Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

ACTA UNIVERSITATIS LATVIENSIS, 2004, Vol. 679, pp. 14-21
Microrelief on the exoskeleton of some early
osteostracans (Agnatha): preliminary analysis of
its significance
OLGA B. AFANASSIEVA
Olga B. Afanassieva, Paleontological Institute of Russian Academy of Sciences, 123,
Profsoyuznaya St., Moscow 117997, Russia; oafan@paleo.ru
The surface of the osteostracan exoskeleton has been studied using the SEM on isolated
microremains, and small fragments taken from complete cephalothoracic shields. The material
comes from the Silurian and Lower Devonian deposits of Severnaya Zemlya Archipelago, Russia,
and Saaremaa Island, Estonia. Imprints of epidermal cells on the exoskeleton surface are described
for the first time in osteostracans. It is concluded that the sculpture on the osteostracan exoskeleton,
both macrosculpture and microsculpture, reflects processes of the probable mode of ossification
of the osteostracan hard cover. On the other hand, various types of microsculpture (microtubercles,
fine ribs or stripes, microapertures) in general are related to the functional peculiarities responsible
for animals’ adaptation to the ambient environment, and were necessary for the implementation
of metabolic processes in different covering tissues of early vertebrates.
Key words: Palaeozoic agnathans, osteostracans, exoskeleton, surface sculpture.
Introduction
In the last few decades considerable progress has been made in the study of the dermal
skeleton of Palaeozoic vertebrates. Use of the scanning electron microscope (SEM)
produced new interesting data on the exoskeleton microstructure of different groups,
including the fine sculpture of the exoskeleton surface (Smith 1977; Schultze 1977;
Deryck and Chancogne-Weber 1995; Märss 2002; Beznosov 2003; also see references
in Märss 2002). For instance, fine sculptural elements, about ten microns in diameter,
were found on the exoskeleton surface in different groups (in chondrichthyans,
acanthodians, and dipnoans) and were explained as imprints of the epidermal cells of
integument. However, for osteostracans little is known about the exoskeleton microrelief,
and there are no special papers on the subject. The present paper attempts to analyze
some of the relevant data for the osteostracan exoskeleton.
Material and methods
Isolated microremains of exoskeleton, extracted by V.N. Karatajute-Talimaa from rock
samples by dissolving with formic acid, were determined as Oeselaspis pustulata
(Patten), Tremataspis obruchevi Afanassieva et Karatajute-Talimaa, Tremataspis cf.
schmidti, T. cf. milleri, and Tremataspis sp. (Afanassieva and Märss 1999; Afanassieva
2000). The material comes from the Ust’-Spokojnaya Formation, Ludlow, Upper Silurian
of October Revolution Island, Severnaya Zemlya Archipelago, Russia.