Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ... Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
16 ADVANCES IN PALAEOICHTHYOLOGY A B C D E F
Olga Afanassieva. Microrelief on the exoskeleton of early osteostracans 17 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.
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Olga Afanassieva. Microrelief on the exoskeleton of early osteostracans<br />
17<br />
arranged in longitudinal rows (diameter at the base about 1 mm), tubercles of medium<br />
size with straight tips (about 0,3 mm), <strong>and</strong> numerous small (0,03-0,15 mm) ribbed tubercles<br />
(Afanassieva 1985). A row (sometimes two or three rows) of shiny flattened tubercles is<br />
arranged along the anterior <strong>and</strong> lateral edges of the cephalic shield. As in Ungulaspis<br />
arctoa, thin ribbing of Thyestes verrucosus is located on the surface of the tubercles of<br />
small size, in the lower parts of medium-sized tubercles, <strong>and</strong> on the surface of the<br />
marginal tubercles. The hypermineralized tissue of the superficial layer is present only<br />
in the apical parts (tips) of the large tubercles, of medium-sized tubercles, <strong>and</strong> of marginal<br />
ones. The basal layer is poorly developed. The exoskeleton mainly consists of relatively<br />
friable bony tissue of the middle layer, with numerous cavities of various sizes.<br />
The assumption that the exoskeleton of this type was covered by soft tissue (Stensiö<br />
1927, 1932) is supported by the position of the elements of the sensory system in the<br />
covering of Thyestes verrucosus (Denison 1951a, b). In my opinion, the arrangement of<br />
the main sensory lines in the exoskeleton of this species is marked by the elongated<br />
tubercles of medium size, disposed in pairs along the conjectural sensory canals<br />
(Afanassieva 1985, 1991, pl. VI, fig. 6). The set of the main sensory lines <strong>and</strong> their<br />
distribution pattern, marked by the tubercles of that type, are characteristic of this<br />
osteostracan group. It is possible to identify the infraorbital, postorbital, transversal<br />
<strong>and</strong> main lateral sensory lines in the Thyestes exoskeleton. The sensory lines of Thyestes<br />
were located between the “sensory” tubercles, i.e. superficially to the exoskeleton. In<br />
addition, I suppose that variously sized cavities at the bases of marginal flattened<br />
tubercles in Thyestes verrucosus <strong>and</strong> Aestiaspis viitaensis contained soft tissues of the<br />
cutaneous covering.<br />
Thus an analysis of mutual arrangement of the exoskeletal structures in Thyestes<br />
verrucosus reveals that the external skeleton was covered by soft tissue. In this<br />
connection it is important to note that the fine ribbing is fo<strong>un</strong>d on the surface of the<br />
exoskeletal structures that were surro<strong>un</strong>ded by soft tissue (small tubercles, lower parts<br />
of medium-sized tubercles <strong>and</strong> of marginal ones). I suppose the fine ribbing was used<br />
for optimal conj<strong>un</strong>ction between the layers of the soft <strong>and</strong> hard tissues within the cover.<br />
The same assumption st<strong>and</strong>s for the external skeleton of Ungulaspis, Aestiaspis,<br />
Septaspis <strong>and</strong> other osteostracans, <strong>and</strong> early vertebrates, with the similar type of<br />
exoskeleton microrelief.<br />
As is known, numerous microapertures (2-5 microns in diameter) are located on the<br />
surface of the exoskeleton of some early osteostracans. They are often grouped in pore<br />
Fig. 1. A-D, surface of the Tremataspis exoskeleton: A, smooth surface of Tremataspis cf. milleri<br />
Patten, specimen PIN 4765/30, Ust’-Spokojnaya Formation, Ludlow, Upper Silurian; 14, locality<br />
47, Spokoinaya River, October Revolution Isl<strong>and</strong>, Severnaya Zemlya Archipelago, Russia; B,<br />
C, Tremataspis sp., specimen PIN 4765/20, Ust’-Spokojnaya Formation, Ludlow, Upper Silurian;<br />
locality 31 (talus), Ushakov River, October Revolution Isl<strong>and</strong>; D, Tremataspis milleri, fragment<br />
of the specimen PIN 4219/ 7, Kuusnymme Beds of Rootsiküla Regional Stage, Upper Wenlockian,<br />
Lower Silurian; Elda Cliff, Saaremaa Isl<strong>and</strong>, Estonia. E, F, Ungulaspis arctoa Afanassieva et<br />
Karatajute-Talimaa, Severnaya Zemlya Formation, Lochkovian, Lower Devonian; Pod’emnaya<br />
River, October Revolution Isl<strong>and</strong>, Severnaya Zemlya Archipelago, Russia; microfragment (PIN<br />
4766/1) of the holotype LIG 35-670; tubercles with fine ribbing on the surface of the cephalic<br />
shield.