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

Olga Afanassieva. Microrelief on the exoskeleton of early osteostracans
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arranged in longitudinal rows (diameter at the base about 1 mm), tubercles of medium
size with straight tips (about 0,3 mm), and numerous small (0,03-0,15 mm) ribbed tubercles
(Afanassieva 1985). A row (sometimes two or three rows) of shiny flattened tubercles is
arranged along the anterior and lateral edges of the cephalic shield. As in Ungulaspis
arctoa, thin ribbing of Thyestes verrucosus is located on the surface of the tubercles of
small size, in the lower parts of medium-sized tubercles, and on the surface of the
marginal tubercles. The hypermineralized tissue of the superficial layer is present only
in the apical parts (tips) of the large tubercles, of medium-sized tubercles, and of marginal
ones. The basal layer is poorly developed. The exoskeleton mainly consists of relatively
friable bony tissue of the middle layer, with numerous cavities of various sizes.
The assumption that the exoskeleton of this type was covered by soft tissue (Stensiö
1927, 1932) is supported by the position of the elements of the sensory system in the
covering of Thyestes verrucosus (Denison 1951a, b). In my opinion, the arrangement of
the main sensory lines in the exoskeleton of this species is marked by the elongated
tubercles of medium size, disposed in pairs along the conjectural sensory canals
(Afanassieva 1985, 1991, pl. VI, fig. 6). The set of the main sensory lines and their
distribution pattern, marked by the tubercles of that type, are characteristic of this
osteostracan group. It is possible to identify the infraorbital, postorbital, transversal
and main lateral sensory lines in the Thyestes exoskeleton. The sensory lines of Thyestes
were located between the “sensory” tubercles, i.e. superficially to the exoskeleton. In
addition, I suppose that variously sized cavities at the bases of marginal flattened
tubercles in Thyestes verrucosus and Aestiaspis viitaensis contained soft tissues of the
cutaneous covering.
Thus an analysis of mutual arrangement of the exoskeletal structures in Thyestes
verrucosus reveals that the external skeleton was covered by soft tissue. In this
connection it is important to note that the fine ribbing is found on the surface of the
exoskeletal structures that were surrounded by soft tissue (small tubercles, lower parts
of medium-sized tubercles and of marginal ones). I suppose the fine ribbing was used
for optimal conjunction between the layers of the soft and hard tissues within the cover.
The same assumption stands for the external skeleton of Ungulaspis, Aestiaspis,
Septaspis and other osteostracans, and early vertebrates, with the similar type of
exoskeleton microrelief.
As is known, numerous microapertures (2-5 microns in diameter) are located on the
surface of the exoskeleton of some early osteostracans. They are often grouped in pore
Fig. 1. A-D, surface of the Tremataspis exoskeleton: A, smooth surface of Tremataspis cf. milleri
Patten, specimen PIN 4765/30, Ust’-Spokojnaya Formation, Ludlow, Upper Silurian; 14, locality
47, Spokoinaya River, October Revolution Island, Severnaya Zemlya Archipelago, Russia; B,
C, Tremataspis sp., specimen PIN 4765/20, Ust’-Spokojnaya Formation, Ludlow, Upper Silurian;
locality 31 (talus), Ushakov River, October Revolution Island; D, Tremataspis milleri, fragment
of the specimen PIN 4219/ 7, Kuusnymme Beds of Rootsiküla Regional Stage, Upper Wenlockian,
Lower Silurian; Elda Cliff, Saaremaa Island, Estonia. E, F, Ungulaspis arctoa Afanassieva et
Karatajute-Talimaa, Severnaya Zemlya Formation, Lochkovian, Lower Devonian; Pod’emnaya
River, October Revolution Island, Severnaya Zemlya Archipelago, Russia; microfragment (PIN
4766/1) of the holotype LIG 35-670; tubercles with fine ribbing on the surface of the cephalic
shield.