Sunday, 1 November 2015

Pentecopterus decorahensis: A Eurypterid 'Sea Scorpion' from the Middle Ordovician of Iowa.

The Eurypterids were a curious group of Chelicerate Arthropods (the group that also includes Horseshoe Crabs, Trilobites and Arachnids). They were often formidable in size (the largest were over two meters in length) had clawed forelimbs and sported a spiked tail, which it has been suggested may have been use to inject venom (though there is no evidence of this), which has led to them being given the popular name 'Sea Scorpions' or 'Giant Sea Scorpions'. The group are well recorded from Silurian and Devonian deposits, where they form a major part of the fauna of both marine and freshwater ecosystems, but are rarer in Ordovician deposits. Despite this the group are believed to have originated and diversified largely within this period, as the group appears to have divided into numerous lineages prior to becoming numerous in the fossil record in the Silurian.

In a paper published in the journal BMC Evolutionary Biology on 1 September 2015, James Lamsdell of the Department of Geology and Geophysics at Yale University, Derek Briggs of the Department of Geology and Geophysics and Yale Peabody Museum of Natural History at Yale University, Huaibao Liu of the Iowa Geological Survey, Brian Witzke of the Departmentof Earth and Environmental Sciences at the University of Iowa, and Robert McKay, also of the Iowa Geological Survey, describe a new species of Eurypterid from the Middle Ordovician Winneshiek Lagerstätte of Winneshiek County in northeastern Iowa.

The Winneshiek Lagerstätte is a 18-27 m laminated greenish brown and dark grey shale laid down in a circular depression 5.6 km in diameter, thought to have been formed by a meteorite impact, the Decorah Impact Structure. The shale outcrops only at a single location, but has been found in boreholes across the basin. The deposits have been interpreted as having formed in a tidal nearshore environment with low oxygen levels and brackish water in a tropical climate. The outcrop has produced numerous Arthropods and Conodonts, Plants, Algae and possible Jawless Fish. From the Conodonts it has been assigned an age of 467.3-458.4 million years, at least nine million years older than the oldest Eurytripid-bearing deposits known from elsewhere, though possible Eurytripids from the Early Ordovician Fezouata Formation of Morocco are currently under investigation.

Portion of a Eurypterid from the Middle Ordovician Winneshiek Lagerstätte of Iowa. Rostrum and linguoid posterior projection underlain by left and right prosomal appendages II and III. (a) Specimen. (b) Interpretive drawing: red = ventral plate, green = appendage II, and blue = appendage III, II-2–II-4 = appendage II podomeres 2–4, III-2 = appendage III podomere 2. The coxa are angled anteriorly and covered by the ventral plate, which can be peeled back to reveal their position. Scale bar is 10 mm. Lamsdell et al. (2015).

These deposits produce a large amount of (largely disarticulated) Eurytperid material, preserved as organic cuticle remains. These have been reconstructed to construct the complete external morphology of the living animal, which is formally described and named Pentecopterus decorahensis, where 'Pentecopterus' means 'penteconter-wing', the penteconter being an early Greek warship with a profile similar to that of the Eurypterid and '-pterus' (-wing) being a common suffix for Eurypterid genera, and 'decorahensis' means 'from Decorah'.

Reconstruction of Pentecopterus decorahensis in dorsal view. Scale bar is 10 mm. Lamsdell et al. (2015).

Pentecopterus decorahensis is interpreted as having reached at least 144 mm in length, based upon the reconstruct specimens. A phylogenetic analysis suggests that it is a member of the Megalograptidae, a group of mostly large Eurypterids with small claws, and that within this group it is closest to the common ancestor of all members. However the Megalograptidae are not considered to be among the earliest Eurypterids, suggesting that the origin of the group as a whole occurred considerably earlier than the Mddle Ordovician.

See also...

Trilobites dominated the seas of the early Palaeozoic, but suffered a major loss of diversity in the Devonian... extinct Arachnid orders in three-dimensions.                                           There are currently sixteen recognized orders of Arachnids, twelve of which are extant (have living relatives) and four extinct, the Trigonotarbids... new species of Horseshoe Crab from the Late Jurassic Owadów-Brzezinki Lagerstätte of Central Poland.                                     Horseshoe Crabs, Xiphosurida, first appeared in the fossil record during the Ordovician, at least 480 million years ago, and are still found today in Southeast...
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