Fossil Lagerstätten are deposits
which produce exceptional fossils, providing us with valuable insights into
vanished ecosystems. There are a number of Ordovician Fossil Lagerstätten known
which produce invertebrate fossils with soft tissues, providing information on
organisms otherwise not seen in the fossil record. These are the Early
Ordovician Fezouata Formation of Morocco, the Middle Ordovician Winneshiek
Lagerstätte in northeast Iowa and the Late Ordovician Beecher’s Trilobite Bed
of New York, Llanfawr Mudstone of Wales, Soom Shale of South Africa and Late
Ordovician deposits from Manitoba.
In a
paper published in the
Chinese Science Bulletin on 17 March 2015,
Andrzej Baliński of the
Instytut Paleobiologii and Yuanlin Sun of the
Key Laboratory of Orogenic Belts and Crustal Evolution at
Peking University discuss a new Early Ordovician Fossil Lagerstätten
from Hubei Province in China. These fossils come from the Fenxiang Formation, a
deepwater shale, and show fossils phosphatised in a similar manner to the
Burgess Shale and Chengjiang Cambrian Fossils, a form of preservation extremely
rare in post-Cambrian deposits (so rare that until quite recently such
preservation was not thought to be seen in post-Cambrian deposits at all). The
fossils were deposited in a deepwater environment, but many are of
shallow-water organisms, thought to have been moved there by a submarine
landslip, in a similar way to the shallow water fossils seen in the Burgess
Shale. They were deposited about 470 million years ago, slightly before the
Great Ordovician Biodiversification Event, in which the Cambrian Evolutionary
Fauna (dominated organisms such as Trilobites, Polychaete Worms, Monoplacophorans and Inarticulate Brachiopods) was largely replaced by the Palaeozoic
Evolutionary Fuana (dominated by organisms such as Articulate Brachiopods, Crinoids, Cephalopod Molluscs and Anthozoan Corals), and contains organisms
from both faunas.
The first fossils described are
Linguloid Brachiopods assigned to the genus Leontiella
with soft tissues preserved. Linguloids are considered to have been the first
Brachiopods to have appeared, and are documented with soft tissue preservation
from the Burgess Shale, but reliably interpreted soft-body preservation in
post-Cambrian Linguloid Brachiopods is extremely rare, with the earliest unquestionable
examples coming from the Early Lower Devonian Hunsrück Slate of Germany, and
possible examples from the Devonian of England, Middle Ordovician of New York
State and Late Ordovician of South Africa. The Fenxiang Brachiopods show three-dimensional
preservation of the pedicle, showing details of the external morphology. Leontiella has a streamlined shell with radial
ornamentation, characteristics associated with true burrowing behaviour in
Brachiopods (unlike the simpler shells of the Cambrian Linguloids); the Fenxiang
specimens have long, vermiform pedicles, which appears to support this,
suggesting that Linguloid Brachiopods had indeed evolved true burrowing
behaviour by this point.
Linguloid brachiopod Leontiella
sp. with preserved pyritized vermiform pedicle. Balinski & Sun (2015).
The Fenxiang Biota also contains
a variety of trace fossils, which also show evidence of burrowing behaviour not
seen in Cambrian deposits. These include numerous sinusoidal
horizontally-orientated cylindrical burrows 20–60 μm in diameter, tightly
packed with framboid structures, thought to be replacement minerals occupying
spaces which formerly contained pyrite crystals, which themselves would have
been formed by bacterial decomposition of an organic infilling of the burrows,
such as fecal pellets. Balinski and Sun interpret these as the burrows of
free-living Nematodes, which form very similar burrows in modern environments.
These traces considerably predate the first Nematode body-fossils, which are
found in the Early Devonian Rhynie Chert of Scotland, and the earliest trace
fossils previously ascribed to Nematodes, from the Middle Triassic of Germany.
Nematodes are thought to have diversified from a common ancestor before the
appearance of the earliest Arthropods and Velvet Worms, both groups which
appear in the Early Cambrian, so the presence of such organisms in the Early
Ordovician is not surprising, however Nematodes are not thought to have been
able to adopt a burrowing lifestyle until larger burrowing and bioturbating
organisms had broken up a layer of microbial matting which covered most of the
seafloor in the Cambrian and prevented much oxygen from penetrating to the
sediments bellow.
Sinusoidal trace fossil from the Fenxiang Formation interpreted as a
Nematode burrow. Balinski & Sun (2015).
The Fenxiang Biota has also
produced an upright colonial Hydroid, possibly belonging to the Haleciidae,
preserved as part and counterpart on a split rock. Hydras and Medusas
(Jellyfish) are thought to have been the earliest Cnidarians to appear, due to
their simple body-plans and placement on genetically-based phylogenetic trees,
and are expected in the fossil record very early. However numerous fossils
assigned to these groups from Ediacaran, Cambrian, Ordovician and Silurian
deposits have subsequently been re-assigned to different groups, leaving the
earliest confidently assigned Medusas coming from the Late Carboniferous Mazon
Creek Formation and the earliest known Hydras those from the Late Ordvician of
Kentucky and (possibly) Wales. The Fenxiang specimen represents an advanced
colonial Hydroid, the earliest known example of such an organism.
Pyritized colony of Hydroid. Balinski & Sun (2015).
The Fenxiang Biota also includes
a variety of other Cnidarians, including Conulariids, an extinct group of Palaeozoic
Scyphozoans, and numerous Black Corals (Antipatharia), an important group of
modern Corals, particularly in deeper waters, but until know not known from the
fossil record at all (though they have been predicted to have been an ancient
group from genetic studies of their phylogenetic position). These Fenxiang
fossils have been processed in acetic acid revealing extremely well preserved
details of their anatomy, which leave little doubt on their assignment to the
group.
Phosphatic coralla of Antipatharian Coral showing basal part of a colony with crowded spines (l), spinose
branch (m), and partly preserved spinose basal part of colony and erect stem (n).
Balinski & Sun (2015).
The deposits also contain
numerous fossils which have yet to be identified or studied in detail,
including branching feather-like fossils, which may prove to be another form of
colonial Hydroid, numerous Arthropod fragments, and a possible Scalidophoran Worm
embryo similar to those known from Cambrian deposits in China, Russia and
Australia and earliest Ordovician of the United States.
See also…
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