The oldest gnathostome teeth.

Mandibular teeth and dentitions are features of jawed vertebrates that were first acquired by the Palaeozoic ancestors1-3 of living chondrichthyans and osteichthyans. The fossil record currently points to the latter part of the Silurian period4-7 (around 425 million years ago) as a minimum date for the appearance of gnathostome teeth and to the evolution of growth and replacement mechanisms of mandibular dentitions in the subsequent Devonian period2,8-10. Here we provide, to our knowledge, the earliest direct evidence for jawed vertebrates by describing Qianodus duplicis, a new genus and species of an early Silurian gnathostome based on isolated tooth whorls from Guizhou province, China. The whorls possess non-shedding teeth arranged in a pair of rows that demonstrate a number of features found in modern gnathostome groups. These include lingual addition of teeth in offset rows and maintenance of this patterning throughout whorl development. Our data extend the record of toothed gnathostomes by 14 million years from the late Silurian into the early Silurian (around 439 million years ago) and are important for documenting the initial diversification of vertebrates. Our analyses add to mounting fossil evidence that supports an earlier emergence of jawed vertebrates as part of the Great Ordovician Biodiversification Event (approximately 485-445 million years ago).

Spiny chondrichthyan from the lower Silurian of South China.

Modern representatives of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods) have contrasting skeletal anatomies and developmental trajectories1-4 that underscore the distant evolutionary split5-7 of the two clades. Recent work on upper Silurian and Devonian jawed vertebrates7-10 has revealed similar skeletal conditions that blur the conventional distinctions between osteichthyans, chondrichthyans and their jawed gnathostome ancestors. Here we describe the remains (dermal plates, scales and fin spines) of a chondrichthyan, Fanjingshania renovata gen. et sp. nov., from the lower Silurian of China that pre-date the earliest articulated fossils of jawed vertebrates10-12. Fanjingshania possesses dermal shoulder girdle plates and a complement of fin spines that have a striking anatomical similarity to those recorded in a subset of stem chondrichthyans5,7,13 (climatiid 'acanthodians'14). Uniquely among chondrichthyans, however, it demonstrates osteichthyan-like resorptive shedding of scale odontodes (dermal teeth) and an absence of odontogenic tissues in its spines. Our results identify independent acquisition of these conditions in the chondrichthyan stem group, adding Fanjingshania to an increasing number of taxa7,15 nested within conventionally defined acanthodians16. The discovery of Fanjingshania provides the strongest support yet for a proposed7 early Silurian radiation of jawed vertebrates before their widespread appearance5 in the fossil record in the Lower Devonian series.

The rapid evolution of lungfish durophagy.

Innovations relating to the consumption of hard prey are implicated in ecological shifts in marine ecosystems as early as the mid-Paleozoic. Lungfishes represent the first and longest-ranging lineage of durophagous vertebrates, but how and when the various feeding specializations of this group arose remain unclear. Two exceptionally preserved fossils of the Early Devonian lobe-finned fish Youngolepis reveal the origin of the specialized lungfish feeding mechanism. Youngolepis has a radically restructured palate, reorienting jaw muscles for optimal force transition, coupled with radiating entopterygoid tooth rows like those of lungfish toothplates. This triturating surface occurs in conjunction with marginal dentition and blunt coronoid fangs, suggesting a role in crushing rather than piercing prey. Bayesian tip-dating analyses incorporating these morphological data indicate that the complete suite of lungfish feeding specializations may have arisen in as little as 7 million years, representing one of the most striking episodes of innovation during the initial evolutionary radiations of bony fishes.

Scale morphology and squamation pattern of Guiyu oneiros provide new insights into early osteichthyan body plan.

