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. However, evidence for regulated tooth addition in a precise spatiotemporal order can be observed in placoderms, but significantly, only within the group Arthrodira. In these fossils, as in other jawed vertebrates with statodont, non-replacing dentitions, new teeth are added at the ends of rows below the bite, but in line with biting edges of the dentition. The pattern is different on each gnathal bone and probably arises from single odontogenic primordia on each, but tooth rows are arranged in a distinctive placoderm pattern. New teeth are made of regular dentine comparable to that of crown-gnathostomes, formed from a pulp cavity. This differs from semidentine previously described for placoderm gnathalia, a type present in the external dermal tubercles. The Arthrodira is a derived taxon within the Placodermi, hence origin of teeth in placoderms occurs late in the phylogeny and teeth are convergently derived, relative to those of other jawed vertebrates. More basal placoderm taxa adopted other strategies for providing biting surfaces and these vary substantially, but include addition of denticles to the growing gnathal plates, at the margins of pre-existing denticle patches. These alternative strategies and apparent absence of regular dentine have led to previous interpretations that teeth were entirely absent from the placoderm dentition. A consensus view emerged that a dentition, as developed within a dental lamina, is a synapomorphy characterising the clade of crown-group gnathostomes. Recent comparisons between sets of denticle whorls in the pharyngeal region of the jawless fish Loganellia scotica (Thelodonti) and those in sharks suggest homology of these denticle sets on gill arches. Although the placoderm pharyngeal region appears to lack denticles (placoderm gill arches are poorly known), the posterior wall of the pharyngeal cavity, formed by a bony flange termed the postbranchial lamina, is covered in rows of patterned denticle arrays. These arrays differ significantly, both in morphology and arrangement, from those of the denticles located externally on the head and trunkshield plates. Denticles in these arrays are homologous to denticles associated with the gill arches in other crown-gnathostomes, with pattern similarities for order and position of pharyngeal denticles. From their location in the pharynx these are inferred to be under the influence of a cell lineage from endoderm, rather than ectoderm. Tooth sets and tooth whorls in crown-group gnathostomes are suggested to derive from the pharyngeal denticle whorls, at least in sharks, with the patterning mechanisms co-opted to the oral cavity. A comparable co-option is suggested for the Placodermi.     

A new scolenaspidid (Osteostraci, Vertebrata) from the Lower Devonian of Podolia, Ukraine

Anew osteostracan genus and species,Victoraspis longicornualis n. gen., n. sp., is described based on material from Rakovets’, present day Ukraine. This new taxon shares characters with the two generaStensiopelta Denison, 1951 andZychaspis Janvier, 1985. A phylogenetic analysis supports the position ofVictoraspis as the sister group to a monophyleticStensiopelta, while the interrelationships of the various species ofZychaspis are poorly resolved. A morphometric analysis is carried out in an attempt to further resolve the taxonomic affinity. This analysis groups all examinedZychaspis species closely together, and further supports the establishment ofVictoraspis as separate genus.      

Categorical versus geometric morphometric approaches to characterizing the evolution of morphological disparity in Osteostraci (Vertebrata,stem Gnathostomata)

Morphological variation (disparity) is almost invariably characterized by two non-mutually exclusive approaches: (1) quantitatively, through geometric morphometrics; and (2) in terms of discrete, ‘cladistic’, or categorical characters. Uncertainty over the comparability of these approaches diminishes the potential to obtain nomothetic insights into the evolution of morphological disparity and the few benchmarking studies conducted so far show contrasting results. Here, we apply both approaches to characterizing morphology in the stem-gnathostome clade Osteostraci in order to assess congruence between these alternative methods as well as to explore the evolutionary patterns of the group in terms of temporal disparity and the influence of phylogenetic relationships and habitat on morphospace occupation. Our results suggest that both approaches yield similar results in morphospace occupation and clustering, but also some differences indicating that these metrics may capture different aspects of morphology. Phylomorphospaces reveal convergence towards a generalized ‘horseshoe’-shaped cranial morphology and two strong trends involving major groups of osteostracans (benneviaspidids and thyestiids), which probably reflect adaptations to different lifestyles. Temporal patterns of disparity obtained from categorical and morphometric approaches appear congruent, however, disparity maxima occur at different times in the evolutionary history of the group. The results of our analyses indicate that categorical and continuous data sets may characterize different patterns of morphological disparity and that discrepancies could reflect preservational limitations of morphometric data and differences in the potential of each data type for characterizing more or less inclusive aspects of overall phenotype.     

