vertebrates. Understanding how this acellular (anosteocytic) bone appeared and was maintained in such an important vertebrate group has important implications for our understanding of the function and evolution of osteocytes. Nevertheless, although it is clear that cellular bone is ancestral for teleosts, it has not been clear in which specific subgroup the osteocytes were lost. This review aims to clarify the phylogenetic distribution of cellular and acellular bone in teleosts, to identify its precise origin, reversals to cellularity, and their implications. We surveyed the bone type for more than 600 fossil and extant ray‐finned fish species and optimised the results on recent large‐scale molecular phylogenetic trees, estimating ancestral states. We find that acellular bone is a probable synapomorphy of Euteleostei, a group uniting approximately two‐thirds of teleost species. We also confirm homoplasy in these traits: acellular bone occurs in some non‐euteleosts (although rarely), and cellular bone was reacquired several times independently within euteleosts, in salmons and relatives, tunas and the opah (Lampris sp.). The occurrence of peculiar ecological (e.g. anadromous migration) and physiological (e.g. red‐muscle endothermy) strategies in these lineages might explain the reacquisition of osteocytes. Our review supports that the main contribution of osteocytes in teleost bone is to mineral homeostasis (via osteocytic osteolysis) and not to strain detection or bone remodelling, helping to clarify their role in bone physiology