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A new Late Devonian acanthodian fish from Mt. Howitt, Victoria, Australia, with remarks on acanthodian biogeography
Abstract
A new acanthodiform acanthodian, Howittacanthus kentoni gen. et sp.nov. is described from the Frasnian lacustrine Mt. Howitt site, eastern Victoria. Howittacanthus is characterized by: a palatoquadrate ossified in three divisions with both otic and auxiliary otic cotyli in contact with the brain-case; autopalatine with posterior basal process; slender body form with small pelvic fins closer to anal than to pectoral fins; and all fin-spines ornamented with a single ridge. Ontogenetic data obtained from Howittacanthus agree with the findings of Zidek for Acanthodes. The family Acanthodidae is redefined. Howittacanthus is placed as the possible sister taxon to a group containing Acanthodes and Acanthodopsis by shared characters of the jaw cartilages. A Euroamerican centre of origin is proposed for acanthodiforms, gyracanthids and diplacanthoids. These groups probably entered the East Gondwana Province following faunal interchange with the Eurameican Province sometime in the Middle Devonian.-Author
... Unlike the other structures discussed here, it did not lie within the gape, and likely reinforced the lower jaw. Mandibular splints are present in Acanthodes, Acanthodopsis, Halimacanthodes, Howittacanthus and Protogonacanthus [56,58,76,77]. They have also been incorrectly identified in a variety of other taxa. ...
The teeth of sharks famously form a series of transversely organized files with a conveyor-belt replacement that are borne directly on the jaw cartilages, in contrast to the dermal plate-borne dentition of bony fishes that undergoes site-specific replacement. A major obstacle in understanding how this system evolved is the poorly understood relationships of the earliest chondrichthyans and the profusion of morphologically and terminologically diverse bones, cartilages, splints and whorls that they possess. Here, we use tomographic methods to investigate mandibular structures in several early branching ‘acanthodian’-grade stem-chondrichthyans. We show that the dentigerous jaw bones of disparate genera of ischnacanthids are united by a common construction, being growing bones with non-shedding dentition. Mandibular splints, which support the ventro-lateral edge of the Meckel's cartilage in some taxa, are formed from dermal bone and may be an acanthodid synapomorphy. We demonstrate that the teeth of Acanthodopsis are borne directly on the mandibular cartilage and that this taxon is deeply nested within an edentulous radiation, representing an unexpected independent origin of teeth. Many or even all of the range of unusual oral structures may be apomorphic, but they should nonetheless be considered when building hypotheses of tooth and jaw evolution, both in chondrichthyans and more broadly.
... In T. affinis, it commences from the base of the caudal peduncle and, then, expands anteriorly and posteriorly (Upeniece and Chevrinais, 2014). In H. kentoni, the smallest specimens already have a completely developed squamation (Long, 1986). Thus, the development of squamation in A. lopatini ( Fig. 1) is similar to that of the majority of acanthodids and mesacanthids; however, it is much rapider, making it close to A. lundi and H. kentoni. ...
The sequence of ossification of skeletal elements in the growth series of the acanthodian Acanthodes lopatini Rohon from the Lower Tournaisian of the Minusa Trough (Siberia) is described. Four formal onto-genetic stages are recognized. It is shown that even 20-30-mm-long juveniles have well-developed fin spines, scapulocoracoids, mandibular bones, branchiostegal rays, and incipient gill rakers. At this stage, scales cover the posterior part of the body and the orbits are marked by dark "eye stains" of uncertain nature. Further ontogenetic changes are connected with the development of sclerotic ring elements, sensory line system, squamation of the head region and fin webs, perichondral ossifications of the Meckel's, quadrate, and pala-tine cartilages, and, finally, mineralization of the hyoid and gill arches. Ontogenetic features of A. lopatini are compared with those in other members of the genus and other acanthodiforms. Some developmental features shared by A. lopatini and Lodeacanthus gaujicus Upeniece suggest that acanthodids could have evolved from mesacanthids.
The Siluro-Devonian adaptive radiation of jawed vertebrates, which underpins almost all living vertebrate biodiversity, is characterized by the evolutionary innovation of the lower jaw. Multiple lines of evidence have suggested that the jaw evolved from a rostral gill arch, but when the jaw took on a feeding function remains unclear. We quantified the variety of form in the earliest jaws in the fossil record from which we generated a theoretical morphospace that we then tested for functional optimality. By drawing comparisons with the real jaw data and reconstructed jaw morphologies from phylogenetically inferred ancestors, our results show that the earliest jaw shapes were optimized for fast closure and stress resistance, inferring a predatory feeding function. Jaw shapes became less optimal for these functions during the later radiation of jawed vertebrates. Thus, the evolution of jaw morphology has continually explored previously unoccupied morphospace and accumulated disparity through time, laying the foundation for diverse feeding strategies and the success of jawed vertebrates.
