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Origin
and distribution of some shallow and deep water chitons
This
work is dedicated to origin and distribution of several groups
of chitons which have disjunct areas. These groups differ both
in the position in system and in the hypothetical time of origin.
The genus Ferreiraella
(order Lepidopleurida) includes chitons with the most primitive
features, originated apparently in the end of Paleozoic, whereas
the genera Lepidochitona and Placiphorella
belong to the advanced order Chitonida. The origin of Lepidochitona was probably in the end of Mesozoic, but the origin of
Placiphorella – in
the second half of Cenozoic. Probably the genus Ferreiraella
originated from one of the representatives of the genus Glaphurochiton that in the Upper Carboniferous apparently inhabited
sunken plant remains and fed on them, since plates of its shell
were found in dark grey shales. The first Ferreiraella
species that originated in the Upper Carboniferous was possibly F.
caribbensis, a lineage presently represented in the
Caribbean Sea. Later on, the representatives of Ferreiraella
spread with waters of the Paleotethys westwards to the region of
present California and Panama Bay and eastwards to the region of
present Japan, Indonesia and New Zealand. The supercontinent
Pangea that was formed
in the Permian divided the common distribution range of the
genus Ferreiraella
into two parts. In the Jurassic, the Pangea began to split into
blocks, and marine organisms inhabiting shelf and slope of these
blocks moved with them. Since then Ferreiraella
remained in the region of the former supercontinent Pangea only
in those areas where sunken wood was accumulated.
The origin of the genus Lepidochitona
undoubtedly was correlated to the Tethys Ocean. It is very
likely that the first species of Lepidochitona
originated in the Upper Cretaceous because three species of the
genus are known from the Lower Paleocene in Europe. The united
Circumequatorial system of streams was able to carry larvae of
chitons mainly in west direction. The first species of the genus
were distributed in the shallow waters of Europe from recent
France to Ukraine where more than 10 fossil species from Eocene,
Oligocene and Miocene are known. The nearness of America and
Europe allowed species of Lepidochitona
to penetrate in the Caribbean Sea and then through the strait
between South and North America in the Pacific coast of North
America. Another way of possible migration of species of the
genus Lepidochitona
was along the North and East coast of Africa from Tethys to
South Africa where at present time four species inhabit. In the
Pliocene, as a result of the closure of the strait between South
and North America and the simultaneous origin of the
Mediterranean Sea, the united circumequatorial stream has not
ceased to exist. The former area of genus Lepidochitona
was split into five pieces.
The
genus Placiphorella
originated in the North Pacific. There are all shallow water
species in this genus but two deepwater species. The first
species of the genus originated probably after the cooling in
the Oligocene. Later their carnivorous habit relieved to
assimilate to the deep waters. At first, the deep water Placiphorella pacifica
originated in the north-western Pacific. Later this species
could distribute eastward and gave origin to the deep water Placiphorella
sp. which at present lives along the Pacific coast of North
America from Aleutian Is. to California. Its descendants
probably penetrated along South America in the South Atlantic.
One portion distributed to the North Atlantic (P.
atlantica); another portion moved eastward to Prince Eduard
and Crozet Islands and through the South Indian Ocean to
Tasmania and New Zealand in the Pacific. |
