Stem Chondrichthyans Research Articles

First shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia sheds new light on the development of tessellated calcified cartilage.

BackgroundLiving gnathostomes (jawed vertebrates) comprise two divisions, Chondrichthyes (cartilaginous fishes, including euchondrichthyans with prismatic calcified cartilage, and extinct stem chondrichthyans) and Osteichthyes (bony fishes including tetrapods). Most of the early chondrichthyan (‘shark’) record is based upon isolated teeth, spines, and scales, with the oldest articulated sharks that exhibit major diagnostic characters of the group—prismatic calcified cartilage and pelvic claspers in males—being from the latest Devonian, c. 360 Mya. This paucity of information about early chondrichthyan anatomy is mainly due to their lack of endoskeletal bone and consequent low preservation potential.Methodology/Principal FindingsHere we present new data from the first well-preserved chondrichthyan fossil from the early Late Devonian (ca. 380–384 Mya) Gogo Formation Lägerstatte of Western Australia. The specimen is the first Devonian shark body fossil to be acid-prepared, revealing the endoskeletal elements as three-dimensional undistorted units: Meckel’s cartilages, nasal, ceratohyal, basibranchial and possible epibranchial cartilages, plus left and right scapulocoracoids, as well as teeth and scales. This unique specimen is assigned to Gogoselachus lynnbeazleyae n. gen. n. sp.Conclusions/SignificanceThe Meckel’s cartilages show a jaw articulation surface dominated by an expansive cotylus, and a small mandibular knob, an unusual condition for chondrichthyans. The scapulocoracoid of the new specimen shows evidence of two pectoral fin basal articulation facets, differing from the standard condition for early gnathostomes which have either one or three articulations. The tooth structure is intermediate between the ‘primitive’ ctenacanthiform and symmoriiform condition, and more derived forms with a euselachian-type base. Of special interest is the highly distinctive type of calcified cartilage forming the endoskeleton, comprising multiple layers of nonprismatic subpolygonal tesserae separated by a cellular matrix, interpreted as a transitional step toward the tessellated prismatic calcified cartilage that is recognized as the main diagnostic character of the chondrichthyans.

Oldest near-complete acanthodian: the first vertebrate from the Silurian Bertie Formation Konservat-Lagerstätte, Ontario.

BackgroundThe relationships between early jawed vertebrates have been much debated, with cladistic analyses yielding little consensus on the position (or positions) of acanthodians with respect to other groups. Whereas one recent analysis showed various acanthodians (classically known as ‘spiny sharks’) as stem osteichthyans (bony fishes) and others as stem chondrichthyans, another shows the acanthodians as a paraphyletic group of stem chondrichthyans, and the latest analysis shows acanthodians as the monophyletic sister group of the Chondrichthyes.Methodology/Principal FindingsA small specimen of the ischnacanthiform acanthodian Nerepisacanthus denisoni is the first vertebrate fossil collected from the Late Silurian Bertie Formation Konservat-Lagerstätte of southern Ontario, Canada, a deposit well-known for its spectacular eurypterid fossils. The fish is the only near complete acanthodian from pre-Devonian strata worldwide, and confirms that Nerepisacanthus has dentigerous jaw bones, body scales with superposed crown growth zones formed of ondontocytic mesodentine, and a patch of chondrichthyan-like scales posterior to the jaw joint.Conclusions/SignificanceThe combination of features found in Nerepisacanthus supports the hypothesis that acanthodians could be a group, or even a clade, on the chondrichthyan stem. Cladistic analyses of early jawed vertebrates incorporating Nerepisacanthus, and updated data on other acanthodians based on publications in press, should help clarify their relationships.

Cretaceous stem chondrichthyans survived the end-Permian mass extinction

Cladodontomorph sharks are Palaeozoic stem chondrichthyans thought to go extinct at the end-Permian mass extinction. This extinction preceded the diversification of euselachians, including modern sharks. Here we describe an outer-platform cladodontomorph shark tooth assemblage from the Early Cretaceous of southern France, increasing the fossil record of this group by circa 120 million years. Identification of this material rests on new histological observations and morphological evidence. Our finding shows that this lineage survived mass extinctions most likely by habitat contraction, using deep-sea refuge environments during catastrophic events. The recorded gap in the cladodontomorph lineage represents the longest gap in the fossil record for an extinct marine vertebrate group. This discovery demonstrates that the deep-sea marine diversity, poorly known during most of the fish evolutionary history, contains essential data for a complete understanding of the long-term evolution of marine fish paleobiodiversity.