Quantitative palaeobiogeography: GIS, phylogenetic biogeographical analysis, and conservation insights

Aim  The utility of GIS-based and phylogenetic biogeographical analysis in palaeobiogeography is reviewed with reference to its ability to elucidate patterns of interest for modern conservation biology, specifically the long-term effects of invasive species.
Location  Emphasis is on biogeographical patterns in the Appalachian basin and mid-continent of North America during the Devonian. Global palaeobiogeographical patterns of the Cambrian are also considered.
Methods  Palaeobiogeographical patterns are assessed within a GIS framework, including both direct range reconstruction and niche modelling methods, and within phylogenetic biogeographical analysis. Biogeographical patterns are considered within multiple clades of fossil invertebrates, including trilobites, crustaceans, brachiopods, and bivalves.
Results  GIS-based analysis (including niche modelling methods) of Devonian invertebrates demonstrates a tightly correlated relationship between sea-level rises and range expansion, dispersal events, and species invasions. The predominance of range expansion and species invasions during the Late Devonian reduced opportunities for vicariant speciation during this interval. Comparison of phylogenetic biogeographical patterns between Cambrian and Devonian trilobites allows discernment of the relative roles of tectonics and eustacy in driving biogeographical patterns.
Main conclusions  GIS analysis and phylogenetic biogeography are powerful tools for analysing the coevolution of the Earth and its biota. Analyses can identify episodes of vicariance and geo-dispersal and produce testable hypotheses for further analysis within the fossil record.     

Biostratigraphy and palaeobiogeography of the Cambrian genusHipponicharion (Ostracoda)

The bradoriid genusHipponicharion Matthew, 1886 is one of the oldest known Cambrian ostracods. Until now, five species have been documented and another three taxa have been tentatively assigned to one or another species.Hipponicharion has been recorded from Poland, Germany, Morocco as well as from Canada and Great Britain.Hipponicharion hispanicum n. sp. represents the first Cambrian ostracod documented from Spain.Hipponicharion taidaltensis n. sp., formerlyH. aff.geyeri Hinz-Schallreuter, 1993 andH. elickii n. sp., formerlyH. cf.eos sensuElicki, 1994 have also been restudied in this paper.Hipponicharion seems to be restricted to the Acadobaltic Province sensuSdzuy (1972). The systematic relationships of the Bradoriida, which is controverse, is briefly outlined, emphasizing close affinities to the Ostracoda.      

Protopteraspis Gosseleti (Vertebrata: Pteraspidomorphi: Heterostraci) from the Lower Devonian of Shropshire, England

New Protopteraspis material from Shropshire is sufficiently similar to Protopteraspis leathensis from the Welsh Borderland, and the type-species Protopteraspis gosseleti from France, to justify transfering P. leathensis to P. gosseleti . The Anglo-Welsh, biostratigraphic leathensis Zone is changed to the Protopteraspis Zone ( sensu Blieck and Janvier 1989). Possible lifestyles are discussed for Protopteraspis gosseleti .     

Conch size evolution of Silurian–Devonian tentaculitoids

Conch size changes of tentaculitoids are reported from the Silurian and Devonian strata. The combination of measurements of diameters and apical angles allows the reconstruction of tentaculitoid volumes. The conch wall thickness and number of septa can be investigated through observations of thin sections. Within the examined groups, the average conch volume (including the adult part and the whole conch) of tentaculitids increased from the Llandovery to the Pragian and then decreased until the Frasnian. A similar trend was observed in the Devonian dacryoconarids, which peaked for average conch volume in the Emsian. Sea‐level change was probably the main factor controlling the conch volume of tentaculitids and dacryoconarids. The continuous decrease in the average number of septa and the average volume of tentaculitid and dacryoconarid juvenile part from the Pragian (or the Emsian) to the Frasnian indicates a reduction of juvenile duration and earlier reproductive behaviour. The ratio of adult part to juvenile part volumes increased from the Emsian to the Frasnian in tentaculitids, suggesting that the angle between the conch and the water–sediment interface decreased. The conch volume and thickness of the juvenile part and the adult part in all tentaculitoid groups demonstrated similar trends. The average volume of the initial chamber and the adult part of dacryoconarids changed synchronously, with the peak value in the Emsian. Homoctenids, with the smallest conch and the thinnest wall, probably lived in deeper water than dacryoconarids.     

