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

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