A Silurian placoderm with osteichthyan-like marginal jaw bones

@article{Zhu2013ASP,
  title={A Silurian placoderm with osteichthyan-like marginal jaw bones},
  author={Min Zhu and Xiaobo Yu and Per Erik Ahlberg and Brian Choo and Jing Lu and Tuo Qiao and Qingming Qu and Wenjin Zhao and Liantao Jia and Henning Blom and You-an Zhu},
  journal={Nature},
  year={2013},
  volume={502},
  pages={188-193}
}
The gnathostome (jawed vertebrate) crown group comprises two extant clades with contrasting character complements. [...] Key Result A phylogenetic analysis places the new form near the top of the gnathostome stem group but does not fully resolve its relationships to other placoderms. The analysis also assigns all acanthodians to the chondrichthyan stem group. These results suggest that the last common ancestor of Chondrichthyes and Osteichthyes had a macromeric dermal skeleton, and provide a new framework for…Expand
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A second Silurian maxillate placoderm is described that more securely bridges the jawless toothlike plates of placoderms to the development of the jawed condition that ultimately led to the three-boned jaw in ancestors of modern vertebrates.
Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia
TLDR
The discovery of extensive endochondral bone in Minjinia turgenensis, a new genus and species of ‘placoderm’-like fish from the Early Devonian of western Mongolia described using x-ray computed microtomography, provides direct support for theories of generalised bone loss in chondrichthyans.
Endochondral bone in an Early Devonian 'placoderm' from Mongolia.
TLDR
The discovery of extensive endochondral bone in Minjinia turgenensis, a new genus and species of 'placoderm'-like fish from the Early Devonian of western Mongolia described using X-ray computed microtomography, provides direct support for theories of generalized bone loss in chondrichthyans.
Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome
TLDR
A conjunction of well-developed cranial processes in Janusiscus helps unify the comparative anatomy of early jawed vertebrate neurocrania, clarifying primary homologies in ‘placoderms’, osteichthyans and chondrichthyans.
An early chondrichthyan and the evolutionary assembly of a shark body plan
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Preliminary phylogenetic results highlight the likely convergent evolution of conventional chondrichthyan conditions among earliest members of this primary gnathostome division, while skeletal morphology points towards the likely suspension feeding habits of Gladbachus, suggesting a functional origin of the gill slit condition characteristic of the vast majority of living and fossil chondRichthyans.
A Palaeozoic shark with osteichthyan-like branchial arches
TLDR
A 325 million year (Myr)-old Palaeozoic shark-like fossil is described that represents the earliest identified chondrichthyan in which the complete gill skeleton is three-dimensionally preserved in its natural position, and its visceral arch arrangement is remarkably osteichthyan-like, suggesting that this may represent the common ancestral condition for crown gnathostomes.
Morphology and histology of acanthodian fin spines from the late Silurian Ramsasa E locality, Skane, Sweden
TLDR
The results support the current hypothesis of gnathostome phylogeny, which places "acanthodians" on the chondrichthyan stem and emphasize the need for further study of vertebrate fin spines using 3D approaches.
Endocast and bony labyrinth of a stem gnathostome shed light on the earliest diversification of jawed vertebrates
TLDR
The endocranial cavity and inner ear of Brindabellaspis stensioi is described, commonly recovered as a taxon of uncertain affinity branching near the base of ‘placoderms’, and its results call into question the appropriateness of fusiform ’placoderm’ as models of primitive gnathostome anatomy and raise questions of homology relating to key cranial features.
Acanthodian dental development and the origin of gnathostome dentitions.
TLDR
It is shown that ischnacanthid marginal dentitions were statodont, composed of multicuspidate teeth added in distally diverging rows and through proximal superpositional replacement, while their symphyseal tooth whorls are comparable to chondrichthyan and osteichthyan counterparts.
Endocast and Bony Labyrinth of a Devonian “Placoderm” Challenges Stem Gnathostome Phylogeny
TLDR
The results call into question the appropriateness of arthrodire-like placoderms as models of primitive gnathostome anatomy and raise questions of homology relating to key cranial features.
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