Redescription of Parexus recurvus, an Early Devonian acanthodian from the Midland Valley of Scotland

Parexus Agassiz was one of the first Early Devonian ‘spiny sharks’ to be described. The genus is readily recognized by the large size and ornament of its anterior dorsal fin spine. Although two species were erected, reappraisal of all known specimens indicate they should be synonymized in the type species Parexus recurvus. Farnellia tuberculata Traquair, originally described as a vertebral column, is actually tooth rows of jaw dentition, and is also now considered to be a junior synonym of P. recurvus. Parexus has a perichondrally ossified scapulocoracoid of typical acanthodian shape, and diagnostic features of the family Climatiidae, but has distinctive scales comprising appositional growth zones that closely resemble those of the putative stem chondrichthyan Seretolepis elegans Karatajute-Talimaa.

Pattern formation in development of chondrichthyan dentitions: a review of an evolutionary model

The mode of tooth development displayed in Chondrichthyans (sharks, rays and holocephalans), one of frequent tooth replacement, was possible once a dental lamina had evolved, and since 1982 this has been known as the odontode regulation theory after Reif. Today, Reif's concepts need to be transformed into those of modern biology, the crosstalk between epithelium and mesenchyme, for the regulation of timing, spacing and shape of vertebrate teeth. Although Reif's proposed ‘primordial tissue’ may be the only site of progenitor cells, to restrict odontogenic potential to time-specific sites (protogerms), as has been suggested in the sequential addition tooth (SAT) model, very little data are available. Here, his model of alternate tooth replacement files has been interpreted as an integrated tooth addition unit of two adjacent files (SAT) unit for alternate replacement of teeth, regulated by putative, precisely timed gene expression for activation and inhibition. We have provided new data on patterns of tooth succession in dentitions of extant sharks and rays to compare with those of Reif. Using a phylogeny combined from molecular and morphological data, it is suggested that the alternate tooth addition and replacement model is derived within Chondrichthyes, and diversified from single file tooth addition of the stem chondrichthyans.

Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes

Acanthodians, an exclusively Palaeozoic group of fish, are central to a renewed debate on the origin of modern gnathostomes: jawed vertebrates comprising Chondrichthyes (sharks, rays and ratfish) and Osteichthyes (bony fishes and tetrapods). Acanthodian internal anatomy is primarily understood from Acanthodes bronni because it remains the only example preserved in substantial detail, central to which is an ostensibly osteichthyan braincase. For this reason, Acanthodes has become an indispensible component in early gnathostome phylogenies. Here we present a new description of the Acanthodes braincase, yielding new details of external and internal morphology, notably the regions surrounding and within the ear capsule and neurocranial roof. These data contribute to a new reconstruction that, unexpectedly, resembles early chondrichthyan crania. Principal coordinates analysis of a character-taxon matrix including these new data confirms this impression: Acanthodes is quantifiably closer to chondrichthyans than to osteichthyans. However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes. As such, perceived chondrichthyan features of the Acanthodes cranium represent shared primitive conditions for crown group gnathostomes. Moreover, this increasingly detailed picture of early gnathostome evolution highlights ongoing and profound anatomical reorganization of vertebrate crania after the origin of jaws but before the divergence of living clades.

A New Paleozoic Symmoriiformes (Chondrichthyes) from the Late Carboniferous of Kansas (USA) and Cladistic Analysis of Early Chondrichthyans

BackgroundThe relationships of cartilaginous fishes are discussed in the light of well preserved three-dimensional Paleozoic specimens. There is no consensus to date on the interrelationship of Paleozoic chondrichthyans, although three main phylogenetic hypotheses exist in the current literature: 1. the Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are grouped along with the modern sharks (neoselachians) into a clade which is sister group of holocephalans; 2. the Symmoriiformes are related to holocephalans, whereas the other Paleozoic shark-like chondrichthyans are related to neoselachians; 3. many Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are stem chondrichthyans, whereas stem and crown holocephalans are sister group to the stem and crown neoselachians in a crown-chondrichthyan clade. This third hypothesis was proposed recently, based mainly on dental characters.Methodology/Principal FindingsOn the basis of two well preserved chondrichthyan neurocrania from the Late Carboniferous of Kansas, USA, we describe here a new species of Symmoriiformes, Kawichthys moodiei gen. et sp. nov., which was investigated by means of computerized X-ray synchrotron microtomography. We present a new phylogenetic analysis based on neurocranial characters, which supports the third hypothesis and corroborates the hypothesis that crown-group chondrichthyans (Holocephali+Neoselachii) form a tightly-knit group within the chondrichthyan total group, by providing additional, non dental characters.Conclusions/SignificanceOur results highlight the importance of new well preserved Paleozoic fossils and new techniques of observation, and suggest that a new look at the synapomorphies of the crown-group chondrichthyans would be worthwhile in terms of understanding the adaptive significance of phylogenetically important characters.

