Biomedical subjects

Moya Meredith Smith

Publications and source records attributed to Moya Meredith Smith.

3 recordsLinked to original sources

PubMed2003

Separate evolutionary origins of teeth from evidence in fossil jawed vertebrates.

Placoderms are extinct jawed fishes of the class Placodermi and are basal among jawed vertebrates. It is generally thought that teeth are absent in placoderms and that the phylogenetic origin of teeth occurred after the evolution of jaws. However, we now report the presence of tooth rows in more derived placoderms, the arthrodires. New teeth are composed of gnathostome-type dentine and develop at specific locations. Hence, it appears that these placoderm teeth develop and are regulated as in other jawed vertebrates. Because tooth development occurs only in derived forms of placoderms, we suggest that teeth evolved at least twice, through a mechanism of convergent evolution.

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PubMed2002

Developmental constraints conserve evolutionary pattern in an osteichthyan dentition.

The lungfish dentition is different from other osteichthyan fish because it has a characteristic and unique pattern of teeth arranged as toothplates. Growth, addition of teeth, and retention as part of a statodont dentition are determined by the initiation pattern. In adult lungfish new teeth are only added laterally to each radial row in the dentition. This is in marked contrast to marginal rows of teeth with regular, alternating replacement in most osteichthyans. We analyze development from fossil hatchling forms of the Late Devonian dipnoan Andreyevichthys and compare with those of Neoceratodus, the Australian lungfish. The specific pattern of development, unique within lungfish, is also present in the transitory, marginal, anterior dentition in both, reflecting a strongly conserved developmental pattern. These marginal teeth form but are then lost in both, so that also this program of development is conserved within lungfish for 360 million years, from the earliest known form.

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PubMedDate not supplied

Vertebrate dentitions at the origin of jaws: when and how pattern evolved.

New evidence shows that teeth evolved with a greater degree of independence from jaws than previously considered. Pharyngeal denticles occur in jawless fish and also in early gnathostomes and precede jaw teeth in phylogeny. Many of these denticles form joined polarized sets on each branchial arch; these resemble whorl-shaped tooth sets on the jaws of stem and crown gnathostomes and are proposed as homologous units. Therefore, the source of patterning of these pharyngeal denticle and tooth sets is conserved from jawless conditions. It is proposed that developmental regulatory systems, responsible for all such tooth patterns on the jaws, are co-opted from the pharyngeal region and not from the skin as classically understood. This strongly implicates embryonic endoderm as opposed to ectoderm in the genetic control of dentition patterning. New interpretations of ontogenetic data on patterning dentitions of extant sharks are proposed, together with those of osteichthyan fish. Two entirely fossil groups, placoderms and acanthodians, at the base of gnathostome phylogeny are reassessed on the basis of a new model. It is concluded that within stem group and crown group gnathostomes several different strategies, unique to each taxon, were adopted to produce different developmental models of dentition patterning from pharyngeal denticles. One shared developmental pattern is that of initiation from primordial tooth sites, independently in each dentate zone of the jaws. The new model is proposed as a framework for data on evolutionary developmental genetics.

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