The morphology of Yujiangolepis liujingensis (Placodermi,Arthrodira) from the Pragian of Guangxi (south China) and its phylogenetic significance

The morphology of the arthrodire placoderm Yujiangolepis liujingensis Wang, Pan & Wang, 1998, from the Lower Devonian of Guangxi is redescribed, and compared to that of the basal arthrodires Antarctaspis White, 1968 from Antarctica, Toombalepis Young & Goujet, 2003 and Wuttagoonaspis Ritchie, 1973 from Australia, and Yiminaspis Dupret, 2008 from south China. A very small plate of the skull roof at the junction of the nuchal, marginal, postorbital, and paranuchal plates is considered as the vestigial homologue of the anterior paranuchal plate in the Petalichthyida and Acanthothoraci. Yujiangolepis is attributed to the family ‘Antarctaspididae’ White, 1968, together with the genera Antarctaspis and Toombalepis. A computerized phylogenetic analysis resolved Yujiangolepis as the most basal arthrodire, and the family ‘Antarctaspididae’ appears paraphyletic, Yujiangolepis being possibly the most basal. The family Wuttagoonaspididae contains the genera Wuttagoonaspis and Yiminaspis, and remains monophyletic. A Chinese origin of the ‘Antarctaspididae’ is proposed, together with a southward dispersal into Gondwana during the Early Emsian.     

Early Cretaceous calcareous nannofossils from high latitudes: implications for palaeobiogeography and palaeoclimate

In order to better understand the palaeoceanography and palaeoclimate of the Early Cretaceous, nannofossil data have been obtained from high-latitudinal sites from offshore mid-Norway and from the Barents Sea. No consistent nannofossil data are yet available for Early Cretaceous high latitudes, which should more clearly reflect possible palaeoclimatic changes, as recorded in fluctuations of diversity and abundance. Existing data from France, Italy, Romania, Poland and NW Europe (Germany, North Sea, England) have been complemented by material from higher latitudes. In order to record the Arctic–Boreal nannofossil assemblages, 400 samples from the Norwegian Shelf and the Barents Sea have been analysed. Sixty samples, covering the Berriasian–Barremian interval, yielded calcareous nannofossils. These are derived from cores between 63 and 73°N of present-day latitudes. The nannofossil assemblages recorded are generally of a low diversity and characterized by abundant Watznaueria barnesae and Crucibiscutum salebrosum, whereas Biscutum constans is less common. C. salebrosum, rare or absent at low latitudes, is extremely common in the Norwegian samples. It shows a bipolar distribution especially during Valanginian–Hauterivian times. In the early Valanginian, a distinctive latitudinal gradient in the abundance of C. salebrosum, impoverished in low latitudes and abundant in high latitudes, possibly reflects latitudinal differences in temperature. Restricted palaeoceanographic settings, caused by a sea-level lowstand, may have amplified these thermal gradients. Palaeoclimatically this implies the existence of climatic belts throughout parts of the Early Cretaceous (early Valanginian, Hauterivian), resulting from considerable temperature gradients from north to south. The palaeobiogeographic patterns discussed resulted in the formation of distinctive latitudinally bound nannofossil zones, which, to a certain extent, are similar to those of modern oceans. The palaeobiogeographic patterns described support the idea of an ice-house phase for the early Valanginian and disagree with the suggestion of a globally warm equable climate during that time.     

Cuticles of Nematothallus: a further enigma

EDWARDS, D. & ROSE, V. 1984. Cuticles of Nematothallus: a further enigma. Cuticles of Nematothallus Lang scnsu Edwards are described from a Lower Devonian (Ditton) exposure on the M50 motorway (Hereford and Worcester, England). Some bear highly distinctive cuticular thickenings sometimes surrounding perforations. The affinities of the cuticles, and the development and possible functions of the perforations are considered.     

