Alex Ritchie

The most primitive osteichthyan braincase

Most living vertebrates, from teleosts to tetrapods, are osteichthyans (bony fishes), but the origin of this major group is poorly understood. The actinopterygians (ray-finned bony fishes) are the most successful living vertebrates in terms of diversity. They appear in the fossil record in the Late Silurian but are poorly known before the Late Devonian. Here we report the discovery of the oldest and most primitive actinopterygian-like osteichthyan braincase known, from 400–million-year-old limestone in southeastern Australia. This specimen displays previously unknown primitive conditions, in particular, an opening for a cartilaginous eyestalk. It provides an important and unique counterpart to the similarly aged and recently described Psarolepis from China and Vietnam. The contrasting features of these specimens, and the unusual anatomy of the new specimen in particular, provide new insights into anatomical conditions close to the evolutionary radiation of all modern osteichthyan groups.

The braincase and palate of the tetrapodomorph sarcopterygian Mandageria fairfaxi: morphological variability near the fish-tetrapod transition

The braincase of the Late Devonian tristichopterid sarcopterygian Mandageria fairfaxi, from Canowindra, NSW, Australia, differs radically from the conservative pattern present in other ‘osteolepiforms’ (stem–group tetrapodomorph fishes) and non–dipnoan sarcopterygian fishes in general. The basioccipital region is short, displaced anteriorly, and either unossified or loosely articulated to the exoccipital, leaving most or all of the notochordal tunnel open ventrally. The exoccipital complex, which is developed into a large saddle that would have rested on top of the notochord, carries large, triangular articular facets on its posterior face and appears to have formed part of a functional neck joint, a synovial articulation between the skull and vertebral column that allows the former to rotate against the latter. Such a joint is characteristic of post–Devonian tetrapods, but unknown in other sarcopterygians. We infer that the ventrally open notochordal tunnel allowed gentle flexion of the cranial notochord during (predominantly vertical) rotational movement at the occiput; this is a mechanically unique solution to the problem of creating a mobile neck. Other unusual features of Mandageria include a posteriorly located lateral commissure, and structures on the entopterygoid and lateral commissure that may have been associated with an elaborate spiracular tract.

The Phyllolepid Placoderm Cowralepis mclachlani: Insights into the Evolution of Feeding Mechanisms in Jawed Vertebrates

Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215–259) (Phyllolepida, Placodermi) represent a unique ontogenetic sequence adding to our understanding of anatomy, function, and phylogeny among basal jawed vertebrates (gnathostomes). A systematic review demonstrates that the Phyllolepida are a subgroup of the Arthrodira. Consideration of visceral and neurocranial characters supports the hypothesis that placoderms are the sister group to remaining gnathostomes. Placoderms possess, as adult plesiomorphic features, a number of characters that are only seen in the development of extant gnathostomes—a peramorphic shift relative to placoderms. Developmental evidence in vertebrates leads to a revised polarity of character transitions. These include 1) hyomandibula‐neurocranium and ventral parachordal‐palatoquadrate articulations (vertebrate synapomorphies); 2) jointed pharynx, paired basibranchials, anterior ethmoidal‐palatoquadrate articulation, short trabeculae cranii, and anterior and posterior neurocranial fissures (gnathostome synapomorphies); and 3) fused basibranchials, dorsal palatoquadrate‐neurocranium articulation, loss of the anterior neurocranial fissure, elongated trabeculae cranii, and transfer of the ventral parachordal‐palatoquadrate articulation to the trabeculae (crown group gnathostomes). The level of preservation in C. mclachlani provides the basis for a reinterpretation of phyllolepid anatomy and function. Cowralepis mclachlani possesses paired basibranchials allowing the reinterpretation of the visceral skeleton in other placoderms. Mandible depression in C. mclachlani follows an osteichthyan pattern and the ventral visceral skeleton acts as a functional unit. Evidence for hypobranchial musculature demonstrates the neural crest origin of the basibranchials and that Cowralepis was a suction feeder. Finally, the position of the visceral skeleton relative to the neurocranium in placoderms parallels the condition in selachians and osteichthyans, but differs in the elongation of the occiput. The cucullaris fossa of placoderms (interpreted as a site of muscle attachment) is shown to represent, in part, the parabranchial chamber. J. Morphol., 2009.

