Per Ahlberg

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.

Lower jaws, lower tetrapods–a review based on the Devonian genus Acanthostega

The lower jaw of the Devonian tetrapod Acanthostega is described for the first time. Redescriptions are provided for the lower jaws of the elpistostegid Panderichthys , the Devonian tetrapods Elginerpeton, Obruchevichthys, Metaxygnathus, Ventastega and Ichthyostega , and the Carboniferous tetrapods Crassigyrinus, Megalocephalus and Gephyrostegus . The character distri- butions thus revealed differ considerably from previous accounts, particularly in the wide distribution of certain primitive characters. Meckelian ossification in the middle part of the jaw is widespread among Devonian tetrapods, being demonstrably absent only in Acanthostega. Among Carboniferous tetrapods, a tooth-bearing parasymphysial plate is shown to be present in Crassigyrinus and Megalocephalus (having already been demonstrated by other authors in Whatcheeria and Greererpeton). A phylogenetic analysis of 26 early tetrapods including all the aforementioned genera, scored for 51 lower jaw characters, produces at least 2,500 equally parsimonious trees. However, the lack of resolution lies largely in a big top end polychotomy containing anthracosaurs, temnospondyls, seymouriamorphs, microsaurs and a nectridean-amniote clade. Below this polycho- tomy, which may correspond approximately to the tetrapod crown group, there is a well-resolved stem-group containing, in descending order, Megalocephalus, Greererpeton, Crassigyrinus , (jaws associated with) Tulerpeton, Whatcheeria, Acanthostega, Metaxygnathus, Ichthyostega, Ventastega and Metaxygnathus (unresolved), an Elginerpeton-Obruchevichthys clade, and Panderichthys. This conflicts with recently published phylogenies by Coates and Lebedev & Coates, which place Tulerpeton and all post-Devonian tetrapods in the amphibian or amniote branches of the tetrapod crown group.

A primitive sarcopterygian fish with an eyestalk

The discovery of two Early Devonian osteichthyan (bony fish) fossils has challenged established ideas about the origin of osteichthyans and their divergence into actinopterygians (teleosts and their relatives) and sarcopterygians (tetrapods, coelacanths, lungfishes and related groups). Psarolepis from China and an unnamed braincase from Australia combine derived sarcopterygian and actinopterygian characters with primitive features previously restricted to non-osteichthyans, suggesting that early osteichthyan evolution may have involved substantial parallellism between sarcopterygians and actinopterygians. But interpretation of these fossils has been hampered by poor phylogenetic resolution. Here we describe a basal sarcopterygian fish,xa0Achoania gen, et sp. nov., that fills the morphological gap between Psarolepis and higher sarcoptergyians. We also report the presence of eyestalk attachments in both Achoania and Psarolepis, showing that this supposedly non-osteichthyan feature occurs inxa0basal sarcopterygians as well as the actinoptergyian-like Australian braincase.

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.

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...

The late Devonian lungfish Soederberghia (Sarcopterygii, Dipnoi) from Australia and North America, and its biogeographical implications

Abstract A new species of rhynchodipterid lungfish, Soederberghia simpsoni, sp. nov., is described on the basis of a complete skull roof, and an incomplete but articulated head plus body, from the Upper Devonian Mandagery Sandstone near Canowindra, New South Wales (NSW, Fig. 1A,), Australia. It is compared with Soederberghia material from the upper part of the Catskill Formation, Pennsylvania (Fig. 1B), and the Cloghnan Shale at Jemalong near Forbes, NSW, Australia. The Catskill and Cloghnan Shale material, consisting of two incomplete skull roofs, appears to be conspecific with the type species, Soederberghia groenlandica from the Famennian Remigolepis Series of Greenland. This is congruent with a recent suggestion that the Mandagery Sandstone is late Frasnian in age, whereas the upper Catskill Formation and Cloghnan Shale are Famennian. At the Famennian localities, Soederberghia is associated with tetrapods. Articulated postcranial material of Soederberghia, including the pectoral girdle, cranial and pleural ribs, unpaired fins and fin supports, is described for the first time. The presence of cranial ribs and a parasphenoid with a posterior stalk suggest that Soederberghia gulped air and probably inhabited a non-marine or shallow near-shore marine environment. The occurrence of Soederberghia groenlandica in the Famennian Old Red Sandstone of North America, Greenland and Australia thus furnishes evidence for contact or close proximity between Gondwana and Laurussia at this time, in conflict with some recent paleomagnetic data. The sister group of Soederberghia is Rhynchodipterus elginensis from the Famennian of Rosebrae near Elgin, Scotland; together they form the Family Rhynchodipteridae. Griphognathus, previously included in the Rhynchodipteridae, lacks a stalked parasphenoid and cranial ribs, and has a quite different dermal bone pattern from Soederberghia and Rhynchodipterus. We remove it from the group on these grounds, and interpret it as less crownward than the Rhynchodipteridae.

