Median Dorsal Plate Research Articles

Silurian phyllocarid crustaceans (Phyllocarida, Archaeostraca) from South China

In the Silurian, the suborder Caryocaridina of the phyllocarids was replaced by the suborder Ceratiocaridina as the dominant group. The latter did not achieve a global distribution until the late Silurian. In the early Silurian, ceratiocarids were a little-diversified group and palaeogeographically restricted mainly to Laurussia. Although previous studies have mentioned phyllocarid fossils from the Silurian of the South China plate, these have never been systematically described. This study describes three genera and four species (including one new genus, two new species, and two undetermined species), i.e. Cugocaris future gen. et sp. nov., Gonatocaris wuhanensis sp. nov., Gonatocaris sp. and Warneticaris sp., based on 38 phyllocarid specimens from the Fentou Formation in Wuhan. Cugocaris future gen. et sp. nov. is distinguished by an elliptic carapace ornamented by sinuous and anastomosing longitudinal striae, a styliform telson, and slender furcal rami. Gonatocaris wuhanensis sp. nov. is characterized by anastomosing longitudinal striae on the carapace, greatest carapace width in the centre of the carapace, and three different types of ornamentations on the surface of the abdominal segments. A phylogenetic analysis of 41 Palaeozoic phyllocarids reveals that Gonatocaris, Rolfecaris and Cugocaris gen. nov. form a monophyletic group. Thus, a new family, Gonatocarididae fam. nov., is proposed. This family is characterized by a carapace lacking median dorsal plate, rostral plate, a well-developed anterior carapace horn, and ornamented with prominent raised, wavy, anastomosing longitudinal striae. A palaeoecological analysis suggests that G. wuhanensis was a fairly active swimmer, while C. future was necto-benthic with relatively weak swimming abilities. This study not only extends the known morphological range of phyllocarids, but also has implications for the functional morphology, taxonomy, and evolution of archaeostracans as a whole. http://zoobank.org/urn:lsid:zoobank.org:pub:4B6B60B6-19F1-4A58-AC65-3B1AEE5D0939

A new selenosteid placoderm from the Late Devonian of the eastern Anti-Atlas (Morocco) with preserved body outline and its ecomorphology

Placoderms are an extinct group of early jawed vertebrates that play a key role in understanding the evolution of the gnathostome body plan, including the origin of novelties such as jaws, teeth, and pelvic fins. As placoderms have a poorly ossified axial skeleton, preservation of the mainly cartilaginous axial and fin elements is extremely rare, contrary to the heavily mineralized bones of the skull and thoracic armor. Therefore, the gross anatomy of the animals and body shape is only known from a few taxa, and reconstructions of the swimming function and ecology are speculative. Here, we describe articulated specimens preserving skull roofs, shoulder girdles, most fins, and body outlines of a newly derived arthrodire. Specimens of the selenosteid Amazichthys trinajsticae gen. et sp. nov. display a skull roof with reticular ornamentation and raised sensory lines like Driscollaspis, a median dorsal plate with a unique sharp posterior depression, the pelvic girdle, the proportions and shape of the pectoral, dorsal, and caudal fins as well as a laterally enlarged region resembling the lateral keel of a few modern sharks and bony fishes. Our new phylogenetic analyses support the monophyly of the selenosteid family and place the new genus in a clade with Melanosteus, Enseosteus, Walterosteus, and Draconichthys. The shape of its body and heterocercal caudal fin in combination with the pronounced “lateral keel” suggest Amazichthys trinajsticae was an active macropelagic swimmer capable of reaching high swimming speeds.

The oldest complete jawed vertebrates from the early Silurian of China.

