Data agreement and model geometry x ray3/28/2023 ![]() The regions of interest were scanned in I12 Experimental Hutch One using the beamline's modular imaging system. The X-ray beam was set to a monochromatic energy of 90 keV (double bent Laue Si 111 monochromator). We performed synchrotron X-ray micro-computed tomography at the I12 beamline of the Diamond Light Source, United Kingdom. Furthermore, we clarify the evolution of gnathostome dentitions by revising the interpretation of unusual dental morphology in a previously described ‘acanthothoracid’. This allows us to reconstruct and compare the orientation and angle of the bite in these ‘placoderms’, and we show that a highly similar jaw articulation and bite occurs between three groups of ‘placoderms’ with disparate presumed ecomorphologies: ‘acanthothoracids’, rhenanids and arthrodires, suggesting conservation of a shared ancestral morphology among these groups. The specimen reveals the most complete quadrate (articular condyle connecting the lower jaw) of any ‘acanthothoracid’ known to date. Here we describe a nearly complete, fully three-dimensional upper jaw (palatoquadrate and suborbital plate) of an ‘acanthothoracid’ from the Early Devonian (Pragian) of Mongolia. Understanding the relationship of ‘placoderm’ dentitions as well as overall jaw morphology and functional diversity are thus key to understanding the origin of modern jaws and teeth. More recently, it has been proposed that some ‘acanthothoracids’ bore marginal teeth aligned in rows along the jaw edge, as in most crown-group gnathostomes. Despite this simplicity, ‘placoderm’ dentitions are diverse, and detailed anatomical investigations in recent decades have highlighted similarities between some ‘placoderm’ groups (such as arthrodires) and crown-group gnathostomes. Most ‘placoderm’ dentitions were relatively simple: tubercles added in rows (or files) or centripetally to growing jaw plates. These range from diverse biting modes, suspension feeding and possible grazing in arthrodires durophagy in ptyctodonts ambush predation in rhenanids and benthic (detritus?) feeding in antiarchs. ‘Placoderms’ exhibit diverse jaw morphologies and presumed feeding ecologies. Owing to a lack of fossilized mandibular arch material in jawless fishes, the primitive structure of the gnathostome jaw has been difficult to reconstruct using palaeontological evidence. This has transformed perspectives of both jaw and tooth evolution, which have long been modelled on shark-like conditions. There is now a growing consensus that the last common ancestor of crown gnathostomes possessed dermal jaw bones, a trait potentially extending to the earliest appearance of jaws. Recent discoveries from the Silurian and Early Devonian of south China and Czechia have delivered significant advances that diminish the morphological gap between ‘placoderm’ and crown-group vertebrate jaw morphology. ‘Placoderms’ occupy a central role in debates on the origin of both jaws and teeth-major gnathostome innovations that led to diverse feeding ecologies. They are thus key to reconstructing character transitions leading to modern (crown-group) gnathostomes. Although their monophyly is highly debated, it is widely agreed that they are the only known jaw-bearing stem-group gnathostomes. ‘Placoderms’ are extinct fishes that ranged from the Silurian to the end of the Devonian period (444 to 359 Ma). Irrespective of current phylogenetic uncertainty, the new data here resolve the likely general condition for ‘placoderms’ as a whole, and as such, ancestral morphology of known jawed vertebrates. It appears that ‘acanthothoracid’ dentitions were fundamentally similar in location to that of arthrodire ‘placoderms’, rather than resembling bony fishes. ![]() Incorporation of the dermal skeleton appears to provide a sound biomechanical basis for jaw origins. We clarify that the bite position is located on the upper jaw cartilage rather than on the dermal cheek and thus show that there is a highly conserved bite morphology among most groups of ‘placoderms’, regardless of their overall cranial geometry. Here we describe a near-complete ‘acanthothoracid’ upper jaw, allowing us to reconstruct the likely orientation and angle of the bite and compare its morphology with that of other known ‘placoderm’ groups. The structure of the jaws-particularly the jaw hinge-is poorly known, leaving open questions about their jaw function and comparison with other placoderms and modern gnathostomes. However, they are so far known mainly from disarticulated skeletal elements that are typically incomplete. ‘Acanthothoracids’ are generally considered the most primitive ‘placoderms’. ‘Placoderms’ (Silurian-Devonian armoured jawed fishes) are central to debates on the origins of these anatomical structures. The origin of jaws and teeth remains contentious in vertebrate evolution. ![]()
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