Sunday 6 September 2015

Exceptional preservation in Early Ordovician fossils from Hubei Province, China.

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…

Reworked Late Ordovician Sponges have been collected from Miocene to Pleistocene across a wide area of northern Europe for over two centuries. These are associated with the course of the Baltic River System, which drained much of northern Europe for...

The enigmatic fossil Furca bohemica was first recorded from Late Ordovician deposits at Veselá Gorge, in what is now the Czech Republic, by the French Palaeontologist Joachim Barrande in 1846, though he never formally described the specimens. A formal description was provided by Antonin Fritsch in...

Brachiopods (or Lampshells) superficially resemble Bivalve Molluscs, though they are not closely related. They have a filter feeding apparatus called a lophophore, unlike anything found in any Mollusc, but also found in Bryozoans and Phoronid Worms. This is encased with in a shell with two valves, each symmetrical about a...

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