Molecular studies suggest that the origin of jawed vertebrates was no later than the Late Ordovician period (around 450 million years ago (Ma))1,2. Together with disarticulated micro-remains of putative chondrichthyans from the Ordovician and early Silurian period3-8, these analyses suggest an evolutionary proliferation of jawed vertebrates before, and immediately after, the end-Ordovician mass extinction. However, until now, the earliest complete fossils of jawed fishes for which a detailed reconstruction of their morphology was possible came from late Silurian assemblages (about 425 Ma)9-13. The dearth of articulated, whole-body fossils from before the late Silurian has long rendered the earliest history of jawed vertebrates obscure. Here we report a newly discovered Konservat-Lagerstätte, which is marked by the presence of diverse, well-preserved jawed fishes with complete bodies, from the early Silurian (Telychian age, around 436 Ma) of Chongqing, South China. The dominant species, a 'placoderm' or jawed stem gnathostome, which we name Xiushanosteus mirabilis gen. et sp. nov., combines characters from major placoderm subgroups14-17 and foreshadows the transformation of the skull roof pattern from the placoderm to the osteichthyan condition10. The chondrichthyan Shenacanthus vermiformis gen. et sp. nov. exhibits extensive thoracic armour plates that were previously unknown in this lineage, and include a large median dorsal plate as in placoderms14-16, combined with a conventional chondrichthyan bauplan18,19. Together, these species reveal a previously unseen diversification of jawed vertebrates in the early Silurian, and provide detailed insights into the whole-body morphology of the jawed vertebrates of this period.

Asterolepis alticristata n. sp. (Antiarchi) from the Upper Devonian (Frasnian) of Nunavut, Canada, and a report on the antiarch diversity of the Fram Formation

A large collection of fossils from the NV2K17 locality, Upper Devonian (Frasnian) Fram Formation, Ellesmere Island, Nunavut, Canada, represents at least two species of Asterolepis. The first, Asterolepis alticristata n. sp., is diagnosed by a tall midline crest that rises sharply from the tergal angle of the anterior median dorsal plate, reaches maximum height at around the position of the lateral corners, and then drops to form a low midline crest along the posterior median dorsal plate. The second species is identified as Asterolepis cf. radiata according to an ornament of radiating rows of conjoined tubercles and ridges on the available thoracic plates (anterior median dorsal, posterior median dorsal, and mixilateral). We describe additional antiarch material from the NV2K17 locality, including an anterior median dorsal plate of Bothriolepis sp. indet., and from localities throughout the Fram Formation. The antiarch occurrences in the Fram Formation conform to a possible faunal turnover from one dominated by asterolepidoids low in the formation to one dominated by bothriolepidids high in the Fram Formation. At the NV2K17 locality, in the middle of the Fram Formation, members of the two groups appear together.

Reappraisal of the Silurian placoderm Silurolepis and insights into the dermal neck joint evolution.

Silurolepis platydorsalis, a Silurian jawed vertebrate originally identified as an antiarch, is here redescribed as a maxillate placoderm close to Qilinyu and is anteroposteriorly reversed as opposed to the original description. The cuboid trunk shield possesses three longitudinal cristae, obstanic grooves on the trunk shield and three median dorsal plates, all uniquely shared with Qilinyu. Further preparation reveals the morphology of the dermal neck joint, with slot-shaped articular fossae on the trunk shield similar to Qilinyu and antiarchs. However, new tomographic data reveal that Qilinyu uniquely bears a dual articulation between the skull roof and trunk shield, which does not fit into the traditional ‘ginglymoid’ and ‘reverse ginglymoid’ categories. An extended comparison in early jawed vertebrates confirms that a sliding-type dermal neck joint is widely distributed and other types are elaborated in different lineages by developing various laminae. Nine new characters related to the dermal neck joint are proposed for a new phylogenetic analysis, in which Silurolepis forms a clade with Qilinyu. The current phylogenetic framework conflicts with the parsimonious evolution of dermal neck joints in suggesting that the shared trunk shield characters between antiarchs and Qilinyu are independently acquired, and the sliding-type joint in Entelognathus is reversely evolved from the dual articulation in Qilinyu.