The postorbital palatoquadrate articulation in elasmobranchs

Although modern hexanchiforms are the only extant elasmobranchs with a postorbital articulation, according to most morphological and molecular cladistic analyses they are not basal, suggesting that Huxley (1876 Proc Zool Soc 1876;24-59) correctly identified this articulation as "an altogether secondary connection." A postorbital articulation is present in many Paleozoic sharks, but differs from that found in hexanchiforms in its morphology, topographic position on the braincase, and inferred ontogenetic origins. Furthermore, a postorbital articulation is absent in hybodonts (the putative extinct sister group to neoselachians). It is proposed that the term amphistylic should be restricted to the modern hexanchiform condition, where the articular facet is located on the primary postorbital process. An identical articulation probably existed in some extinct galeomorphs (e.g., daggerSynechodus dubrisiensis, daggerParaorthacodus), but is not widespread within elasmobranchs generally. The term archaeostylic ("ancient pillar") is proposed here for the suspensorial arrangement in Paleozic sharks with a postorbital articulation on the ventrolateral part of the lateral commissure. Such an articulation is not known in other gnathostomes and may represent a basal chondrichthyan synapomophy (especially if daggerPucapampella is a stem chondrichthyan), suggesting that the autodiastylic pattern is not primitive for chondrichthyans and that holocephalans have secondarily lost a postorbital articulation. The amphistylic condition may have arisen from the archaeostylic, or it could have been acquired independently within neoselachians, but in either case it is most parsimoniously viewed as apomorphic.

Jaws and teeth of the earliest bony fishes

Extant jawed vertebrates, or gnathostomes, fall into two major monophyletic groups, namely chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). Fossil representatives of the osteichthyan crown group are known from the latest Silurian period, 418 million years (Myr) ago, to the present. By contrast, stem chondrichthyans and stem osteichthyans are still largely unknown. Two extinct Palaeozoic groups, the acanthodians and placoderms, may fall into these stem groups or the common stem group of gnathostomes, but their relationships and monophyletic status are both debated. Here we report unambiguous evidence for osteichthyan characters in jaw bones referred to the late Silurian (423-416-Myr-old) fishes Andreolepis hedei and Lophosteus superbus, long known from isolated bone fragments, scales and teeth, and whose affinities to, or within, osteichthyans have been debated. The bones are a characteristic osteichthyan maxillary and dentary, but the organization of the tooth-like denticles they bear differs from the large, conical teeth of crown-group osteichthyans, indicating that they can be assigned to the stem group. Andreolepis and Lophosteus are thus not only the oldest but also the most phylogenetically basal securely identified osteichthyans known so far.

HEADS AND TAILS: A CHORDATE PHYLOGENY.

Abstract- A cladistic analysis of chordates is presented, based on some 320 nested characters. All the principal higher taxa are defined by synapomorphies, including extinct acanthodians and placoderms. The data base draws broadly from adult anatomy (including osteological data for Recent and fossil taxa), embryology, physiology, and biochemistry. A conventional sequence of chordate higher taxa is generated (hemichordates, urochordates, cephalochordates, craniates). Among the craniates, cyclostomes are considered paraphyletic. Gnathostomes are monophyletic, but two fossil "agnathan" groups (galeaspids, osteostracans) are regarded as stem gnathostomes. Chondrichthyans and osteichthyans are monophyletic. New arguments for osteichthyan affinity of acanthodians are presented. The phylogenetic position of placoderms is still problematic, but they can no longer be perceived as stem chondrichthyans or even as "elasmobranchiomorphs." Recent dipnoans and tetrapods are sister groups, but new paleontological discoveries refute many of their supposed osteological synapomorphies, thereby reopening the possibility of a closer relationship between tetrapods and osteolepiform rhipidistians.