Rhizodont crossopterygian fish from the Dinantian of Foulden, Berwickshire, Scotland, with a re-evaluation of this group

Evolution of median fin patterning and modularity in living and fossil osteichthyans

Morphological and developmental similarities, and interactions among developing structures are interpreted as evidences of modularity. Such similarities exist between the dorsal and anal fins of living actinopterygians, on the anteroposterior axis: (1) both fins differentiate in the same direction [dorsal and anal fin patterning module (DAFPM)], and (2) radials and lepidotrichia differentiate in the same direction [endoskeleton and exoskeleton module (EEM)]. To infer the evolution of these common developmental patternings among osteichthyans, we address (1) the complete description and quantification of the DAFPM and EEM in a living actinopterygian (the rainbow trout Oncorhynchus mykiss) and (2) the presence of these modules in fossil osteichthyans (coelacanths, lungfishes, porolepiforms and 'osteolepiforms'). In Oncorhynchus, sequences of skeletal elements are determined based on (1) apparition (radials and lepidotrichia), (2) chondrification (radials), (3) ossification (radials and lepidotrichia), and (4) segmentation plus bifurcation (lepidotrichia). Correlations are then explored between sequences. In fossil osteichthyans, sequences are determined based on (1) ossification (radials and lepidotrichia), (2) segmentation, and (3) bifurcation of lepidotrichia. Segmentation and bifurcation patterns were found crucial for comparisons between extant and extinct osteichthyan taxa. Our data suggest that the EEM is plesiomorphic at least for actinopterygians, and the DAFPM is plesiomorphic for osteichthyans, with homoplastic dissociation. Finally, recurrent patterns suggest the presence of a Lepidotrichia Patterning Module (LPM).

Fin ray patterns at the fin-to-limb transition

The fin-to-limb transition was marked by the origin of digits and the loss of dermal fin rays. Paleontological research into this transformation has focused on the evolution of the endoskeleton, with little attention paid to fin ray structure and function. To address this knowledge gap, we study the dermal rays of the pectoral fins of 3 key tetrapodomorph taxa-Sauripterus taylori (Rhizodontida), Eusthenopteron foordi (Tristichopteridae), and Tiktaalik roseae (Elpistostegalia)-using computed tomography. These data show several trends in the lineage leading to digited forms, including the consolidation of fin rays (e.g., reduced segmentation and branching), reduction of the fin web, and unexpectedly, the evolution of asymmetry between dorsal and ventral hemitrichia. In Eusthenopteron, dorsal rays cover the preaxial endoskeleton slightly more than ventral rays. In Tiktaalik, dorsal rays fully cover the third and fourth mesomeres, while ventral rays are restricted distal to these elements, suggesting the presence of ventralized musculature at the fin tip analogous to a fleshy "palm." Asymmetry is also observed in cross-sectional areas of dorsal and ventral rays. Eusthenopteron dorsal rays are slightly larger than ventral rays; by contrast, Tiktaalik dorsal rays can be several times larger than ventral rays, and degree of asymmetry appears to be greater at larger sizes. Analysis of extant osteichthyans suggests that cross-sectional asymmetry in the dermal rays of paired fins is plesiomorphic to crown group osteichthyans. The evolution of dermal rays in crownward stem tetrapods reflects adaptation for a fin-supported elevated posture and resistance to substrate-based loading prior to the origin of digits.

A Tournaisian (earliest Carboniferous) conglomerate-preserved non-marine faunal assemblage and its environmental and sedimentological context

