Saturday 9 May 2015

Priapulid Worms from the Middle Cambrian of Canada.

Priapulid Worms are a form of marine worms with denticle-covered pharynxes which can be everted to form proboscises. They are a minor element of modern marine faunas, largely restricted to deep marine muds and other environments where oxygen levels are low and they face less competition from other forms of Worms. However in the Cambrian Period they appear to have been a major component of marine faunas in many areas. The palaeontologist Charles Doolittle Walcott described the Priapulid Worm genus Ottoia from the Middle Cambrian Burgess Shale in 1911, originally designating three species, Ottoia prolifica, Ottoia minor and Ottoia tenuis, however Ottoia minor has since been moved to a separate genus, Ancalagon, and Ottoiatenuis has been recognized as a (quite unrelated) Enteropneust, leaving only a single species in the genus. A variety of Priapulid fossils from other Cambrian deposits have also been assigned to the genus, though most of these are somewhat dubious.

In a paper published in the journal Palaeontology on 6 May 2015, Martin Smith of the Department of Earth Sciences at the University of Cambridge, Thomas Harvey of the Department of Geology at the University of Leicester and Nicholas Butterfield, also of the Department of Earth Sciences at the University of Cambridge re-examine the genus Ottoia, concluding that there are in fact two species present in the Burgess Shale, and examine the status of Priapulid denticles from other Cambrian localities.

Smith et al. paid particular attention to the denticles of Ottoia, noting that it has several regions on its pharynx, each with denticles with a different morphology. While these were consistent across all the Burgess Shale fossils, one horizon within the deposits contained (exclusively) specimens with a distinctive morphology present in one of these bands, which they refer to as the Type B teeth.

The new species is named Ottoia tricuspida, in reference to its Type B teeth, which have three cusps. In the other described species the Type B teeth have a large central cusp surrounded by four to eight smaller ones, decreasing in size away from the centre. In addition the new species has a double row of Type A teeth, whereas Ottoia prolifica has only a single row.

Ottoia tricuspida. (A) Complete specimen. (B) Enlargement of proboscis. Scale bars represent 10 mm (A); 2 mm (B). Smith et al. (2015).

All of the known specimens of Ottoia tricuspida come from a single horizon within the Burgess Shale, 120–130 cm below the base of the Phyllopod Bed, with a single possible occurrence in the Upper Walcott Quarry. Smith et al. consider the possibilities that it may represent members of the species Ottoia prolifica of a different sex or developmental stage as all the specimens on this horizon appear to belong to Ottoia tricuspida with no specimens of Ottoia prolifica present, whereas on other horizons the reverse is true, and because a wide range of specimen sizes are found in both morphologies.

Schematic diagrams of Ottoia sclerite morphologies. Smith et al. (2015).

Specimens assigned to Ottoiahave also been reported from the Pioche Shale in Nevada, the Marjum Formation and Spence Shale of Utah, the Conasauga Formation of Georgia, the Chancellor Basin of Canada and the Kaili Formation of China. However none of these fossils has been fully described, and most are somewhat dubious, making confident assignment to the genus impossible until such time as these fossils can be properly examined and assessed.

However disarticulated Priapulid denticles are a common feature of Cambrian deposits, and the potential exists to detect Ottoia from such denticles. Smith et al. examined denticles from the Pika Formation of westernmost Jasper National Park, Alberta, the Deadwood Formation of Saskatchewan, and in both cases determined that Ottoia prolifica was present.

Small Carbonaceous Fossils from the Deadwood and Pika formations assigned to Ottoia prolifica. (A–M) Teeth from a single sample of the Deadwood Formation (Riley Lake drillcore at 1300 m), interpreted as tail hooks (A–B), introvert hooks (C–D), a coronal spine (E), a Type A tooth (F), a Type B tooth (G), a Type B tooth with surrounding cuticle (H), morphologies intermediate between tooth types B, C and D (I–L), and a Type D tooth (M). (N–U)Teeth from the Pika Formation, interpreted as an introvert hook (N), an introvert hook without submarginal denticles (O), a Type A-like tooth (P), a Type B tooth (Q), a tooth possibly intermediate between Type B and Type C (R), a Type C tooth (S), an intermediate between Type C and Type D morphologies (T), and a Type D tooth (U). Scale bar represents 50 μm. Smith et al. (2015).

See also…

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