in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
226
Phylogeny of some Oxytomidae and the Lower/Middle Jurassic
Boundary in the North of Russian Asia
O.A. Lutikov 1 , I.E. Temkin 2 , B.N. Shurygin 3
1. Research Institute of Paleontology, Stratigraphy and Sedimentology, Novosibirsk 630110, Russia (E-mail: niipss@mail.ru)
2. Department of Invertebrate Zoology, National Museum of Natural History, 10th and Constitution Ave, NW MRC 163, W-306,
Washington DC 20560-0163 (E-mail: temkini@si.edu)
3. Institute of Petroleum Geology and Geophysics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090, Russia
(E-mail: shuryginBN@ipgg.nsc.ru)
Representatives of the bivalve family Oxytomidae
Ichikawa, 1958 are widely distributed in the Jurassic
and the Cretaceous sediments of Russia, North America,
Europe, Asia, Australia, and New Zealand. They
include species of the genera Oxytoma Meek, 1864,
Meleagrinella Whitfield, 1885, and Arctotis Body-
levsky, 1960.
The hinge morphogenesis of the Oxitomidae and
the phylogeny of select representatives of the family
were discussed in detail elsewhere (Lutikov, Shurygin,
2010; Lutikov, Tёmkin, Shurygin, 2010). Our analysis
of the ontogeny and phylogenty of the ligamental and
byssal character complexes allowed for a taxonomic
revision of some oxytomids, that included redefining
the concepts of the genera Meleagrinella Whitfield,
1885 and Arctotis Bodylevsky, 1960, and recognizing
two new subgenera, Meleagrinella (Praemeleagrinella)
subgen. nov. (Hettangian-Aalenian) and Arctotis
(Praearctotis) subgen. nov. (Toarcian-Aalenian).
The phylogenetic consideration regarding Melea-
grinella and Arctotis were based on the comparative
analysis of the ligament ontogeny in the Hettangian-
Sinemurian, Pliensbachian, Toarcian, Aalenian, Bajo-
cian, Callovian, Oxfordian, Kimmeridgian, Volgian
and Valanginian forms. Phylogenetic recon- struction
of the supraspecific groups of the Oxytomidae were
based on the combination of the cladistic analysis and
stratigraphic considerations (Fig.1 A).
The direction of the evolutionary transformation
of the ligamental and byssal character complexes in the
Oxytomidae was reconstructed according to the results
of the cladistic analysis (Fig.1 B, C). In the phyletic
leneage Praemeleagrinella-Arctotis, the change in the
morphogenesis of the ligamental complex led to the
origin of a complex, spoon-like resilifer. In the Bajo-
cian Arctotis s.str., the evolution of the morphogenesis
of the byssal complex resulted in the progressive
closure of the byssal notch (from the opened byssal
notch in the Hettangian Praemeleagrinella to closed
(overgrown and fused)). The outline of the auricle of
the right valve has shifted from subquadrate to
subtriangular (Fig.1 D).
The appearance of new phenes in large and
geographically widespread populations was critical for
the segregation of Arctotis. The oblique resilifer of
Praearctotis was the precursor of the developed spoon-
like resilifer of Arctotis. This type of the resilifer was
recorded in multiple individuals from the regions
corresponding to the Vilui (Siberia, temperate) and
Nordvick (Siberia, north temperate) epicontinental seas
in the Late Toarcian (Pseudolioceras wurttenbergeri
phase) (Knyazev et al., 2003). The spoon-like resilifer
in Praearctotis evolved somewhat later on the territory
of the Zhigansk (Siberia, temperate) and East-Taimyr
(Siberia, north temperate) epicontinental seas.
We recorded the appearance of the developed
spoon-like resilifer in Arctotis str. on the territory of the
Zhigansk Sea in the Middle Jurassic sediments (Pseu-
dolioceras maclintocki phase, Early Aalenian).
The evolution of the new genus-group taxon did
not involve an abrupt adaptive change. The transition
from the exclusively byssal attachment (Praemelea-
grinella) to the attachment by the byssus-anchor com-
plex (Arctotis str.) progressed from the Early Toarcian
to the Bajocian. In the Late Toarcian, the represen-
tatives of the genus Praearctotis characterized by the
transitional states of the genus-level characters were
dominant. The integration of several phenes during the
evolution of the adaptive morphological structures
ensured the definitive formation of the supraspecific
taxon Arctotis and its diversification during the general
regression in the Aalenian.
Consequently, the presence of Arctotis str. with a
well-developed spoon-like resilifer marks the strati-
graphic boundary between the lower- and middle
Jurassic strata on the wide territory of Central Siberia.
Acknowledgements: This job was supported by
RFFI (Russian Foundation for Basic Research) (project
№ 09-05-00136) and RAS (Russian Academy of the
Sciences), programs 15 and 17.
Key words: Jurassic; Bivalvia; Taxonomy; Bio-
stratigraphy