New information on Titanichthys (Placodermi, Arthrodira) from the Cleveland Shale (Upper Devonian) of Ohio, USA

AbstractThe placoderm Titanichthys from the Late Devonian (Famennian) is based on incomplete and fragmentary specimens that have hindered understanding of its overall anatomy and phylogenetic relationships. A new, nearly complete, articulated specimen from the Upper Devonian Cleveland Shale provides new information about the previously undescribed rostral, postmarginal, postsuborbital, submarginal, posterior superognathal plates, and the nasal capsule. A revised diagnosis is provided for the genus. Three new diagnostic characters are identified, including a transversely elliptical rostral plate that does not contact adjacent plates, a reduced posterior superognathal, and a median dorsal plate that inserts into the posterior dorsal lateral plate. The first comprehensive phylogenetic analysis of Titanichthys indicates that the genus is a basal aspinothoracid arthrodire closely related to the enigmatic taxa Bungartius and Tafilalichthys.

The Late Devonian placoderm <i>Aspidichthys</i> Newberry, 1873 from the Holy Cross Mountains, Poland

Abstract. The reported placoderm remains from the Holy Cross Mountains, Poland, belong to the species Aspidichthys ingens Koenen, 1883. This study focuses on the material described in the past from Wietrznia Quarry but the new specimens were also collected form Kowala and Płucki localities. All specimens presented here differ from others described in the past, mainly by having smaller tubercles and possessing a distinctive crest in the anterior median part of the median dorsal plate, but did not allow the erection of a new taxon. Based on conodont assemblage, all specimens are attributed to a Frasnian age. Aspidichthys occurs in the middle to the upper part of the Frasnian (Palmatolepis punctata to P. linguiformis conodont zone) and is not found in the Famennian boundary. Described specimens also occur in a stepwise deepening environment caused by local synsedimentary tectonics.

A new species of Groenlandaspis Heintz, 1932 (Placodermi, Arthrodira), from the Famennian (Late Devonian) of Belgium

ABSTRACTA new species of the phlyctaeniid arthrodire genus Groenlandaspis from the upper Famennian of Belgium is described. The remains of Groenlandaspis potyi, sp. nov., consist of dissociated thoracic armor elements, but the specimen designated to be the holotype displays the median dorsal, anterior, and posterior dorsolateral plates in articulation. Though incomplete, the new species is characterized by an equilateral triangle-shaped median dorsal plate, a protruding posterodorsal apron of the posterior dorsolateral plate behind the overlap area for the median dorsal plate, and an overall lack of ornamentation. Groenlandaspis potyi, sp. nov., constitutes the second occurrence of a Groenlandaspis species in continental Europe after the description of Groenlandaspis thorezi from upper Famennian quarries of Belgium. Another probable new species of Groenlandaspis is also described, though of unknown locality and horizon; it can, however, be deduced to be from the upper Famennian of Belgium without more precision. Together with some unpublished material of groenlandaspidids from the Famennian tetrapod-bearing locality of Strud, this material highlights the richness of Groenlandaspididae diversity in Belgium. The discovery of Groenlandaspis potyi, sp. nov., in Belgium reinforces the Famennian global distribution of this genus during this period. Also, because these organisms have been considered as non-marine indicators, this material is another argument pleading for close relationships between Euramerica and Gondwana around the Frasnian–Famennian boundary.http://zoobank.org/urn:lsid:zoobank.org:pub:500F0661-2638-4F26-9293-95B89AF0B39B

New arthrodires (placoderm fishes) from the Aztec Siltstone (late Middle Devonian) of southern Victoria Land, Antarctica

A small collection of arthrodire remains is described from the Devonian Aztec Siltstone of southern Victoria Land, Antarctica. Barwickosteus antarcticus, gen. et sp. nov., is a small phlyctaeniid arthrodire probably closely related to Barrydalaspis from the Bokkeveld Group of South Africa. Grifftaylor antarcticus, gen. et sp. nov., is a generalised phlyctaeniid resembling Phlyctaenius and Neophlyctaenius. New specimens of Boomeraspis show that it had a high-spired trunk-armour with a median dorsal plate of similar proportions to Tiaraspis, Mithakaspis, Turrisaspis or Africanaspis. Other fragmentary median dorsal plates are provisionally referred to Turrisaspis and Mulgaspis. With these new taxa the vertebrate assemblage from the Aztec Siltstone comprises at least 37 genera and 50 species, making it one of the most diverse of Middle–Late Devonian age.

