Zerina Johanson

Osteolepiforms and the ancestry of tetrapods

Fossil discoveries and improved phylogenies, have greatly improved our understanding of the origin of tetrapods, making it possible to reconstruct sequences of character change leading to tetrapod morphologies, and to tentatively identify the genetic basis for some of these changes,. However, progress has centred on the upper part of the Tetrapodomorpha which is occupied by Devonian tetrapods such as Acanthostega,, and Ichthyostega. Few advances have been made in improving our understanding of the lower, ‘fish’ part of the group, beyond establishing Elpistostegalia, Osteolepiformes and Rhizodontida as progressively more primitive constituents. It has not been convincingly confirmed or disproved that the Osteolepiformes, a diverse but structurally uniform group that is central to the debate about tetrapod origins, is monophyletic relative to tetrapods (that is, a single side branch on the tetrapod lineage). The earliest steps of the fish–tetrapod transition have thus remained poorly resolved. Here we present the first detailed analysis of the lower part of the Tetrapodomorpha, based on 99 characters scored for 29 taxa. We show that both the Osteolepiformes as a whole and their constituent group Osteolepididae are paraphyletic to tetrapods (that is, each comprises a section of the tetrapod lineage with several side branches), and that their ‘uniting characters’ are attributes of the tetrapodomorph stem lineage. The supposedly discredited idea of osteolepiforms as tetrapod ancestors is, in effect, supported by our analysis. Tetrapod-like character complexes evolved three times in parallel within the Tetrapodomorpha.

Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms

The fossil group Placodermi is the most phylogenetically basal of the clade of jawed vertebrates but lacks a marginal dentition comparable to that of the dentate Chondrichthyes, Acanthodii and Osteichthyes (crown‐group Gnathostomata). The teeth of crown‐group gnathostomes are part of an ordered dentition replaced from, and patterned by, a dental lamina, exemplified by the elasmobranch model. A dentition recognised by these criteria has been previously judged absent in placoderms, based on structural evidence such as absence of tooth whorls and typical vertebrate dentine.

Copulation in antiarch placoderms and the origin of gnathostome internal fertilization

Reproduction in jawed vertebrates (gnathostomes) involves either external or internal fertilization. It is commonly argued that internal fertilization can evolve from external, but not the reverse. Male copulatory claspers are present in certain placoderms, fossil jawed vertebrates retrieved as a paraphyletic segment of the gnathostome stem group in recent studies. This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups. The reproductive biology of antiarchs, consistently identified as the least crownward placoderms and thus of great interest in this context, has until now remained unknown. Here we show that certain antiarchs possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation. These structures are not associated with pelvic fins. The clasper morphology resembles that of ptyctodonts, a more crownward placoderm group, suggesting that all placoderm claspers are homologous and that internal fertilization characterized all placoderms. This implies that external fertilization and spawning, which characterize most extant aquatic gnathostomes, must be derived from internal fertilization, even though this transformation has been thought implausible. Alternatively, the substantial morphological evidence for placoderm paraphyly must be rejected.

Devonian arthrodire embryos and the origin of internal fertilization in vertebrates

Evidence of reproductive biology is extremely rare in the fossil record. Recently the first known embryos were discovered within the Placodermi, an extinct class of armoured fish, indicating a viviparous mode of reproduction in a vertebrate group outside the crown-group Gnathostomata (Chondrichthyes and Osteichthyes). These embryos were found in ptyctodontids, a small group of placoderms phylogenetically basal to the largest group, the Arthrodira. Here we report the discovery of embryos in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia (approximately 380 million years ago), providing the first evidence, to our knowledge, for reproduction using internal fertilization in this diverse group. We show that Incisoscutum and some phyllolepid arthrodires possessed pelvic girdles with long basipterygia that articulated distally with an additional cartilaginous element or series, as in chondrichthyans, indicating that the pelvic fin was used in copulation. As homology between similar pelvic girdle skeletal structures in ptyctodontids, arthrodires and chondrichthyans is difficult to reconcile in the light of current phylogenies of lower gnathostomes, we explain these similarities as being most likely due to convergence (homoplasy). These new finds confirm that reproduction by internal fertilization and viviparity was much more widespread in the earliest gnathostomes than had been previously appreciated.

A complete primitive rhizodont from Australia

Studies of the origin and developmental genetics of tetrapod limbs have focused attention on the need to identify the precise type of sarcopterygian (lobe-finned fish) fin from which limbs evolved. This can only be achieved through a phylogenetic analysis of sarcopterygians. Sarcopterygian fin skeletons vary in structure,; use of an inappropriate fin skeleton as a model limb precursor will lead to erroneous inferences about the evolution of morphology and the developmental pathways at the fish–tetrapod transition. The pectoral fin of the rhizodont sarcopterygian Sauripteris is strikingly limb-like and features prominently in discussions about the origin of limbs,,. It is thus important to establish the phylogenetic position of rhizodonts. However, their anatomy is incompletely known,. Published phylogenetic analyses are based on poorly substantiated characters, such as the alleged presence of two external nostrils in the Australian genus Barameda,. Here we present, from the Upper Devonian period of Canowindra, Australia, the most primitive and by far the most complete rhizodont discovered so far. It has a single external nostril but possesses no other derived tetrapod-like features. Our new evidence shows that rhizodonts are more remote from tetrapods than are osteolepiform and elpistostegid lobe-fin fishes. Similarities between rhizodont fins and tetrapod limbs are thus probably convergent, and the pectoral fin of Sauripteris should not be used as a model limb precursor.

