Exoskeletal ultrasculpture of early vertebrates

ABSTRACT

For the first time, systematic treatment of the ultrasculpture on the exoskeleton of Palaeozoic Agnatha (Thelodonti, Heterostraci, and Osteostraci) is given, supplemented with a discussion of the main published sources on Gnathostomata, such as Acanthodii, Chondrichthyes, and Osteichthyes. Seven main types of ultrasculpture have been determined: (1) fine longitudinal striation; (2) transverse lamellae; (3) short V-shaped slits and branching grooves; (4) polygons separated by walls; (5) polygons separated by intercellular grooves; (6) micronodules; and (7) microtubercles. Five of these types occur on the scales of thelodonts. No ultrasculpture has been found on sandiviiform and loganelliiform thelodonts, anaspids, some Ordovician taxa, and placoderms. A smooth surface might be the most primitive condition for the scale surface. Fine longitudinal striation, transverse lamellae, and polygons separated by walls are the features connected with enameloid tissue. Polygons separated by intercellular grooves may characterize both enameloid and enamel. Microtubercles (as has been shown earlier) characterize true enamel. All ultrasculpture types with certain different features occur in more than one higher taxonomic group. Among Thelodonti these types mirror the classification of the subclass into four higher-rank taxonomic groups. Relationships between thelodonts and chondrichthyans based on ultrasculpture characteristics are much more complicated than previously discussed. Four types of ultrasculpture, found among thelodonts, occur also on the scales of chondrichthyans and/or putative chondrichthyans. Different ultrasculpture types might also point to the polyphyletic nature of the Thelodonti.