Morphology, palaeobiology and phylogeny of Oryctocaris balssi gen. nov. (Arthropoda), a phyllocarid from the Lower Devonian Hunsrück Slate (Germany)

Nahecaris balssi Broili, 1930 is a phyllocarid crustacean known only from the Lower Devonian Hunsrück Slate in Germany. This study redescribes the species on the basis of 22 new specimens and the holotype. Phylogenetic analysis of several Palaeozoic phyllocarids reveals that N. balssi and N. stuertzi Jaeckel, 1921 are not congeneric, as was previously assumed, and that there are significant morphological differences between these taxa. Thus, the new genus Oryctocaris is erected for N. balssi, characterized mainly by the absence of a rostrum and a median dorsal plate. It is suggested that O. balssi was a fairly active swimmer, which was not limited to the benthic zone. Furthermore, the species presumably swam by means of powerful pleopod strokes coupled with downward movements of the abdomen. It is likely the telson spine with its furcal rami acted as a paddle, enhancing propulsion. We suggest that O. balssi was a deposit feeder, foraging by digging in the mud and picking up food particles or small hard-shelled organisms such as molluscs, which were then crushed by the large and robust mandibles.http://zoobank.org/urn:lsid:zoobank.org:pub:7A84A8A9-F9F3-464E-A3B2-24B9056EE0B6

The skull of Hagiangella goujeti Janvier, 2005, a high-crested acanthothoracid (Vertebrata, Placodermi) from the Lower Devonian of northern Vietnam

ABSTRACT The acanthothoracid Hagiangella goujeti Janvier, 2005, has been described exclusively on the basis of isolated thoracic plates from the Lochkovian (Lower Devonian) Khao Loc Formation of Tung Vai, Ha Giang Province, northern Vietnam. It is characterized by a very high, triangular median crest on the median dorsal plate, and has been referred to the Acanthothoraci on the basis of the morphology of its fused anterolateral, spinal and anterior ventrolateral plates, and the characteristic stellate ornamentation of the group. Isolated plates of H. goujeti are relatively abundant at Tung Vai and no other placoderm taxon from this locality seems to share the same type of ornamentation. However, the skull of this species remained elusive. Here we report two well-preserved skull roofs from Tung Vai, which we refer to H. goujeti. They display the same stellate ornamentation and small size as the previously described plates of the thoracic armor of this species. This new material shows that the head of H. goujeti is surprisingly short (i.e., possibly lacking dermal rostral and pineal elements), in contrast to the elongate and narrow skull of all other acanthothoracids. The combination of unique characters (e.g., presence of two pairs of posterior pit lines, two pairs of central and paranuchal plates, etc.) suggests a possible sister group relationship to the placoderm assemblage Petalichthyida + Ptyctodontida + Arthrodira.

Revision of the genusKujdanowiaspisStensiö, 1942 (Placodermi, Arthrodira, “Actinolepida”) from the Lower Devonian of Podolia (Ukraine)

The genus Kujdanowiaspis Stensio, 1942 has long been considered as the archetype of placoderms; hence, it has been often used as outgroup in phylogenetic analyses involving placoderms, or used as a representative of all the placoderms for all early vertebrate works. Nevertheless, there has been no real work on the taxonomy of this genus since Denison (1978). Here we propose a revision of the material of Kujdanowiaspis from the Old Red Sandstone of Podolia (including neurocrania, skull roofs and thoracic armours), together with the description of unpublished specimens of the genus Heightingtonaspis White, 1969. Among the available Podolian material, three species are considered valid: Kujdanowiaspis buczacziensis (Brotzen, 1934), K. podolica (Brotzen, 1934) and Erikaspis zychi (Stensio, 1945) (K. podolica and K. buczacziensis only differ in size and in the density and size of the tuberculated ornamentation; the dermal plate pattern of E. zychi differs from that of the genus Kujdanowiaspis). The axillar area of the scapulocoracoid of K. podolica is compared with those of an osteostracan “agnathan” and of a non-tetrapod sarcopterygian. In the three cases, the articulation of the pectoral fin is of the monobasal type. An analogy with the embryonic development of the pectoral fin of the actinopterygian Danio rerio (Hamilton, 1822) suggests that the monobasal articulation would correspond to the plesiomorphic condition compared with the multibasal one of the adult actinopterygians and some derived brachythoracid placoderms. The suprasynarcual is a newly identified, chondrified element of the vertebral column, supposed to respond to the height of the median dorsal plate in Kujdanowiaspis podolica.