云南早泥盆世一流线型盔甲鱼及其分类学与古生态学意义(英文)

记述了云南昭通早泥盆世布拉格期坡松冲组华南鱼目一新属、新种——剑裂甲鱼(Rhegmaspis xiphoidea gen.et sp.nov.)。新属的主要特征是:具有鱼雷形头甲,细长吻突,眶孔位于头甲腹侧位,鳃穴向头甲腹面弯曲,角、内角和腹环丢失。鉴于以上特征,新属被归到了包括鸭吻鱼和乌蒙山鱼在内的鸭吻鱼科。根据新材料提供的信息,修订了鸭吻鱼科,包括鸭吻鱼和乌蒙山鱼的鳃穴、腹环、角、内角等特征。裂甲鱼属是该地区发现的第一个具有流线形体型的盔甲鱼类,指示了该地区一些盔甲鱼类可能开始了一种营上底栖的生活方式,具有更加主动的取食行为。新属的发现不仅丰富了该地区华南鱼目的多样性,而且提供了盔甲鱼类在早泥盆世布拉格期通过占据新的生态位而发生的最后一次辐射演化的证据。     

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. © 2009 Wiley‐Liss, Inc.     

The fossil record of Glypturus Stimpson, 1866 (Crustacea,Decapoda, Axiidea,Callianassidae) revisited,with notes on palaeoecology and palaeobiogeography

Abstract: The fossil record of the callianassid genus Glypturus (Decapoda, Axiidea) is re‐evaluated. Our systematic revision, both of extant and fossil taxa, is based on major cheliped morphology only, thus providing an important impetus for palaeontological studies. Both spination and tuberculation of chelipeds are herein considered of great taxonomic importance. Presence of spines on the upper margins of the merus and propodus and the lower margin of the carpus are significant for generic assignment, whereas the extent of tuberculation on lateral surfaces of the propodus is important for assignment at the species level. Altogether, four extant and six exclusively fossil species of Glypturus are recognized. Several extinct callianassid taxa are now transferred to the genus, namely Callianassa berryi, Callianassa fraasi, Callianassa munieri, Callianassa pugnax and Callianassaspinosa; Callianassa pseudofraasi is considered a junior synonym of C. fraasi. Based on a comparison of ecological preferences of extant representatives, the presence of Glypturus in the fossil record is considered to be linked with tropical to subtropical, nearshore carbonate environments of normal salinity. We argue that Glypturus is of Tethyan origin, with a stratigraphical range going as far back as the Eocene. Since then, the genus migrated both westwards and eastwards, establishing present‐day communities in the western Atlantic and Indo‐West Pacific which both comprise several distinct species. In the presumed area of origin, the genus does no longer occur today. The exlusively fossil (middle Eocene) genus Eoglypturus from Italy is considered closely related to Glypturus and is thus assigned to the subfamily Callichirinae as well.     

Traquairaspididae and Cyathaspididae (Heterostraci) from the Lower Devonian of Poland

Lower Devonian strata from the drill cores from Poland have yielded new Heterostraci material. One ventral shield of Toombsaspis (Traquairaspididae, Phialaspidinae) discovered in the Białopole IG-1 drill core, and a fragment of dorsal shield of cf. Paraliliaspis (Cyathaspididae, Anglaspidinae) with the impressions of internal structures from Ciepielów IG-1 are described and provide new information on paleogeographic disparity of these groups.     

Biogeography of Triassic tetrapods: evidence for provincialism and driven sympatric cladogenesis in the early evolution of modern tetrapod lineages

Triassic tetrapods are of key importance in understanding their evolutionary history, because several tetrapod clades, including most of their modern lineages, first appeared or experienced their initial evolutionary radiation during this Period. In order to test previous palaeobiogeographical hypotheses of Triassic tetrapod faunas, tree reconciliation analyses (TRA) were performed with the aim of recovering biogeographical patterns based on phylogenetic signals provided by a composite tree of Middle and Late Triassic tetrapods. The TRA found significant evidence for the presence of different palaeobiogeographical patterns during the analysed time spans. First, a Pangaean distribution is observed during the Middle Triassic, in which several cosmopolitan tetrapod groups are found. During the early Late Triassic a strongly palaeolatitudinally influenced pattern is recovered, with some tetrapod lineages restricted to palaeolatitudinal belts. During the latest Triassic, Gondwanan territories were more closely related to each other than to Laurasian ones, with a distinct tetrapod fauna at low palaeolatitudes. Finally, more than 75 per cent of the cladogenetic events recorded in the tetrapod phylogeny occurred as sympatric splits or within-area vicariance, indicating that evolutionary processes at the regional level were the main drivers in the radiation of Middle and Late Triassic tetrapods and the early evolution of several modern tetrapod lineages.     