A conglomerate bed from the Tournaisian Ballagan Formation of Scotland preserves a rich array of vertebrate and other nonmarine fossils providing an insight into the wider ecosystem and paleoenvironment that existed during this pivotal stage of Earth history. It challenges hypotheses of a long-lasting post-extinction trough following the end-Devonian extinction event. The fauna recovered includes a wide size range of tetrapods, rhizodonts, and dipnoans, from tiny juveniles or small-bodied taxa up to large adults, and more than one taxon of each group is likely. Some fauna, such as actinopterygians and chondrichthyans, are rare as macrofauna but are better represented in the microfossil assemblage. The fauna provides evidence of the largest Carboniferous lungfish ever found. The specimens are preserved in a localized, poorly-sorted conglomerate which was deposited in the deepest part of a river channel, the youngest of a group of channels. In addition to the fossils (micro- and macro-), the conglomerate includes locally-derived clasts of paleosols and other distinctive elements of the surrounding floodplains. Charcoal fragments represent small woody axes and possible larger trunk tissue from arborescent pteridosperms. Preservation of the fossils indicates some aerial exposure prior to transport, with abrasion from rolling. The findings presented here contrast with other published trends in vertebrate size that are used to interpret a reduction in maximum sizes during the Tournaisian. The richness of the fauna runs counter to the assumption of a depauperate nonmarine fauna following the end-Devonian Hangenberg event, and charcoal content highlights the occurrence of fire, with the requisite levels of atmospheric oxygen during that stage.

Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning

The fossil fish Rhizodus hibberti, a member of the tetrapod stem group, shows a unique skeletal pattern in the pelvic fin. Rather than the highly conserved one-to-two pattern of a femur, tibia, and fibula (seen in all known tetrapods, including the extinct, fishlike members of the group), the fin of Rhizodus comprises a femur articulating distally with three bones, each with a distinct morphology. This reveals an early stage in the evolution of limb development, in which the processes patterning the proximal parts of the embryonic fin/limb (the stylopod and zeugopod) were not constrained in the way seen in living tetrapods and could produce more varied skeletal patterns in the adult.

All living tetrapods have a one-to-two branching pattern in the embryonic proximal limb skeleton, with a single element at the base of the limb (the humerus or femur) that articulates distally with two parallel radials (the ulna and radius or the tibia and fibula). This pattern is also seen in the fossilized remains of stem-tetrapods, including the fishlike members of the group, in which despite the absence of digits, the proximal parts of the fin skeleton clearly resemble those of later tetrapods. However, little is known about the developmental mechanisms that establish and canalize this highly conserved pattern. We describe the well-preserved pelvic fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian (lobe-finned) fish, and member of the tetrapod stem group. In this specimen, three parallel radials, each robust with a distinct morphology, articulate with the femur. We review this unexpected morphology in a phylogenetic and developmental context. It implies that the developmental patterning mechanisms seen in living tetrapods, now highly constrained, evolved from mechanisms flexible enough to accommodate variation in the zeugopod (even between pectoral and pelvic fins), while also allowing each element to have a unique morphology.

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

A new member of the Tristichopteridae (=Eusthenopteridae), Mandageria fairfaxi gen. et sp. nov., is described from the Late Devonian (Famennian) Mandagery Sandstone outcropping near Canowindra, NSW, Australia. It is represented by several complete or partial heads and bodies, preserved as natural moulds. Mandageria shares derived characters with the Late Famennian tristichopterid Eusthenodon wängsjöi from East Greenland; the pineal plate series is large and kite-shaped and is posterior to the middle of the parietals, the intertemporal does not contact the posterior supraorbital, a posterior process of the premaxilla divides the apical fossa, the anteriormost premaxillary tooth is enlarged, the postorbital is excluded from the orbit by a supraorbital–lacrimal contact, and the coronoids lack marginal teeth except posteriorly. Mandageria fairfaxi differs from Eusthenodon in superficial fusion of the supratemporal, tabular and postparietals, in the lateral extrascapulars being separated by only 2–3 mm in the midline anteriorly, and in having proportionately smaller scales. It also has an elongate supracleithrum, which is probably autapomorphic. The postcranial skeleton is comparable to that of the Frasnian genus Eusthenopteron, but differs in the more posterior position of the median fins, the poorly ossified vertebral column, and the flattened ectepicondyle. Mandageria fairfaxi is the second osteolepiform described from Canowindra (the first, Canowindra grossi) and, other than the now-reinterpreted Marsdenichthys, the first tristichopterid described from Australia.