Developmental plasticity and disparity in early dipnoan (lungfish) dentitions.

SUMMARY Although the lungfish (Dipnoi) belong within the Osteichthyes, their dentitions are radically different from other osteichthyans. Lungfish dentitions also show a uniquely high structural disparity during the early evolution of the group, partly owing to the independent variation of odontogenic and odontoclastic processes that are tightly and stereotypically coordinated in other osteichthyans. We present a phylogenetic analysis of early lungfishes incorporating a novel approach to coding these process characters in preference to the resultant adult dental morphology. The results only partially resolve the interrelationships of Devonian dipnoans, but show that the widely discussed hypothesis of separate tooth‐plated, dentine‐plated, and denticulated lineages is unlikely to be true. The dipnoan status of Diabolepis is corroborated. Lungfish dentitions seem to have undergone extensive and nonparsimonious evolution during the early history of the group, but much of the resulting disparity can be explained by a modest number of evolutionary steps in the underlying developmental processes, those for dental formation (odontogenic) and those for the remodeling of dentine tissue (odontoclastic). Later in lungfish evolution, this disparity was lost as the group settled to a pattern of dental development that is just as stereotypic as, but completely different from, that of other osteichthyans.

Devonian rhizodontids and tristichopterids (Sarcopterygii; Tetrapodomorpha) from East Gondwana.

Devonian rhizodontid fishes from East Gondwana include Gooloogongia loomesi Johanson & Ahlberg 1998 from near Canowindra, New South Wales (NSW), Australia and Aztecia mahalae gen. nov., sp. nov. from southern Victoria Land, Antarctica. Gooloogongia loomesi is known from well-preserved cranial and postcranial material, and as such represents the most complete rhizodont known. Newly discovered elements (braincase, pelvic girdle) are described for the first time. Aztecia mahalae gen. nov., sp. nov. is based on shoulder girdle material formerly assigned to Notorhizodon Young et al. 1992, which, based on skull and lower jaw morphology, is a member of the Tristichopteridae. The presence of several plesiomorphic characters suggests that Gooloogongia occupies a basal phylogenetic position within the Rhizodontida, while Aztecia n. gen. possesses a more derived shoulder girdle. We argue that the Rhizodontida evolved on the Gondwanan landmass. Notorhizodon occupies a relatively derived position within the Tristichopteridae, but is contemporary with the earliest and phylogenetically most basal Laurussian members of the group. This shows that the tristichopterids achieved a worldwide distribution considerably earlier than previously thought.

Development of teeth and jaws in the earliest jawed vertebrates

Teeth and jaws constitute a model of the evolutionary developmental biology concept of modularity and they have been considered the key innovations underpinning a classic example of adaptive radiation. However, their evolutionary origins are much debated. Placoderms comprise an extinct sister clade or grade to the clade containing chondrichthyans and osteichthyans, and although they clearly possess jaws, previous studies have suggested that they lack teeth, that they possess convergently evolved tooth-like structures or that they possess true teeth. Here we use synchrotron radiation X-ray tomographic microscopy (SRXTM) of a developmental series of Compagopiscis croucheri (Arthrodira) to show that placoderm jaws are composed of distinct cartilages and gnathal ossifications in both jaws, and a dermal element in the lower jaw. The gnathal ossification is a composite of distinct teeth that developed in succession, polarized along three distinct vectors, comparable to tooth families. The teeth are composed of dentine and bone, and show a distinct pulp cavity that is infilled centripetally as development proceeds. This pattern is repeated in other placoderms, but differs from the structure and development of tooth-like structures in the postbranchial lamina and dermal skeleton of Compagopiscis and other placoderms. We interpret this evidence to indicate that Compagopiscis and other arthrodires possessed teeth, but that tooth and jaw development was not developmentally or structurally integrated in placoderms. Teeth did not evolve convergently among the extant and extinct classes of early jawed vertebrates but, rather, successional teeth evolved within the gnathostome stem-lineage soon after the origin of jaws. The chimaeric developmental origin of this model of modularity reflects the distinct evolutionary origins of teeth and of component elements of the jaws.

A new tristichopterid (Osteolepiformes: Sarcopterygii) from the Mandagery Sandstone (Late Devonian, Famennian) near Canowindra, NSW, Australia

New trischopterygid (Osteoplepiformes: Sarcopterygii) from the Mandagery Sandstone (Famennian) near Canowindra, N.S.W. Australia

Second tristichopterid (Sarcopterygii, Osteolepiformes) from the Upper Devonian of Canowindra, New South Wales, Australia, and phylogeny of the Tristichopteridae

ABSTRACT A new tristichopterid, Cabonnichthys burnsi gen. et sp. nov. from the Upper Devonian Mandagery Sandstone near Canowindra, New South Wales, Australia, is the second tristichopterid to be described from this locality. The first, Mandageria fairfaxi, was interpreted as the sister group of the Laurussian taxon Eusthenodon. Cabonnichthys burnsi gen. et sp. nov. is similar to these two taxa in possessing a posterior supraorbital extending ventrally behind the orbit to exclude the postorbital from the orbital margin, a posteriorly positioned pineal surrounded by kite or teardrop shaped bones, and an enlarged fang-like tooth at the anterior end of the premaxillary tooth row. Cabonnichthys burnsi differs from Mandageria and Eusthenodon in retaining a contact between the intertemporal and posterior supraorbital, and in possessing “horn-like” processes on the postparietal shield that notch into the posterior margin of the postorbital. The Tristichopteridae is a clade characterized inter alia by the possessi...