A NEW COELACANTH FROM THE MIDDLE DEVONIAN OF LATVIA

Abstract We describe a new species of coelacanth, Miguashaia grossi sp. nov., recognized from isolated cranial, shoulder girdle bones and scales from the Upper Givetian Gauja and Lode Formations of two Latvian localities. This coelacanth, together with comparisons with other Devonian coelacanths, allows a more precise description of the plesiomorphic condition of the lower jaw, shoulder girdle, and palate. These plesiomorphic conditions are as follows. The dentary is shallow throughout, the articular-retroarticular is deep posteriorly where it is developed as two processes—one for the symplectic articulation and the other demarcating the posteroventral angle of the jaw. In the shoulder girdle plesiomorphic attributes include the large triangular clavicle, a cleithrum which is directed anterodorsally and possibly a large and polygonal extracleithrum. In the palate the pterygoid shows a dorsal process of unknown function mid-way along the dorsal margin and this is also a plesiomorphic feature of coelacanths.

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.

New light on the earliest known tetrapod jaw

The Late Devonian (late Frasnian) vertebrate material from Scat (or Scaat) Craig near Elgin, Scotland, includes the earliest known tetrapod limb bones and limb girdle fragments (Ahlberg, 1991, 1998). (The term “tetrapod” is used throughout in the sense of “vertebrate with limbs”. “Stem tetrapod” implies a tetrapod that does not belong to the crown group.) Associated skull bones and lower jaw fragments, comprising the premaxilla plus median rostral and all of the lower jaw except the articular and mesial wall of the adductor fossa, form the type material of the stem tetrapod Elginerpeton pancheni Ahlberg (Ahlberg, 1995; Ahlberg and Clack, 1998). A probable tetrapod postorbital (Ahlberg, 1998) is also provisionally referred to this taxon although it lacks diagnostic features at the generic level. The postcranial tetrapod material from Scat Craig comprises the dorsal part of the scapulocoracoid plus the ventral part of the cleithrum (four specimens), the ilium (two specimens), and one specimen each of the humerus, femur, tibia and neural arch (Ahlberg, 1998). The identification of the humerus has recently been challenged (Shubin et al., 2004; see Ahlberg, 2004 for a contrasting view). The four shoulder-girdle fragments are very similar, as are the two ilia, suggesting that each set represents a single tetrapod taxon; for this reason, and because of the good size match between the jaws and postcranial elements, the postcranial material is provisionally referred to Elginerpeton (Ahlberg, 1998). The Scat Craig assemblage not only represents the earliest known remains of tetrapods (in the sense of tetrapodomorphs with limbs rather than paired fins; see Ahlberg and Clack, 1998:11) but is actually one of the largest suites of tetrapod material from the Devonian. Only the Famennian Ichthyostega, Acanthostega, and Ventastega are known in significantly more detail. By contrast, the late Frasnian Obruchevichthys from Latvia (and possibly Russia), which is contemporary with Elginerpeton and may be closely related to it (Ahlberg, 1995), is known only from two small jaw fragments (Ahlberg and Clack, 1998). The Scat Craig material is thus very important for our understanding of early tetrapod evolution. Although Elginerpeton was described as recently as 1995, most of the Scat Craig tetrapod material was collected during the 19 century (Andrews, 1982) and found its way, unrecognised, into the Devonian fish collections of several national and local museums in Britain. Most of these specimens have been gathered up and described by Ahlberg in a series of papers (Ahlberg, 1991, 1995, 1998; Ahlberg and Clack, 1998). However, on a recent visit to the National Museums of Scotland, Edinburgh, two of us (Friedman and Blom) discovered an overlooked specimen, NMS G 1861.46.67, in the Scat Craig lungfish material. This specimen comprises the posterior end of the lower jaw, including most of the surangular and part of the articular, and thus fills one of the last major gaps in our knowledge of the mandible of Elginerpeton. Institutional Abbreviations—ANSP, Academy of Natural Sciences, Philadelphia, PA, USA; BMNH, The Natural History Museum, London, UK; LEICS, New Walk Museum & Art Gallery, Leicester, UK; MGUH, Geological Museum of the University of Copenhagen, Denmark; NMS, National Museum of Scotland, Edinburgh, UK. DESCRIPTION