Palaeoecology of the Dinantian of Foulden, Berwickshire, Scotland

The Foulden Fish Bed was deposited in a semi-permanent brackish water lake on a floodplain between a northerly mountain range and the sea. The lake supported a rich biota mainly of swimming animals (fish and crustaceans) with a benthos of trace fossils only in the earlier and later stages of the lake's development. Xiphosurans are very rare. This biota changed according to the physical conditions of the lake. Salinity, fluctuating according to water level, was a dominant control. Early in the lake's history there was a dominant benthos of euryhaline bivalves; at the same time, large rhizodonts lived in the lake as the top carnivores. Subsequently, crustaceans were the dominant nektobenthos, in a probable brackish-water regime, the rare Bairdops preying on the common Belotelson; there was a limited infauna. Following this, palaeoniscids abound in the middle part of the sequence, perhaps due to a higher water level but crustaceans are absent at this horizon. During the last stages before final silting-up following a mass-mortality episode, biotic diversity reached a maximum with many fishes and crustaceans and some trace fossils, due to optimal salinity conditions.

Comparisons are made with other Carboniferous biotas and with the Jurassic Solnhofen limestone.

The postcranial skeleton of the Middle Devonian lungfish Dipterus valenciennesi

The axial skeleton and fin supports of the Middle Devonian lungfish Dipterus are described from a range of specimens from the Orcadian Basin of Scotland. The axial skeleton of the trunk region consists of intercentra, relatively short ribs, and neural arches bearing two rows of supraneural spines; posteriorly, the intercentra are replaced by haemal arches. The notochord seems to have been essentially unconstricted. There may have been two pairs of cranial ribs. In general terms the vertebral column resembles that of Neoceratodus but is more fully ossified. The only known paired fin support of Dipterus is an archipterygial structure resembling those of Neoceratodus, with at least seven mesomeres and jointed pre- and postaxial radials. The pectoral fin is rotated as in modern lungfishes. The median fin supports all consist of basal plates supporting radials. In the posterior dorsal fin support the posterior radials form a branched structure. The heterocercal tail is supported by segmental radials. A new body reconstruction is presented.

The characters possessed by Dipterus offer support for a lungfish–porolepiform sistergroup relationship. All known lungfishes fit into a single cumulative series of postcranial morphologies. The evidence for a paedomorphic trend in lungfish phylogeny is interesting but inconclusive.

Cope's Rule and Romer's theory: patterns of diversity and gigantism in eurypterids and Palaeozoic vertebrates

Gigantism is widespread among Palaeozoic arthropods, yet causal mechanisms, particularly the role of (abiotic) environmental factors versus (biotic) competition, remain unknown. The eurypterids (Arthropoda: Chelicerata) include the largest arthropods; gigantic predatory pterygotids (Eurypterina) during the Siluro-Devonian and bizarre sweep-feeding hibbertopterids (Stylonurina) from the Carboniferous to end-Permian. Analysis of family-level originations and extinctions among eurypterids and Palaeozoic vertebrates show that the diversity of Eurypterina waned during the Devonian, while the Placodermi radiated, yet Stylonurina remained relatively unaffected; adopting a sweep-feeding strategy they maintained their large body size by avoiding competition, and persisted throughout the Late Palaeozoic while the predatory nektonic Eurypterina (including the giant pterygotids) declined during the Devonian, possibly out-competed by other predators including jawed vertebrates.

An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes

Although the superbly preserved specimens of Onychodus jandemarrai have greatly advanced our understanding of the Onychodontiformes, a primitive sarcopterygian group with large parasymphysial tooth whorls, the scarcity of the otoccipital material in the group hampers further morphological comparisons between onychodonts and other sarcopterygian groups. Here we report a new onychodont Qingmenodus yui gen. et sp. nov. from the Early Devonian (Pragian) of South China that comprises well-ossified otoccipital and upper and lower jaw material. As one of the oldest known onychodonts, Qingmenodus shows for the first time the nearly complete structure of the otoccipital in onychodonts and provides an additional basis to address the phylogenetic position of the group. Its elongated otic shelf exhibits the posterior shift of the attachment for the basicranial muscle as in coelacanths and sheds light on the feeding mechanism of onychodonts. Qingmenodus displays a mosaic of primitive and derived onychodont features. The phylogenetic analysis places Qingmenodus immediately basal to the clade comprising Onychodus and Grossius.