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Olga Afanassieva. Microrelief on the exoskeleton of early osteostracans
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Ungulaspis arctoa, described by Afanassieva and Karatajute-Talimaa (1998), occurs
in the upper part of the Severnaya Zemlya Formation deposits of the Lower Devonian
of October Revolution Island. Exoskeleton microfragments belonging to the holotype
of Ungulaspis arctoa were taken from the anterolateral and marginal parts of the dorsal
shield.
All fragments were studied by the present author using the SEM technique. Complete
and fragmented shields of osteostracans from the collections of the Paleontological
Institute of the Russian Academy of Sciences in Moscow (mainly, Tremataspidoidei
from Silurian of Saaremaa Island) were used for comparison.
Material described in this paper is housed in the Paleontological Institute of the
RAS (PIN) and in the Institute of Geology and Geography of Lithuania in Vilnius
(LIGG).
Description and discussion
The surface of the cephalothoracic shields of Tremataspis species is smooth and shiny
(Robertson 1938; Denison 1951a; Afanassieva and Karatajute-Talimaa 1998). There are
only a few, low tubercles on the dorsal side of the interzonal part of the shield. The
tubercles have smooth, non-perforated walls and tops. The sensory-line system opens
on the surface through the relatively wide pores. The diameter of the pores varies from
10-15 microns (T. milleri Patten) up to 40 microns (T. obruchevi Afanassieva et Karatajute-
Talimaa).
The surface of trunk scales of Tremataspis is usually shiny and smooth (Fig. 1 A).
The scales are thick, with all three layers of exoskeleton well developed. The basal layer
has a typical cross-laminated structure and forms a significant part of the exoskeleton.
No fragments and scales with tubercles were found. Some scales demonstrate an
infrequent type of fine sculpture. On the surface of the scale (specimen PIN 4765/20)
irregular polygons reminding a honeycomb pattern are clearly visible (Fig. 1 B, C). Their
diameter is about 10 microns. Similar ultrasculpture has been found on the surface of
cephalothoracic shields, and of several scales of Tremataspis species from the Silurian
deposits of Saaremaa Island, Estonia (Fig. 1 D).
The surface of the cephalic shield of Ungulaspis arctoa in the anteromedial parts of
the dorsal side are covered with small rounded and elongated tubercles (Afanassieva
and Karatajute-Talimaa 1998; Afanassieva 1999, pl. 1, fig. 1). The tubercle sides are
composed of relatively compact tissue without any foramina. The surface of the tubercles
is covered with fine ribs which form a peculiar microrelief (Fig. 1 E, F). The distance
between the ribs is about 5 microns. The ribs usually meet at the top of the rounded
tubercle and at the apical part (ridge) of the elongated one. Similar ribbing is found on
the side surfaces of the large (about 1 mm in length) elongated tubercles situated along
the anterior and lateral edges of the cephalic shield of Ungulaspis arctoa (Afanassieva
1999, pl. 1, fig. 4).
To clarify the significance of the thin ribbing of the osteostracan exoskeleton we
should focus our attention on the external skeleton of Thyestes verrucosus Eichwald.
The surface of the Thyestes shield is covered with numerous tubercles (Stensiö 1932;
Denison 1951b) that can be divided into three types: large tubercles with curved tips,