Evolution and development of the synarcual in early vertebrates

The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occurs in vertebrate taxa such as the fossil group Placodermi and the Chondrichthyes (Holocephali, Batoidea). Although the synarcual varies morphologically in these groups, it represents the first indication, phylogenetically, of a differentiation of the vertebral column into separate regions. Among the placoderms, the synarcual of Cowralepismclachlani Ritchie, 2005 (Arthrodira) shows substantial changes during ontogeny to produce an elongate, spool-shaped structure with a well-developed dorsal keel. Because the placoderm synarcual is covered in perichondral bone, the ontogenetic history of this Cowralepis specimen is preserved as it developed anteroposteriorly, dorsally and ventrally. As well, in the placoderm Materpiscis attenboroughi Long et al., 2008 (Ptyctodontida), incomplete fusion at the posterior synarcual margin indicates that both neural and haemal arch vertebral elements are added to the synarcual. A survey of placoderm synarcuals shows that taxa such as Materpiscis and Cowralepis are particularly informative because perichondral ossification occurs prior to synarcual fusion such that individual vertebral elements can be identified. In other placoderm synarcuals (e.g. Nefudinaqalibahensis Lelièvre et al., 1995; Rhenanida), cartilaginous vertebral elements fuse prior to perichondral ossification so that individual elements are more difficult to recognize. This ontogenetic development in placoderms can be compared to synarcual development in Recent chondrichthyans; the incorporation of neural and haemal elements is more similar to the holocephalans, but differs from the batoid chondrichthyans.

A new Late Devonian acanthodian fish from the Hunter Formation near Grenfell, New South Wales

The ischnacanthid acanthodian Grenfellacanthus zerinae gen. et sp. nov. is described on the basis of two large jaw bones from the Late Devonian (late Famennian) Hunter Formation, near Grenfell, N.S.W. The new species is the youngest known ischnacanthid, and the largest ischnacanthid from Gondwana. As for many ischnacanthids, the structure of the jaws and teeth indicate that Grenfellacanthus was probably an ambush predator.

Ichnofacies of the Stairway Sandstone fish-fossil beds (Middle Ordovician, Northern Territory, Australia)

The Stairway Sandstone is a 30–560 m thick succession of Middle Ordovician siliciclastic sedimentary rocks within the Amadeus Basin of central Australia, deposited in the epeiric Larapintine Sea of northern peri-Gondwana. The Stairway Sandstone is significant as one of only two known Gondwanan successions to yield articulated arandaspid (pteraspidomorph agnathan) fish. Herein we use the ichnology of the Stairway Sandstone to reveal insights into the shallow marine habitat of these early vertebrates, and compare it with that of other known pteraspidomorph-bearing localities from across Gondwana. The Stairway Sandstone contains a diverse Ordovician ichnofauna including 22 ichnotaxa of Arenicolites, Arthrophycus, Asterosoma, Cruziana, Didymaulichnus, Diplichnites, Diplocraterion, ?Gordia, Lockeia, Monocraterion, Monomorphichnus, Phycodes, Planolites, Rusophycus, Skolithos and Uchirites. These ichnofauna provide a well-preserved example of a typical Ordovician epeiric sea assemblage, recording the diverse ethologies of tracemakers in a very shallow marine environment of flashy sediment accumulation and regularly shifting sandy substrates. New conodont data refine the age of the Stairway Sandstone to the early Darriwilian, with ichnostratigraphic implications for the Cruziana rugosa group and Arthrophycus alleghaniensis.

Fusion, gene misexpression and homeotic transformations in vertebral development of the gnathostome stem group (Placodermi)

Development of the vertebral column is controlled by a complex of pleiotropic and polygenetic phenomena, in the mouse and chick regulating formation of different parts of individual vertebrae and morphological identity along the column (Hox code). In mouse and chick, experimental misexpression, including upstream and downstream genes, results in shifts in vertebral identity, loss of particular parts of individual vertebrae or vertebral fusion. Axial skeleton homologies across the Vertebrata allow these observations to be extended to taxa such as Homo sapiens, Chondrichthyes and Placodermi, the latter an entirely fossil group. Misexpression phenotypes among fossil taxa illuminate the phylogenetic history of these regulatory mechanisms. Phenotypes associated with genes originating via genomic duplication can determine the historical depth for these duplication events. Analysis of an ontogenetic sequence for the occipital-synarcual complex in the placoderm Cowralepis mclachlani provides the basis for comparison of this early gnathostome with other placoderms, chondrichthyans and amniotes. The occipital-synarcual patterns in placoderms parallel the phenotypic misexpression in mice and chicks (fusion and homeotic mutation) and the varying degrees of fusion in the Type I-III human Klippel-Feil syndrome. The association of these phenotypes to Hoxd regulatory complexes indicates that the gnathostome genomic duplication occurred at the base of the gnathostome stem group. Given the conservative nature of regulatory genes and the homology of vertebral elements, the presence of fusion in stem gnathostomes implies that the mechanism of fusion in mouse and chick models can be extrapolated to extant chondrichthyans (testable) and accounts for the phenotypic similarity across gnathostomes. The presence of these phenotypes in fossils indicates the antiquity of these regulatory mechanisms and of genomic duplication.