Morphology and phylogenetic relationships of fossil snake mackerels and cutlassfishes (Trichiuroidea) from the Eocene (Ypresian) London Clay Formation

‘Gempylids’ (snake mackerels) and trichiurids (cutlassfishes) are pelagic fishes characterized by slender to eel‐like bodies, deep‐sea predatory ecologies, and large fang‐like teeth. Several hypotheses of relationships between these groups have been proposed, but a consensus remains elusive. Fossils attributed to ‘gempylids’ and trichiurids consist almost exclusively of highly compressed body fossils and isolated teeth and otoliths. We use micro‐computed tomography to redescribe two three‐dimensional crania, historically assigned to †Eutrichiurides winkleri and †Progempylus edwardsi, as well as an isolated braincase (NHMUK PV OR 41318). All from the London Clay Formation (Eocene, Ypresian), these specimens represent some of the oldest fossils identified as trichiuroids. We find that †Eutrichiurides winkleri does not show diagnostic characters of †Eutrichiurides, and it is assigned to a new genus. To investigate the placement of these fossils relative to extant lineages, we combine existing morphological character sets for ‘gempylids’ and trichiurids along with published mitogenomic data. Our analyses recover a monophyletic Trichiuridae nested within a paraphyletic ‘Gempylidae’. The taxon formerly known as †Eutrichiurides winkleri is considered Trichiuroidea incertae sedis, while †Progempylus edwardsi and NHMUK PV OR 41318 are recovered within the ‘gempylid’ grade. Using previously published descriptions and character optimizations from our phylogenetic analyses we suggest possible placements for laterally compressed body fossils assigned to Trichiuroidea (†Argestichthys, †Abadzekhia, †Chelifichthys, †Anenchelum, †Eutrichiurides, †Musculopedunculus).Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146609/1/spp21221.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146609/2/spp21221_am.pd

The Synarcual of the Little Skate, Leucoraja erinacea: Novel Development Among the Vertebrates

Evolutionary variation in anteroposterior patterning of the axial skeleton is a major contributor to the evolution of the vertebrate body plan, with five canonical vertebral types in tetrapods (cervical, thoracic, lumbar, sacral, caudal). However, less is known about the evolutionary origin and variation in vertebral regionalization patterns outside of tetrapods where described vertebral types range from as few as two in some chondrichthyans to eight or more in some teleost fishes. The synarcual is a specialized adaptation of the anterior axial skeleton comprising a putatively fused array of vertebral elements characteristic of jawed vertebrate (gnathostome) clades such as batoid and chimaeroid chondrichthyans where they support enlarged pectoral fins and dorsal fin spines respectively, as well as a fossil group known as the placoderms. Placoderms represent the phylogenetically most basal jawed vertebrates and the presence of a synarcual in these and chondrichthyans may suggest a conserved vertebral type for jawed vertebrates, predating the divergence of stem and crown gnathostomes. Alternatively, synarcuals may have evolved independently in these lineages, exhibiting a remarkable case of morphological convergence. In order to address this question, we investigated the early development of the cervicothoracic synarcual of an emerging model chondrichthyan: the Little skate Leucoraja erinacea. By combining x-ray computed tomography, and classical histology, we show that the skate synarcual is a complex composite element which develops from a hollow, continuous cartilaginous element devoid of any vertebral centra anterior to the pectoral girdle, and fusion/remodeling of initially distinct vertebrae posteriorly. A de novo transcriptome assembly for two developmental stages of the skate synarcual and post-synarcual axial skeletal elements supported this two-phase development, with differences in expression levels of several developmental genes, including Hox family transcription factors, which suggest anterior-posterior regionalization along the vertebral column, potentially linked to the synarcual. In addition, multiple genes related to skeletal remodeling were found to be more highly expressed at stage 33, including genes related to osteoclast activity, normally associated with removal of bone, a tissue absent in chondrichthyans. These genes are potentially related to loss of mineralization as vertebral elements are incorporated into the synarcual

How to tuna fish:constraint, convergence, and integration in the neurocranium of pelagiarian fishes