A new antiarch fish from the Upper Devonian Zhongning Formation of Ningxia, China

A new antiarch, Ningxialepis spinosa n. gen. n. sp., is described from the Zhongning Formation (Famennian, Late Devonian) of Shixiagou, Qingtongxia, Ningxia, northwestern China. It is characterized by the presence of X-shaped pit-lines, long obstantic margins, high dorsal median spine of the trunk armour formed from the anterior and posterior median dorsal plates, prominent dorsolateral and ventrolateral ridges of the trunk armour, and the anterior median dorsal plate partly overlapping the anterior dorsolateral plate. Ningxialepis is placed as the sister taxon to Jiangxilepis in the Family Jiangxilepidae from South China, based on a phylogenetic analysis of the Euantiarcha. The Jiangxilepidae is redefined. The close affinity between Ningxialepis and Jiangxilepis further corroborates the geographic proximity between the North China and South China blocks in the Late Devonian.

A basal antiarch (placoderm fish) from the Silurian of Qujing, Yunnan, China

A new basal antiarch, Silurolepis platydorsalis n. gen. n. sp., represented by a large-sized trunk armor and a disassociated anterior median dorsal plate (AMD), is described from the Ludlow (Silurian) of Qujing, Yunnan, southwestern China. It is characterized by its AMD overlapping the posterior median dorsal plate, broad anterior margin of the AMD, and crest-shaped dorsolateral ridge of the trunk armor. As the oldest known articulated placoderm fish to date, the new fish provides new insights into the early evolution of antiarchs.

A new genus and species of Ptyctodont (Placodermi) from the Late Devonian Gneudna Formation, Western Australia, and an analysis of Ptyctodont phylogeny

AbstractAn almost complete but predominantly disarticulated ptyctodont fish, Kimbryanodus williamburyensis n. gen., n.sp. from the Late Devonian Gneudna Formation, is described. The fossils occur as three-dimensionally preserved isolated plates, and this has allowed the reconstruction of the fish. A taxonomic revision of the ptyctodonts was undertaken based on recently described Australian taxa and new reconstructions of Australian, American and European specimens. The phylogenetic analysis supports a threefold division of the ptyctodonts, with Rhamphodopsis being the most basal taxon and the other ptyctodonts divided into those possessing a median dorsal spine, spinal plate and simple V-shaped overlap of the anterior lateral and anterior dorsolateral plates and those taxa which do not.

New arthrodires (Family Williamsaspididae) from Wee Jasper, New South Wales (Early Devonian), with comments on placoderm morphology and palaeoecology

AbstractTwo new arthrodire species represented by articulated trunk armours from the Early Devonian (Emsian) limestones of the Burrinjuck area are placed in a new genus Elvaspis (E. tuberculata, E. whitei), assigned to the Family Williamsaspididae. On new evidence of the dermal neck‐joint and shape of the median dorsal plate this family is reassigned from the Phlyctaeniina to the Brachythoraci. The strongly ornamented post‐branchial lamina of the trunk armour relates to a recent hypothesis that special post‐branchial denticles in placoderms are homologous to pharyngeal denticles of crown‐gnathostomes rather than modified external tubercles. Variable development of the post‐branchial lamina and its ornament in different placoderm subgroups, with specific resemblance to the external ornament characteristic of that subgroup, indicates that modification of normal external dermal ornament is the most parsimonious interpretation. A comparison of fish diversity in modern and ancient tropical reef environments is consistent with an equilibrium model for species diversification through time.

