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

Olga Afanassieva. Microrelief on the exoskeleton of early osteostracans
19
fields. These structures (or perforated septa in species with a well-developed exoskeleton),
connected with the sensory system, are typical for most of the members of the suborder
Tremataspidoidei (Tremataspis, Dartmuthia, Saaremaaspis, Oeselaspis, Procephalaspis,
Thyestes, Aestiaspis, Septaspis). It should be noted that exoskeletal microstructure of
Sclerodus and Tyriaspis (possible Tremataspidoidei) has never been investigated, and in
Witaaspis similar structures were not found (Afanassieva 1991). In my opinion their
absence in Witaaspis is probably due to incomplete exoskeletal development in this form
(the thin cephalothoracic shield is composed only of a part of the middle and basal layers).
In Thyestes verrucosus a large number of pore fields is located on the surface of the
shield and on the slopes of large and medium-sized tubercles. As a rule, no trace of the
polygonal pattern typical of osteostracans is observed. I studied the cephalothoracic
shield of Thyestes verrucosus (specimen PIN 1628/31), in which, as supposed, the
processes of dermal ossification have not been completed. The material comes from the
Viita or the Vesiku Beds of the Rootsiküla Regional Stage. In the posterolateral parts of
the dorsal side of the shield radiating canals were found opening on the surface of the
exoskeleton (Fig. 2 C). It has been determined that pore fields on the slopes of large
tubercles are aligned in rows along radiating canals (Fig. 2 D). Distal parts of these
canals, open from above, form a pattern, typical of osteostracans, and determine
approximate borders of “tesserae” of various sizes. It is assumed that the large tubercles
of longitudinal rows (along the ribs of rigidity of the dorsal shield) emerged first. The
formation of the exoskeleton began with the laying of dentine tips of the tubercles, and
proceeded centripetally. Middle-sized tubercles with thin tips were formed between
them. Every tubercle was laid in the center of an individual “tessera”. Finally, small
tubercles emerged last in ontogenesis, which is proved by their location on the slopes
of larger tubercles. The exoskeleton of Thyestes verrucosus developed relatively rapidly
but slower than in species of Tremataspis. The existence of a system of units (tesserae),
gradually increasing in size, allowed the individual to grow during a longer period of
time up to complete consolidation of the shield, and also distributed the burden on the
organism resulting from a rapid process of shield formation (Afanassieva 2002).
In Oeselaspis pustulata (Patten) the tops of large tubercles are capped with a thick
layer of enameloid tissue and mesodentine (Denison 1951b). Usually the surface of
large tubercles is smooth (Fig. 2 E). The microfragment of the cephalothoracic shield of
Oeselaspis pustulata (specimen PIN 4765/65) is distinguished from the others by the
surface sculpture of one of the large tubercles (Fig. 2 F). A part of the largest tubercle
Fig. 2. A-D, Thyestes verrucosus Eichwald, specimen PIN 1628/31, dorsal part of cephalothoracic
shield; Viita or Vesiku Beds of Rootsiküla Regional Stage, Upper Wenlockian, Lower Silurian;
Saaremaa Island, Estonia; A, fine ribbing on the surface of small tubercle; B, fine ribbing on the
lower part of the medium-size tubercle with broken apical part; C, tubercles of different sizes
and open radiating canals on the surface of the shield; D, pore fields on the slope of the large
tubercle lining up along radiating canals. E, F, Oeselaspis pustulata (Patten), specimen PIN
4765/65, microfragment of cephalothoracic shield; ?upper part of the Samojlovich Formation,
Upper Wenlock, Lower Silurian; sample 5D/76, locality Sosednii, Jungsturm Strait, Pioneer
Island, Severnaya Zemlya Archipelago, Russia; E, smooth surface of the large tubercle; F, surface
of the horizontal section of the large tubercle as a result of acid-etching (or/and abrasion).