Morphological evolution of the vertebrate skull has been explored across a wide range of tetrapod clades using geometric morphometrics, but the application of these methods to teleost fishes, accounting for roughly half of all vertebrate species, has been limited. Here we present the results of a study investigating 3D morphological evolution of the neurocranium across 114 species of Pelagiaria, a diverse clade of open-ocean teleost fishes that includes tuna and mackerel. Despite showing high shape disparity overall, taxa from all families fall into three distinct morphological clusters. Convergence in shape within clusters is high, and phylogenetic signal in shape data is significant but low. Neurocranium shape is significantly correlated with body elongation and significantly but weakly correlated with size. Diet and habitat depth are weakly correlated with shape, and nonsignificant after accounting for phylogeny. Evolutionary integration in the neurocranium is high, suggesting that convergence in skull shape and the evolution of extreme morphologies are associated with the correlated evolution of neurocranial elements. These results suggest that shape evolution in the pelagiarian neurocranium reflects the extremes in elongation found in body shape but is constrained along relatively few axes of variation, resulting in repeated evolution toward a restricted range of morphologies.</p

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is well-preserved only in the early diverging stem-gnathostome heterostracans. However, its anatomy remains poorly understood. Here, we use X-ray microtomography to characterize the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). The apparatus is composed of 13 plates arranged approximately bilaterally, most of which articulate from the postoral plate. Our reconstruction shows that the oral plates were capable of rotating around the transverse axis, but likely with limited movement. It also suggests the nasohypophyseal organs opened internally, into the pharynx. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterized by a directional trend towards increasingly active food acquisition

Development and evolution of dentition pattern and tooth order in the Skates and Rays (Batoidea; Chondrichthyes)

Shark and ray (elasmobranch) dentitions are well known for their multiple generations of teeth, with isolated teeth being common in the fossil record. However, how the diverse dentitions characteristic of elasmobranchs form is still poorly understood. Data on the development and maintenance of the dental patterning in this major vertebrate group will allow comparisons to other morphologically diverse taxa, including the bony fishes, in order to identify shared pattern characters for the vertebrate dentition as a whole. Data is especially lacking from the Batoidea (skates and rays), hence our objective is to compile data on embryonic and adult batoid tooth development contributing to ordering of the dentition, from cleared and stained specimens and micro-CT scans, with 3D rendered models. We selected species (adult and embryonic) spanning phylogenetically significant batoid clades, such that our observations may raise questions about relationships within the batoids, particularly with respect to current molecular-based analyses. We include developmental data from embryos of recent model organisms Leucoraja erinacea and Raja clavata to evaluate the earliest establishment of the dentition. Characters of the batoid dentition investigated include alternate addition of teeth as offset successional tooth rows (versus single separate files), presence of a symphyseal initiator region (symphyseal tooth present, or absent, but with two parasymphyseal teeth) and a restriction to tooth addition along each jaw reducing the number of tooth families, relative to addition of successor teeth within each family. Our ultimate aim is to understand the shared characters of the batoids, and whether or not these dental characters are shared more broadly within elasmobranchs, by comparing these to dentitions in shark outgroups. These developmental morphological analyses will provide a solid basis to better understand dental evolution in these important vertebrate groups as well as the general plesiomorphic vertebrate dental condition

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

The clavobranchialis musculature in sarcopterygian fishes, and contribution to osteichthyan feeding and respiration

Various fossil lungfish taxa preserve distinct depressions on the smooth postbranchial lamina of the dermal pectoral girdle. These depressions are largely unknown in other sarcopterygian fishes, but are present in the rhizodont sarcopterygian Strepsodus. Comparisons with extant actinopterygian fishes suggest these depressions mark the point of origin for the clavobranchialis musculature, extending anterodorsally into the gill chamber to insert on the ventral surface of the ceratobranchial(s). Studios examining feeding and respiratory mechanisms of bony fishes (Osteichthyes) have emphasised the role of mandibular depression in generating negative pressures within the oral cavity to draw in water/air/food via suction. However, phylogenetically basal actinopterygians, fossil lungfish and other fossil sarcoptcrygians (such as Strepsodus) lack the apomorphies that increase suction among bony fishes. In these taxa the clavobranchialis muscles may serve to augment this negative pressure by retracting the ceratobranchials and increasing the size of the oral/ oropharyngeal cavity. A comparable action is performed by the chondrichthyan coracobranchiales muscles, particularly during feeding, and the function of these ventral gill arch muscles is likely to be a synapomorphy of jawed vertebrates (Gnathostomata). This musculature is absent from jawless vertebrates such as the Osteostraci