Scale morphology and squamation play an important role in the study of fish phylogeny and classification. However, as the scales of the earliest osteichthyans or bony fishes are usually found in a disarticulated state, research into squamation patterns and phylogeny has been limited. Here we quantitatively describe the scale morphology of the oldest articulated osteichthyan, the 425-million-year-old Guiyu oneiros, based on geometric morphometrics and high-resolution computed tomography. Based on the cluster analysis of the scales in the articulated specimens, we present a squamation pattern of Guiyu oneiros, which divides the body scales into 4 main belts, comprising 16 areas. The new pattern reveals that the squamation of early osteichthyans is more complicated than previously known, and demonstrates that the taxa near the crown osteichthyan node in late Silurian had a greater degree of squamation zonation compared to more advanced forms. This study offers an important reference for the classification of detached scales of early osteichthyans, provides new insights into the early evolution of osteichthyan scales, and adds to our understanding of the early osteichthyan body plan.

Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates.

The phylogeny of early gnathostomes provides an important framework for understanding one of the most significant evolutionary events, the origin and diversification of jawed vertebrates. A series of recent cladistic analyses have suggested that the placoderms, an extinct group of armoured fish, form a paraphyletic group basal to all other jawed vertebrates. We revised and expanded this morphological data set, most notably by sampling autapomorphies in a similar way to parsimony-informative traits, thus ensuring this data (unlike most existing morphological data sets) satisfied an important assumption of Bayesian tip-dated morphological clock approaches. We also found problems with characters supporting placoderm paraphyly, including character correlation and incorrect codings. Analysis of this data set reveals that paraphyly and monophyly of core placoderms (excluding maxillate forms) are essentially equally parsimonious. The two alternative topologies have different root positions for the jawed vertebrates but are otherwise similar. However, analysis using tip-dated clock methods reveals strong support for placoderm monophyly, due to this analysis favoring trees with more balanced rates of evolution. Furthermore, enforcing placoderm paraphyly results in higher levels and unusual patterns of rate heterogeneity among branches, similar to that generated from simulated trees reconstructed with incorrect root positions. These simulations also show that Bayesian tip-dated clock methods outperform parsimony when the outgroup is largely uninformative (e.g., due to inapplicable characters), as might be the case here. The analysis also reveals that gnathostomes underwent a rapid burst of evolution during the Silurian period which declined during the Early Devonian. This rapid evolution during a period with few articulated fossils might partly explain the difficulty in ascertaining the root position of jawed vertebrates.

A Silurian maxillate placoderm illuminates jaw evolution.

The discovery of Entelognathus revealed the presence of maxilla, premaxilla, and dentary, supposedly diagnostic osteichthyan bones, in a Silurian placoderm. However, the relationship between these marginal jaw bones and the gnathal plates of conventional placoderms, thought to represent the inner dental arcade, remains uncertain. Here we report a second Silurian maxillate placoderm, which bridges the gnathal and maxillate conditions. We propose that the maxilla, premaxilla, and dentary are homologous to the gnathal plates of placoderms and that all belong to the same dental arcade. The gnathal-maxillate transformation occurred concurrently in upper and lower jaws, predating the addition of infradentary bones to the lower jaw.

Early Gnathostome Phylogeny Revisited: Multiple Method Consensus.

A series of recent studies recovered consistent phylogenetic scenarios of jawed vertebrates, such as the paraphyly of placoderms with respect to crown gnathostomes, and antiarchs as the sister group of all other jawed vertebrates. However, some of the phylogenetic relationships within the group have remained controversial, such as the positions of Entelognathus, ptyctodontids, and the Guiyu-lineage that comprises Guiyu, Psarolepis and Achoania. The revision of the dataset in a recent study reveals a modified phylogenetic hypothesis, which shows that some of these phylogenetic conflicts were sourced from a few inadvertent miscodings. The interrelationships of early gnathostomes are addressed based on a combined new dataset with 103 taxa and 335 characters, which is the most comprehensive morphological dataset constructed to date. This dataset is investigated in a phylogenetic context using maximum parsimony (MP), Bayesian inference (BI) and maximum likelihood (ML) approaches in an attempt to explore the consensus and incongruence between the hypotheses of early gnathostome interrelationships recovered from different methods. Our findings consistently corroborate the paraphyly of placoderms, all 'acanthodians' as a paraphyletic stem group of chondrichthyans, Entelognathus as a stem gnathostome, and the Guiyu-lineage as stem sarcopterygians. The incongruence using different methods is less significant than the consensus, and mainly relates to the positions of the placoderm Wuttagoonaspis, the stem chondrichthyan Ramirosuarezia, and the stem osteichthyan Lophosteus-the taxa that are either poorly known or highly specialized in character complement. Given that the different performances of each phylogenetic approach, our study provides an empirical case that the multiple phylogenetic analyses of morphological data are mutually complementary rather than redundant.

