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THE OLDEST FOSSIL EVIDENCE OF A DENTAL LAMINA IN SHARKS

The presence of a specific tooth-producing tissue, or dental lamina, is a synapomorphy of “higher” gnathostomes (chondrichthyans, acanthodians, and osteichthyans). Because the dental lamina is a soft tissue, that basically consists, of a band of epithelial tissue that produces the tooth buds, it does not become fossilized. However, the pattern of adding teeth from precise positions of the jaw have been considered as evidence of its existence (Reif, 1982). In this sense, the successive tooth families of chondrichthyans or the symphyseal tooth-whorls of acanthodians and osteichthyans are widely accepted as diagnostic of a dental lamina (Reif, 1982; Smith and Coates, 2001). More recently, Smith and Johanson (2003 a, b) suggested that teeth (and thus, the dental lamina) evolved independently at least twice in basal jawed vertebrates. These authors claimed the presence of a homoplastic dental lamina in more derived placoderms (arthrodires) based on the presence of patterned and regulated “true” teeth development from specific locations, although their proposal has been thereinafter discussed (Burrow, 2003; Young, 2003). With respect to chondrichthyans, teeth are organized in families, which include functional and replacement teeth, perpendicularly aligned to the jaw axis. Teeth develop in the lingual part of the family and migrate to the labial part until occupying the optimum functional position. Later, the used tooth is shed (sometimes it can be retained) and this position is occupied by the next tooth of the same family. To date, the in situ tooth families preserved in the early Emsian (Lower Devonian), Doliodus problematicus (Woodward 1892), showed the oldest fossil evidence of a dental lamina in sharks (Miller et al., 2003).