Sensory structures of Archaeostraca (Phyllocaridida, Crustacea)

The carapace of some Archaeostraca exhibits two kinds of structures that are best interpreted as corresponding to sensory organs. Dorsal ones are located either along the dorsal margin of the valves of the carapace, or on the median dorsal plate. Anterolateral ones are located in the anterior region of both valves. Both structures are respectively homologous of the anterior and posterior sensory organs known from extant Leptostraca and other Crustacea. These structures are described in adult Archaeostraca from the Ordovician to the Carboniferous. It is expected that they could have baroreceptor, mechanoreceptor, and/or chemoreceptor functions in some cases, and a function in osmotic regulation in others.

Groenlandaspidid placoderm fishes from the Late Devonian of North America

Two new taxa of groenlandaspidid fish (Placodermi: Arthrodira) from non-marine deposits of the Catskill Formation (Upper Devonian) at Red Hill, Clinton County, Pennsylvania, USA, are the first to be described from North America outside of Greenland. Groenlandaspis pennsylvanica n.sp. is known only from isolated trunk elements that are quite similar to Groenlandaspis antarctica Ritchie, 1975. Groenlandaspis pennsylvanica n.sp. is characterized by distinct features of the median dorsal (MD), posterior dorsolateral (PDL), and anterior dorsolateral (ADL) plates. Turrisaspis elektor n.gen. and n.sp. is a smaller form with a high median dorsal plate (MD) and tall, narrow elements of the dorsolateral trunk shield, particularly the PDL, giving this taxon a distinctively short, high trunk shield. The head shield of Turrisaspis elektor has a divided pineal plate and the postmarginal plate makes contact with the postorbital plate, excluding the marginal plate from the lateral margin of the head shield. Groenlandaspis pennsylvanica n.sp. is a rare faunal component at the Red Hill site whereas Turrisaspis elektor is very common. The large sample size of Turrisaspis elektor provides a glimpse at ontogenetic and individual variation within this taxon.

A new antiarch (placoderm fish) from the Early Devonian of South China

A new bothriolepidoid antiarch of small size, Luquanolepis pileos gen. et sp. nov. is described from the Early Devonian of Luquan County, Yunnan Province. Only remains of the trunk armour are known. It is characterised by the long dorsal wall, with a strong posterior process formed by the posterior median dorsal and posterior dorsolateral plates, the posterior median dorsal longer and broader than the anterior median dorsal, a strongly developed posterior internal tranverse crista anteriorly placed at about the middle of the length of the dorsal trunk armour, and an independent posterior lateral plate. It is one of the few antiarchs known from the Early Devonian which has a developed brachial process and segmented pectoral fin. The small unpaired semilunar plate, subanal lamina and other features indicate bothriolepid affinities, and the separate posterior lateral suggests it is a stem taxon within the bothriolepidoids, a group to which 13 genera are assigned. Distribution of 19 characters amongst these taxa is summarised in a data matrix used in a computer-assisted cladistic analysis of the group to produce over 100 equally parsimonious trees. By eliminating poorly known taxa and characters from the analysis and inferring some missing character states 14 equally parsimonious trees were produced for 10 bothriolepidoid taxa. Luquanolepis is regarded as the most primitive member within the clade, consistent with the fact that it is the oldest bothriolepidoid genus so far discovered.

A new bothriolepid antiarch (Pisces, Placodermi) from the Devonian of Dô Son peninsula, northern Vietnam

A new bothriolepid antiarch, Vietnamaspis trii gen. et sp. nov., is described from the Middle-Upper Devonian sandstones of the Dô Son peninsula, near Haiphong, Vietnam. The site is north of the Song Ma suture zone on the South China Terrane, and contains a fauna including Bothriolepis and groenlandaspids. Vietnamaspis gen. nov. is characterized by having a long, parallel-sided posterior median dorsal plate, and an anterior median dorsal plate with a narrow anterior margin. Vietnamaspis gen. nov. is regarded as a sister taxon to Bothriolepis and Grossilepis because it has a relatively large axillary foramen on the anterior ventrolateral plate. The Australian bothriolepid Bothriolepis warreni Long 1983 is referred to a new genus, Briagalepis gen. nov., because it has very short overlap surfaces for the anterior dorsolateral plate on the anterior median dorsal plate, and lacks a reticulate dermal ornament.