A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians.

Crown or modern sarcopterygians (coelacanths, lungfishes, and tetrapods) differ substantially from stem sarcopterygians, such as Guiyu and Psarolepis, and a lack of transitional fossil taxa limits our understanding of the origin of the crown group. The Onychodontiformes, an enigmatic Devonian predatory fish group, seems to have characteristics of both stem and crown sarcopterygians but is difficult to place because of insufficient anatomical information. We describe the new skull material of Qingmenodus, a Pragian (~409-million-year-old) onychodont from China, using high-resolution computed tomography to image internal structures of the braincase. In addition to its remarkable similarities with stem sarcopterygians in the ethmosphenoid portion, Qingmenodus exhibits coelacanth-like neurocranial features in the otic region. A phylogenetic analysis based on a revised data set unambiguously assigns onychodonts to crown sarcopterygians as stem coelacanths. Qingmenodus thus bridges the morphological gap between stem sarcopterygians and coelacanths and helps to illuminate the early evolution and diversification of crown sarcopterygians.

A Silurian placoderm with osteichthyan-like marginal jaw bones.

The gnathostome (jawed vertebrate) crown group comprises two extant clades with contrasting character complements. Notably, Chondrichthyes (cartilaginous fish) lack the large dermal bones that characterize Osteichthyes (bony fish and tetrapods). The polarities of these differences, and the morphology of the last common ancestor of crown gnathostomes, are the subject of continuing debate. Here we describe a three-dimensionally preserved 419-million-year-old placoderm fish from the Silurian of China that represents the first stem gnathostome with dermal marginal jaw bones (premaxilla, maxilla and dentary), features previously restricted to Osteichthyes. A phylogenetic analysis places the new form near the top of the gnathostome stem group but does not fully resolve its relationships to other placoderms. The analysis also assigns all acanthodians to the chondrichthyan stem group. These results suggest that the last common ancestor of Chondrichthyes and Osteichthyes had a macromeric dermal skeleton, and provide a new framework for studying crown gnathostome divergence.

The earliest known stem-tetrapod from the Lower Devonian of China.

Recent discoveries of advanced fish-like stem-tetrapods (for example, Panderichthys and Tiktaalik) have greatly improved our knowledge of the fin-to-limb transition. However, a paucity of fossil data from primitive finned tetrapods prevents profound understanding of the acquisition sequence of tetrapod characters. Here we report a new stem-tetrapod (Tungsenia paradoxa gen. et sp. nov.) from the Lower Devonian (Pragian, ∼409 million years ago) of China, which extends the earliest record of tetrapods by some 10 million years. Sharing many primitive features with stem-lungfishes, the new taxon further fills in the morphological gap between tetrapods and lungfishes. The X-ray tomography study of the skull depicts the plesiomorphic condition of the brain in the tetrapods. The enlargement of the cerebral hemispheres and the possible presence of the pars tuberalis in this stem-tetrapod indicate that some important brain modifications related to terrestrial life had occurred at the beginning of the tetrapod evolution, much earlier than previously thought.

Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian.

