A Silurian placoderm with osteichthyan-like marginal jaw bones

Abstract

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.

Copulation in antiarch placoderms and the origin of gnathostome internal fertilization

“…These structures are not associated with pelvic fins. The clasper morphology resembles that of ptyctodonts, a more crownward placoderm group [7][8] , suggesting that all placoderm claspers are homologous and that internal fertilization characterized all placoderms. This implies that external fertilisation and spawning, which characterizes most extant aquatic gnathostomes, must be derived from internal fertilisation, even though this transformation has been thought implausible.…”

“…The discovery in arthrodiran 3 and ptyctodont placoderms 2,4 of copulatory structures (claspers) originally 3, 12 but incorrectly 4 interpreted as modified pelvic fins implied that any placoderms lacking pelvic appendages would have also lacked claspers, and therefore reproduced externally by spawning. This was thought to be the case with the antiarchs, recently considered the sister group of all other jawed vertebrates [5][6][7][8] . Deposits where small antiarch juveniles suffered mass mortality have been interpreted as nurseries associated with mass spawning events 13 .…”

“…It is commonly argued that internal fertilization can evolve from external, but not the reverse. Male copulatory claspers are present in certain placoderms [2][3][4] , fossil jawed vertebrates retrieved as a paraphyletic segment of the gnathostome stem group in recent studies [5][6][7][8] . This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups.…”

“…This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups. The reproductive biology of antiarchs, consistently identified as the least crownward placoderms [5][6][7][8] and thus of great interest in this context, has until now remained unknown. Here we show that certain antiarchs possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation.…”

Une toute nouvelle tête pour l’ancêtre des vertébrés à mâchoires

“…Comprendre la démarche en biologie évolutive Un fossile de placoderme, Entelognathus primordialis, découvert en Chine et récemment décrit dans la revue Nature [1] vient d'apporter un soutien convaincant à une hypothèse contre-intuitive : l'organisation de la tête de l'ancêtre des gnathostomes (les vertébrés à mâchoires) actuels ressemble plus à celle des ostéichthyens (les vertébrés osseux, un groupe qui comprend l'homme) qu'à celle des chondrichthyens (les « poissons cartilagineux », comprenant entre autres les requins) (Figure 1). Ce fossile est exceptionnel par la qualité de sa conservation, son âge (environ 419 millions d'années), mais surtout par l'hypothèse évolutive que l'on peut déduire de sa comparaison avec d'autres vertébrés, hypothèse qui heurte le sens commun.…”