Determination Methods for the Exoskeletal Remains of Early Vertebrates

Abstract

The exoskeleton, consisting of micromeric elements (odontodes) and their derivatives, is characteristic of the most ancient vertebrates. Great morphological and histological variability of discrete exoskeletal microremains makes it difficult to identify them. It is necessary to study not only separate scales or tesserae, but also to get a picture of the squamation in general, because species determined from discrete elements are understood as an assemblage of morphological types. For determination of discrete exoskeletal elements, their morphology, internal structure, defined tissue types of crown and basal plate, types (way) of their growth, system of vascular canals should be studied in addition changes occuring during ontogenetic development of both the dermal skeletal elements and the squamation should be taken in consideration. The material of different groups of early vertebrates (astraspids, tesakoviaspids, heterostracans, thelodonts, mongolepids, chondrichthyans and acanthodians), which were widely distributed in the Early Palaeozoic, are used as examples.

From body scale ontogeny to species ontogeny: Histological and morphological assessment of the Late Devonian acanthodian Triazeugacanthus affinis from Miguasha, Canada

“…As a result the number of rows as well as the number of scales per row increases during the ontogeny of Triazeugacanthus . This information refutes the hypothesis suggested by Karatajute-Talimaa [88] that the number of scales remained stable during the ontogeny of acanthodians.…”

“…Such variations, revealed by the alternation of short dark zones (rest zones) and large light zones (growth zones), occurred also during Triazeugacanthus life (Fig 3). Based on the growth of complete Triazeugacanthus specimens and that of isolated elements, we showed that the individual growth of scales is correlated to species ontogeny as suggested by Zidek [19] and Karatajute-Talimaa [88]; this is in contrast to Valiukevičius [32] who mentioned that the number of growth lamellae do not reflect the developmental stage of the animal. “box-in-box” growing scales are reliable proxies of species growth.…”

“…each layer is not a separate odontode) but rather an accretion around an initial unit; however, 3D microanatomical and histological data would be necessary to clarify this issue (see [2]). Based on our topology, the “box-in-box” growth pattern, occurring at the base of the total-group chondrichthyan, was replaced by polyodontode scales growing through appositional and areal growth in putative chondrichthyans [12, 29, 86, 87] and chondrichthyans [83, 88]. However, the polyodontode growth was already present prior to the origin of the total-group chondrichthyans since it is present in osteostracans [89, 90], in placoderms [91] and also in basal osteichthyans [2].…”

Scales and Tooth Whorls of Ancient Fishes Challenge Distinction between External and Oral ‘Teeth’

“…Two major processes have been proposed to describe their growth pattern: areal growth (in an upward direction, causing the crown to become thicker) and superpositional growth (in dorsoventral or anteroposterior directions, causing the crown to become wider) [35], [39]. The thin section in Figure 2D indicates that odontode (O2) grows both superpositionally and areally relative to odontode (O1).…”

The enameloid microstructure of euselachian (Chondrichthyes) scales

“…R e m a r k s. According to Reif (1978), the hyb odontid morphogenetic scale type compensates for the growth of the animal by both addition of new scales in the skin and growth of some scales by addi tion of consecutive odontodes (sensu Ørvig, 1967; see also Karataj t Talimaa, 1998). Thus, the squama tion of hybodontids contains nongrowing (single odontode) scales and compound growing scales (fol lowing terminology of Reif, 1978; see also Karataj t Talimaa, 1992,1998.…”