Preservation of Trilobites

P. R. Sheldon (Lethais v.21, p293-306, 1988) notes that the genus Ogyginus from shales and also Llanvirn sandstones at Gilwern Hill Quarry are an exception to the local rule that trilobite pygidial width defines a normal or slightly bimodal unskewed distribution dominate by the final or holaspid stage of growth. According to him the Ogyginus corndensis (Murchison) from Gilwern Hill are “extremely abundant:, and in a previously collected museum collection of 463 intact specimens showed a maximum width of about 97 mm and some 335 specimens in the less than 13 mm width range of small holaspids and merapids (Trilobites by R. Levi-Setti (1993 Univ. of Chicago Press), the head merely being truncated during previous splitting of the poorly defined bedding planes. But the similar-looking, later Ordovician trilobite (Hall) does look abundant on shell beds separated by four mm thick layers of barren oil shale of the Collingwood Member of the Lindsay Formation, on the beach at Craigleith Ontario. But in this case the largest specimens are so fragmented and separated into particular layers that they are not collected or illustrated like the 20 to 45 mm specimens seen in Levi-Setti (pls. 3 and 136), H.B. Whittington ( Trilobites 1992, Boydell Press U.K.) and R. Ludvigsen (Fossils of Ontario, Part 1: The Trilobites Royal Ontario Museum 1979) from that particular site seen on my own photographs


Weberides mucronatus (McCoy) Acre Limst.

Calcareous shales overlying the Mississippian Acre Limestone, 300 m W.S.W. of Cullernose Point U.K. showed ten pygidia and one similarly disarticulated cranidium of Weberides mucronatus (McCoy) in a cluster with an equal number of brachiopods including still articulated, pyrite-filled Rugosochonites Sokolskaya, three small gastropods, one bivalve and four crinoid stem segments. Most of them are seen on the photographed half of the split bedding plane, which is probably the lower surface of the top judging from the greater oxidation of the pyrite on that side. The Pygidium marked b was therefore originally being deposited concave-up like the adjacent relatively smooth glabella seen aligned parallel to it. These shales represent a relatively near-shore marine environment, the trilobites being absent in basins. The cluster was presumably formed by waves despite the probable concave-up orientations. The pygidium marked b has a width and length around 11 mm and has broken through the calcite exoskeleton of 0.08 to 0.10 mm thickness producing a rather poor preservation on both the part and counterpart. The other trilobites were of similar size as are the brachiopods, supporting the idea of current sorting. This would explain the absence of thoracic segments presumably decayed into long harrow strips before the storm.

Pseudogygites latimarginatus (Hall) &Isorthoceras tenuistriatum(Hall) Craigleith Ont.Fragment of pygidium of larger size of 0.3 mm thickness, with same 0.05 mm pits and marginal wrinkles.
Pseudogygites & Isorthoceras, Craigleith Ontario

This and other shell layers of the Ordovician (Maysvillian) Linsay Formation on the beach at Craigleith, Ontario, are separated by three mm thick bands of relatively barren oil shale within the Collingwood Member. Two books entitled trilobites by H. B. Whittington (Boydell Press 1992) and R. Levi-Setti (Univ. of Chicago Press 1993) and also R. Ludvigsen’s Fossils of Ontario Part 1:The Trilobites (Royal Ontario Museum 1979) illustrate specimens of Pseudogygites latimarginatus (Hall) from this site with widths of the tail-plate of up to about 45 mm, but one from Bowmanville in a slightly different facies is seen on plate 136 of Levi-Setti with a greater width of about 61 mm on the tail-plate and about 68 mm on the associated thorax. It would seem from my photographs that specimens of that size are common at Craigleith, but being more disarticulated are seldom photographed. Since it is possible that such breakage is due to cephalopod predation, followed by resedimentation into basinal shell layers, rather than by molting as Ludvigsen proposed for the samll molts there, it is instructive to study exactly what these fragments look like. The associated orthoconic cephalopod Isorthocreas tenuistriatum (Hall), revised from Geisonoceras by R. C. Frey (U.S.G.S. Prof. Pap. 1066-P, p, P59, 1995) is too small to have attacked the adult trilobitesand is represented by a fragment of similar thickness (0.4 mm) to the tail-plate on the edge of the photograph with a similar width to the black separate thorax segment of 40 mm width.


Isorthoceras tenuistriatum (Hall) of 0.4 mm thickness with 0.1 mm transverse growth ridges
Pseudogygites separated fixed cheeks on glabella, Craigleith.

This photograph shows a pair of head fragments consisting of the fixed cheek on one side joined to the posterior axial segments of the cephalon with the other fixed cheek broken off before deposition. The posterior border of the axial segment of the larger fragment appears to be distorted perhaps because it was bitten earlier in life or because it was not fully calcified when attacked. The axial lobe width suggests the head was originally about 60 mm wide. The notch in the edge of the fixed cheek is where the eye separated molting along the facial suture as proposed by Ludvigsen, but the separation of the head from the thorax, and the irregular breakage of the other fixed cheek are not due to molting in his hypothesis. The unusual feature is a posterior margin running at an angle to the segmentation of the head and not bending back at the mid-line.

Fake Calymene blumenbachii Brongniart, Dudley

Pygidium of 21 mm width, 0.08 mm thick

Occipital ring with possible predation scar

Disarticulated thoracic segment of 40 mm width, with pleural furrows on pleural lobes flanking 10 mm wide axial lobe with 0.2 mm thick shell covered with 0.05 mm dorsal pits.

The Dudley Locust was collected when the Wenlock Limestone was mined there and often sold with the disarticulated fragments glued with local rock dust as whole specimens were rare and more expensive. This one looks odd because it lacked the free cheeks and an attempt by the collector or dealer to expose them, or perhaps find others to add there failed. The pygidium has been added in a fake matrix, which now has microscopic cracks in it, these parts being relatively common. The thorax therefore shows ten not 13 segments, divided into two sets of five, which do appear to belong together despite distortion,. However, there is a better type of glue/matrix along the sides suggesting that the thorax was added to the interesting cranidium with a probable cephalopod bite mark in the occipital ring.

Pleurobrachia - Part 2

June 1, 2009

Given one change of the tank water from the sea on May 22, the Pleurobrachia continued to feed on the few available copepods until it was last seen alive at about noon on the 25th. Most of the time it lived at the surface with the two tentacles hanging below and then retracting, but when last seen it was moving down with the cillia beating and the tentacles extended below. On June 1st, the tank sediment was partly removed with the old water and appeared to preserve the corpse, which could not be seen before, as a grey triangular disc of 10 mm length extending upwards into a brown 60 mm filament in the water. It would seem that live Pleurobrachia remain at the surface by active swimming and then sink to die giving them some potential of becoming fossils despite being originally transparent and nearly all composed of water. When a Beröe cucumis Fabricus similarly arrived with the new water by accident on July 12, 2007, it was considerably more active, actively swimming up from the default position on the tank floor, where it presumably died within one day. That seawater did show flashes of light in the night, but they did not come from the moving Beröe and no light was seen at all in the Pleurobrachia water. Beröe are reported to appear later in the year because they eat Pleurobrachia, which can themselves live longer in the tank by feeding on copepods.