Coelacanths are known for their evolutionary conservatism, and the body plan seen in Latimeria can be traced to late Middle Devonian Diplocercides, Holopterygius and presumably Euporosteus. However, the group's early history is unclear because of an incomplete fossil record. Until now, the only Early Devonian coelacanth is an isolated dentary (Eoactinistia) from Australia, whose position within the coelacanths is unknown. Here we report the earliest known coelacanth skull (Euporosteus yunnanensis sp. nov.) from the Early Devonian (late Pragian) of Yunnan, China. Resolved by maximum parsimony, maximum likelihood and Bayesian analyses as crownward of Diplocercides or as its sister taxon, the new form extends the chronological range of anatomically modern coelacanths by about 17 Myr. The finding lends support to the possibility that Eoactinistia is also an anatomically modern coelacanth, and provides a more refined reference point for studying the rapid early diversification and subsequent evolutionary conservatism of the coelacanths.

Fossil fishes from china provide first evidence of dermal pelvic girdles in osteichthyans.

BACKGROUND: The pectoral and pelvic girdles support paired fins and limbs, and have transformed significantly in the diversification of gnathostomes or jawed vertebrates (including osteichthyans, chondrichthyans, acanthodians and placoderms). For instance, changes in the pectoral and pelvic girdles accompanied the transition of fins to limbs as some osteichthyans (a clade that contains the vast majority of vertebrates--bony fishes and tetrapods) ventured from aquatic to terrestrial environments. The fossil record shows that the pectoral girdles of early osteichthyans (e.g., Lophosteus, Andreolepis, Psarolepis and Guiyu) retained part of the primitive gnathostome pectoral girdle condition with spines and/or other dermal components. However, very little is known about the condition of the pelvic girdle in the earliest osteichthyans. Living osteichthyans, like chondrichthyans (cartilaginous fishes), have exclusively endoskeletal pelvic girdles, while dermal pelvic girdle components (plates and/or spines) have so far been found only in some extinct placoderms and acanthodians. Consequently, whether the pectoral and pelvic girdles are primitively similar in osteichthyans cannot be adequately evaluated, and phylogeny-based inferences regarding the primitive pelvic girdle condition in osteichthyans cannot be tested against available fossil evidence. METHODOLOGY/PRINCIPAL FINDINGS: Here we report the first discovery of spine-bearing dermal pelvic girdles in early osteichthyans, based on a new articulated specimen of Guiyu oneiros from the Late Ludlow (Silurian) Kuanti Formation, Yunnan, as well as a re-examination of the previously described holotype. We also describe disarticulated pelvic girdles of Psarolepis romeri from the Lochkovian (Early Devonian) Xitun Formation, Yunnan, which resemble the previously reported pectoral girdles in having integrated dermal and endoskeletal components with polybasal fin articulation. CONCLUSIONS/SIGNIFICANCE: The new findings reveal hitherto unknown similarity in pectoral and pelvic girdles among early osteichthyans, and provide critical information for studying the evolution of pelvic girdles in osteichthyans and other gnathostomes.

The oldest articulated osteichthyan reveals mosaic gnathostome characters.

The evolutionary history of osteichthyans (bony fishes plus tetrapods) extends back to the Ludlow epoch of the Silurian period. However, these Silurian forms have been documented exclusively by fragmentary fossils. Here we report the discovery of an exceptionally preserved primitive fish from the Ludlow of Yunnan, China, that represents the oldest near-complete gnathostome (jawed vertebrate). The postcranial skeleton of this fish includes a primitive pectoral girdle and median fin spine as in non-osteichthyan gnathostomes, but a derived macromeric squamation as in crown osteichthyans, and substantiates the unexpected mix of postcranial features in basal sarcopterygians, previously restored from the disarticulated remains of Psarolepis. As the oldest articulated sarcopterygian, the new taxon offers insights into the origin and early divergence of osteichthyans, and indicates that the minimum date for the actinopterygian-sarcopterygian split was no later than 419 million years ago.