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 ...
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112 ADVANCES IN PALAEOICHTHYOLOGY<br />
they are loosely attached to the head shield. Only three of the shields are complete<br />
consisting of <strong>un</strong>paired Nu, Pp <strong>and</strong> Prm plates, as well as paired Pn, Pmg <strong>and</strong> La<br />
plates. Two head shields lack Pmg from one side, <strong>and</strong> four more shields are without<br />
Pmg plates from both sides. Three head shields have lost the anterior most portion,<br />
<strong>and</strong> the Prm plate is not present. In some cases (specimens LDM 81/283, 81/726) it<br />
seems that the head shield has been disarticulated after burial, since plates are<br />
displaced <strong>and</strong> situated at a distance of 1 to 3 mm from each other. One of the best<br />
preserved portions of the tr<strong>un</strong>k shield is specimen LDM 81/434, consisting of<br />
articulated right ADL, AVL, Cd1 <strong>and</strong> Cv1. Besides this specimen, there are 18<br />
examples of articulated AVL plates <strong>and</strong> proximal segments of pectoral fin. Specimen<br />
No. LDM 81/716, for example, shows articulated AVL plates on both sides with<br />
connected proximal segments. Another seven specimens are articulated proximal<br />
segments of pectoral fin alone. Only two specimens, LDM 81/58-60 <strong>and</strong> 81/83, have<br />
a partially preserved dorsal wall of the tr<strong>un</strong>k shield. Three specimens (LDM 81/41,<br />
81/81 <strong>and</strong> 81/82) constitute an incomplete ventral wall of the tr<strong>un</strong>k shield. By co<strong>un</strong>ting<br />
articulated or partly articulated portions of shields of Bothriolepis one may infer<br />
that the disintegration of placoderm fish skeletons took place quite close to the<br />
place of burial, <strong>and</strong> that the final stages of disintegration continued even after<br />
transportation was finished.<br />
Sarcopterygian <strong>and</strong> tetrapod remains are represented with some completely<br />
disarticulated bones of the shoulder girdle <strong>and</strong> skull, as well as rare scales or sometimes<br />
teeth, but more or less complete lower jaws have often been fo<strong>un</strong>d, e.g. three partial<br />
lower jaws of Holoptychius cf. nobilissimus, five jaws of Cryptolepis grossi, <strong>and</strong> 14<br />
complete or partially complete jaws of Ventastega curonica. There are two almost<br />
complete specimens representing tetrapod cheeks, <strong>and</strong> one skull roof of Ventastega,<br />
which was fo<strong>un</strong>d at a distance of some 45 cm from the left side cheek. Judging from the<br />
size <strong>and</strong> shape of broken edges, both skull <strong>and</strong> cheek belong to the same individual.<br />
Skeletal element representation. The sorting of remains according to their shape,<br />
size <strong>and</strong> weight has been observed. Among the remains, medium-sized (4-6 cm long)<br />
bones prevail, smaller remains such as scales of Cryptolepis grossi are<br />
disproportionally rare, but larger ones such as lower jaws of Ventastega curonica<br />
(about 21 cm long) are considerably more frequently fo<strong>un</strong>d than the other skeletal<br />
elements of the animal (Fig. 7 A).<br />
Voorhies assigned skeletal elements of some modern mammals to one of three<br />
categories in accordance with their ease of fluvial transport: Group I is the most readily<br />
transported <strong>and</strong> Group III the most resistant to transport (Behrensmeyer 1975). It is<br />
impossible to conduct the Voorhies Group analysis of the Devonian vertebrate<br />
assemblages for two reasons. Firstly, skeletons of different taxa in a mixed taxonomic<br />
assemblage yield different numbers of elements for each Voorhies Group, <strong>and</strong> sometimes<br />
homologous elements from even phylogenetically close taxa fall into different Voorhies<br />
Groups (Behrensmeyer 1975). Secondly, placoderms <strong>and</strong> ancient tetrapods have no<br />
direct analogues with homological skeletal elements among modern taxa. Therefore<br />
the abilities of different skeletal elements of vertebrates from the Pavāri oryctocoenosis<br />
to be transported <strong>and</strong> sorted by fluvial processes can only be evaluated. A most<br />
satisfactory analysis is possible only examining representation of Bothriolepis ciecere,<br />
from which almost all bones are represented in the assemblage. Ventastega curonica