Acanthodians are long-extinct fossil fish that stand close to the divergence of cartilaginous and bony fishes. Their morphology has the potential to reveal much about the early evolution of jawed vertebrates. The problem is that their anatomy is poorly known, most of it confined to Acanthodes, a probably unrepresentative genus that lived in the Permian, late in acanthodian history. Martin Brazeau now presents the first detailed description of the braincase of Ptomacanthus, an acanthodian that lived in the Early Devonian acanthodian heyday, about 415 million years ago. The results show that acanthodians were probably not a natural group: Ptomacanthus was either a very early relative of sharks, or close to the common ancestry of all modern jawed vertebrates. This paper presents the first detailed description of the braincase of Ptomacanthus, an acanthodian that lived in the Early Devonian. The results show that acanthodians were probably not a natural group: Ptomacanthus was either a very early relative of sharks, or close to the common ancestry of all modern jawed vertebrates. Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era1, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic ‘acanthodians’ are possibly the earliest2,3 gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution1. 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 anglicus4, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes5, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes5 in exhibiting several plesiomorphic features shared with placoderms6,7 and some early chondrichthyans8,9. 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.

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Devonian sharks from south-eastern Australia and Antarctica

The braincase and jaws of a Devonian “acanthodian” and modern gnathostome origins

Chapter 2 – Phylogenetic Interrelationships of Neoselachians (Chondrichthyes: Euselachii)

Chondrichthyans (including living sharks, skates, rays and chimaeras) have a fossil record of scales and dermal denticles perhaps dating back to the Late Ordovician period, about 455 million years ago. Their fossil tooth record extends to the earliest Devonian period, almost 418 million years ago, whereas the oldest known articulated shark remains date from the Early Devonian period, about 394 million years ago. Here we report the discovery of an articulated shark that is almost 409 million years old from the Early Devonian (early Emsian) period of New Brunswick, Canada. The specimen, identified as Doliodus problematicus (Woodward), sheds light on the earliest chondrichthyans and their interrelationships with basal jawed vertebrates. This species has been truly problematic. Previously known only from isolated teeth, it has been identified as an acanthodian and a chondrichthyan. This specimen is the oldest shark showing the tooth families in situ, and preserves one of the oldest chondrichthyan braincases. More notably, it shows the presence of paired pectoral fin-spines, previously unknown in cartilaginous fishes.

The oldest articulated chondrichthyan from the Early Devonian period

Ventilation and the origin of jawed vertebrates: a new mouth

Chondrichthyans (sharks, rays and holocephalans) are subjected to cladistic analysis in order to identify possible monophyletic groups. Chondrichthyan monophyly is established on the basis of several apomorphic characters, of which the most convincing is the presence of a mineralized layer of prismatic perichondral tissue of a unique type. Modern elasmobranchs are united with the Jurassic Palaeospinax by several synapomorphies. Palaeospinax and some other fossil sharks are sequenced as successive sister-groups to modern elasmobranchs. It is concluded that elasmobranchs and chimaeroids are monophyletic sister-groups, but that sharks are not monophyletic unless holocephalans are included.

Devonian sharks from south-eastern Australia and Antarctica

Citations

The braincase and jaws of a Devonian “acanthodian” and modern gnathostome origins

Chapter 2 – Phylogenetic Interrelationships of Neoselachians (Chondrichthyes: Euselachii)

The oldest articulated chondrichthyan from the Early Devonian period

Ventilation and the origin of jawed vertebrates: a new mouth

Chondrichthyan phylogeny: a look at the evidence

Related Papers (5)

The oldest articulated chondrichthyan from the Early Devonian period

The braincase and jaws of a Devonian “acanthodian” and modern gnathostome origins

Chondrichthyan phylogeny: a look at the evidence

The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia.

II - The Acanthodian fishes