Structure and evolution of the horizontal septum in vertebrates

Although the horizontal septum (HS) has been identified as playing a role in fish biomechanics and in path finding of cells during zebrafish development, its morphology is poorly known. However, it is generally regarded as an evolutionarily conserved structure. To test this idea, we applied a novel combination of techniques to analyse the HS of 35 species from all major gnathostome clades in which is visualized its collagen fibre architecture. Results show that the HS is a conserved trait only with respect to the presence of caudolateral [= epicentral] and craniolateral [= posterior oblique] collagen fibre tracts, but differs remarkably with respect to the specifications of these tracts. Our data revealed several evolutionary changes within vertebrates. In the gnathostome ancestor, the two tracts are represented by evenly distributed epicentral fibres (ECFs) and posterior oblique fibres (POFs). ECFs are condensed to distinct epicentral tendons (ECTs) in the actinopteran ancestor. POFs independently evolved to distinct posterior oblique tendons (POTs) at least two times within teleosts. Within basal teleostomes, POFs as well as ECFs or ECTs were lost two times independently. POTs were lost at least three times independently within teleosts. This view of a homoplastic HS remains stable regardless of the competing phylogenies used for analysis. Our data make problematic any generalization of biomechanical models on fish swimming that include the HS. They indicate that the pathfinding role of the HS in zebrafish may be extended to gnathostome fishes, but not to agnathans, sarcopterygian fishes and tetrapods.     

Miocene snakes from northeastern Kazakhstan: new data on the evolution of snake assemblages in Siberia

Abstract

The Neogene snake fauna from the central and eastern regions of Eurasia is still largely unknown. This paper reports on a unique snake fauna from the late middle Miocene of the Baikadam and Malyi Kalkaman 1 and 2 localities, northeastern Kazakhstan, which represents the best-documented Miocene snake assemblage in Central Asia. Previous studies admitted that snake fauna could be homogeneous over a large part of Eurasia during the Miocene, with the late middle to early late Miocene assemblages similar to snake assemblages that inhabited Europe in the late early and early middle Miocene. This assumption is partially supported by the presence of Texasophis bohemiacus and Coluber cf. hungaricus, as well as vipers of the ‘V. aspis’ complex. However, the presence of taxa which are (1) probably not related to European representatives (‘Colubrinae’ A and B), (2) probably never occurred in Central and Western Europe and (3) are closely related to species recently inhabiting southern Siberia (Elaphe aff. dione, Gloydius sp.) indicates that faunal dissimilarity was relatively high within Eurasia during the late middle Miocene. This assumption is in accordance with studies of small mammal assemblages which show a decreasing homogenity in the Eurasia in the course of the middle Miocene.     

Recurrent gene loss correlates with the evolution of stomach phenotypes in gnathostome history

The stomach, a hallmark of gnathostome evolution, represents a unique anatomical innovation characterized by the presence of acid- and pepsin-secreting glands. However, the occurrence of these glands in gnathostome species is not universal; in the nineteenth century the French zoologist Cuvier first noted that some teleosts lacked a stomach. Strikingly, Holocephali (chimaeras), dipnoids (lungfish) and monotremes (egg-laying mammals) also lack acid secretion and a gastric cellular phenotype. Here, we test the hypothesis that loss of the gastric phenotype is correlated with the loss of key gastric genes. We investigated species from all the main gnathostome lineages and show the specific contribution of gene loss to the widespread distribution of the agastric condition. We establish that the stomach loss correlates with the persistent and complete absence of the gastric function gene kit—H⁺/K⁺-ATPase (Atp4A and Atp4B) and pepsinogens (Pga, Pgc, Cym)—in the analysed species. We also find that in gastric species the pepsinogen gene complement varies significantly (e.g. two to four in teleosts and tens in some mammals) with multiple events of pseudogenization identified in various lineages. We propose that relaxation of purifying selection in pepsinogen genes and possibly proton pump genes in response to dietary changes led to the numerous independent events of stomach loss in gnathostome history. Significantly, the absence of the gastric genes predicts that reinvention of the stomach in agastric lineages would be highly improbable, in line with Dollo''s principle.