The gill apparatus of gnathostomes (jawed vertebrates) is fundamental to feeding and ventilation and a focal point of classic hypotheses on the origin of jaws and paired appendages. The gill skeletons of chondrichthyans (sharks, batoids, chimaeras) have often been assumed to reflect ancestral states. However, only a handful of early chondrichthyan gill skeletons are known and palaeontological work is increasingly challenging other presupposed shark-like aspects of ancestral gnathostomes. Here we use computed tomography scanning to image the three-dimensionally preserved branchial apparatus in Ptomacanthus, a 415 million year old stem-chondrichthyan. Ptomacanthus had an osteichthyan-like compact pharynx with a bony operculum helping constrain the origin of an elongate elasmobranch-like pharynx to the chondrichthyan stem-group, rather than it representing an ancestral condition of the crown-group. A mixture of chondrichthyan-like and plesiomorphic pharyngeal pat-terning in Ptomacanthus challenges the idea that the ancestral gnathostome pharynx conformed to a morphologically complete ancestral type.
Evolution and Development of Fishes - edited by Zerina Johanson January 2019
Fish microfossils - scales, teeth, bone fragments, ornaments, spines, from Silurian to Permian of the Australian region (East Gondwanan) are described and discussed. Their applications for biostratigraphy, palaeoecology and palaeoenvironment analysis are considered.
Euryhaline adaptations in Pennsylvanian vertebrates allowed them to inhabit the marine to freshwater spectrum. This is illustrated by new assemblages of fish and tetrapods from the early Moscovian Minto Formation of New Brunswick, Canada. Fish include chondrichthyans (xenacanthids and the enigmatic Ageleodus), acanthodians (gyracanthids and acanthodiforms), sarcopterygians (rhizodontids, megalichthyids and dipnoans), and actinopterygians (eurynotiforms). Tetrapods include small- to medium-sized, and largely aquatic, stem tetrapods (colosteids) and anthracosaurs (embolomeres). A key finding is that the parautochthonous fossil assemblages are preserved across a salinity gradient, with diversity (measured by the Simpson Index) declining from open marine environments, through brackish embayments, and reaching a nadir in tidal estuaries. Chondrichthyans dominate the entire salinity spectrum (65% of fossils), a distribution that demonstrates a euryhaline mode of life, and one large predatory chondrichthyan, Orthacanthus, may have practised filial cannibalism in coastal nurseries because its heteropolar coprolites contain juvenile xenacanthid teeth. In contrast, other fish communities were more common in open marine settings while tetrapods were more common in coastal brackish waters. While all these faunas were also likely to have been euryhaline, their osmoregulation was, perhaps, less versatile. The demonstration of widespread euryhalinity among fish and aquatic tetrapods explains why Pennsylvanian faunas generally show a cosmopolitan biogeography because taxa were able to disperse via seaways. It also resolves the paradox of enriched strontium isotopic signatures observed in these faunas because organisms would have been, at times, exposed to continental water bodies as well. Therefore, our new findings contribute to the long-running debate about the ecology of Pennsylvanian fishes and tetrapods.
'Elpistostegalid' fishes were the most advanced lobe-finned tetrapodomorph fishes from the uppermost Middle-Late Devonian Period, the immediate sister taxa to the first tetrapods. They have previously been found only in the Northern Hemisphere Old Red Sandstone Continent of Euramerica, where the majority of Late Devonian (Famennian) tetrapod fossils occur. Here we report on the discovery of a new articulated partial sarcopterygian fish from the Middle Devonian of Australia, Howittichthys warrenae gen. et sp. nov., which shows a combination of features found in 'elpistostegalids', but as it is largely incomplete we cannot positively confirm this taxonomic assignment. Alternatively, it might be a Glyptopomus-like basal tetrapodomorph. The new specimen, taken into account with older (now Frasnian) age assessment of Late Devonian tetrapod material from Australia, and evidence supporting a Gondwana-Euramerican migratory event at the end of the Givetian, would lend support for an East Gondwana origin for the 'elpistostegalid'-tetrapod clade.
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