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The braincase and jaws of a Devonian “acanthodian” and modern gnathostome origins
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Physiology and Developmental Biology, Evolutionary Organism Biology.
2009 (English)In: Nature, ISSN 0028-0836, Vol. 457, no 7227, 305-308 p.Article in journal (Refereed) Published
Abstract [en]

Modern gnathostomes ( jawed vertebrates) emerged in the early Palaeozoic era(1), but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic 'acanthodians' are possibly the earliest(2,3) gnathostome group and exhibit amosaic of shark- and bony fish- like characters that has long given them prominence in discussions of early gnathostome evolution(1). 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(4), and re- evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes(5), 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(5) in exhibiting several plesiomorphic features shared with placoderms(6,7) and some early chondrichthyans(8,9). 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.

Place, publisher, year, edition, pages
2009. Vol. 457, no 7227, 305-308 p.
National Category
Biological Sciences
URN: urn:nbn:se:uu:diva-97742DOI: 10.1038/nature07436ISI: 000262440900037OAI: oai:DiVA.org:uu-97742DiVA: diva2:172795
Available from: 2008-10-29 Created: 2008-10-29 Last updated: 2010-08-04Bibliographically approved
In thesis
1. Endocranial Morphology and Phylogeny of Palaeozoic Gnathostomes (Jawed Vertebrates)
Open this publication in new window or tab >>Endocranial Morphology and Phylogeny of Palaeozoic Gnathostomes (Jawed Vertebrates)
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gnathostomes, or jawed vertebrates, make up the overwhelming majority of modern vertebrate diversity. Among living vertebrates, they comprise the chondrichthyans (“cartilaginous fishes” such as sharks, skates, rays, chimaeras) and the osteichthyans (“bony fishes” or bony vertebrates, inclusive of tetrapods). Gnathostomes appear to have originated in the early Palaeozoic Era, but their early fossil record is fairly scant. The best fossils appear first in the Late Silurian and Devonian periods. Much of gnathostome diversity owes to unique adaptations in the internal skeleton of their head (the endocranium). The endocranium is composed of the braincase, jaws, hyoid arch, and branchial arches, which sometimes fossilise when they are composed of bone or calcified cartilage.

The purpose of this thesis is to describe and compare the fossilised cranial endoskeletons of a variety of different Palaeozoic gnathostomes. The objective is to test current conceptions of gnathostome interrelationships (i.e. phylogeny) and infer aspects of key morphological transformations that took place during the evolution of Palaeozoic members of this group. Two key areas are examined: the morphology and interrelationships of Palaeozoic gnathostomes and the morphology of the visceral arches in sarcopterygian fishes.

New data on the visceral arches are described from the stem tetrapods Panderichthys and rhizodontids. These provide insight into the sequence of character acquisition leading to the tetrapod middle ear. Panderichthys shows key features of the tetrapod middle ear chamber were established prior to the origin fo digited limbs. New morphological data are described from the “acanthodian” fish Ptomacanthus. Ptomacanthus provides only the second example of a well-preserved braincase from any member of this group. It shows dramatic differences from that of its counterpart, Acanthodes, providing new evidence for acanthodian paraphyly. New interpretations of basal gnathostome and osteichthyan phylogeny are presented, challenging or enriching existing views of these problems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 50 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 570
systematics, palaeontology, anatomy, evolution
National Category
urn:nbn:se:uu:diva-9360 (URN)978-91-554-7330-3 (ISBN)
Public defence
2008-11-21, Lindahlsalen, Evolutionary Biology Centre, Norbyvägen 18A, Uppsala, Sweden, 09:00 (English)
Available from: 2008-10-29 Created: 2008-10-29 Last updated: 2009-04-03Bibliographically approved

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