Thursday, 11 October 2018

Pahvantia hastata: A small, filter-feeding Radiodont from the Middle Cambrian of Utah.

The first Phytoplankton (photosynthetic organisms living in the water column) are thought to have appeared at least 1.8 billion years ago, when the first simple Acritarchs (organic-walled microfossils thought to have been the resistant cysts of single-celled Green Algae) appeared in the fossil record. The first Animals appeared around 700 million years ago, though for a long while these were non-motile, benthic (bottom dwelling) organisms, apparently ecologically disconnected from the primary producers in the waters above them. Then around 545 million years ago the Cambrian Explosion saw the appearance of a wide range of Animal groups, many of them showing bilateral symmetry, calcareous shells and the ability to move freely. The precise cause of this sudden increase in diversity is unclear, but one factor thought to have been important is the appearance of Zooplankton, tiny animals living in the water column, which feed on the Phytoplankton and provide a far better food source for benthic organisms, raining down a constant source of droppings and bodies, larger than the single-celled Algae. 

The Radiodonts were a group of Arthropods that appeared early in the Cambrian and appear to have been the first free-swimming raptoral predators, a role subsequently taken on by animals such as Sharks, Fish, Squid, and marine Tetrapods, which appeared to place them firmly at the top of the Cambrian food-chain. These Radiodonts had rather complex exoskeletons, which tended to become disarticulated before being preserved, so that for a long time they were not understood by palaeontologists, being interpreted a parts of different Animals. Then in 2014 Tamisiocaris, a large Radiodont from the early Cambrian was discovered to have feeding organs adapted not for raptoral predation but for filter feeding. With hindsight this was not surprising, as filter feeding has repeatedly appeared in the largest members of other groups of raptoral marine predators, including Sharks, Whales and several types of Fish, making this a predictable occurrence in large raptoral Radiodonts, a view that was supported by the discovery of a second such species, the two metre long Aegirocassis from the Early Ordovician.

In a paper published in the journal Nature Communications on 14 September 2018, Rudy Lerosey-Aubril of the Palaeoscience Research Centre at the University of New England, and Stephen Pates of the Department of Zoology at the University of Oxford, and the Institute of Earth Sciences at the University of Lausanne, describe the discovery of filter-feeding in a third species of Radiodont, Pahvantia hastata, from the Middle Cambrian Wheeler Formation of Utah.

Pahvantia hastata has previously been described from three partial fossils, thought to be univalve carapaces of an arthropod of uncertain affinities, plus a fourth specimen named separately as Proboscicaris agnosta, originally thought to be an isolated valve of a bivalved carapace, again of an arthropod of uncertain affinities, then recently identified as a lateral element of a Radiodont cephalic carapace. Lerosey-Aubril and Pates describe a new, more complete (though still dislocated) specimen of Pahvantia hastata, which shows that the specimens previously described as Pahvantia hastata and Proboscicaris agnosta are in fact parts of the same Animal (when this happens the Animal takes the first name given to one of its parts, in this case Pahvantia hastata), and in addition describe the feeding apparatus of this Animal for the first time.

Assemblage of cephalic elements of Pahvantia hastata. Photographs of part and counterpart of new specimen immersed in dilute ethanol (a), (b), (d), (f) or dry (c), (e). (a), (b) General views of part (a) and counterpart (b). (c)–(f) Details of the frontal appendage (c), (d), its setae (e), and its proximalmost two endites (f). Scale bars represent 5mm (a), (b), 2mm (c), (d), 1mm (e), (f). ae anterior extension, as auxiliary spines, ce central element, cpm concave posterior margin, en1–7 endites 1–7, fa frontal appendage, ipm indented posterior margin, ll lateral lip, lle left lateral element, pe peduncle, plc posterolateral constriction, rle right lateral element, se setae, ss anterior sagittal spine. Lerosey-Aubril & Pates (2018). 

The new specimen shows that Pahvantia possesses a well-developed tripartite cephalic carapace, with lanceolate outline, a tiny antero-sagittal spine, well-developed lateral lips, a strong postero-lateral constriction, and an indented posterior margin. Its feeding apparatus projects from under the left side of the dorsal element, and comprises a multitude of long hair-like structures superimposed upon one-another and tentatively interpreted as setae. This apparatus is interpreted as being capable of filtering microscopic food particles (Algae or very small faecal particles) from the water, suggesting an improved filter-feeding capacity over Tamisiocaris and Aegirocassis, which are thought to have been better suited to feeding on macroplankton (small Crustaceans or similar).

Interestingly, Pahvantia, Tamisiocaris and Aegirocassis do not appear to be closely related, but rather to members of distinct Radiodont lineages, suggesting that filter-feeding arose separately at least three times within the Radiodonts, supporting the idea that the development of such a lifestyle is very likely on large raptoral marine predators (at 24 cm Pahvantia does not seem to fit the notion of a large raptoral predator, but it was the largest Animal in its environment, which appears to be a more important factor than absolute size.

Diversity of sizes and feeding habits in radiodonts. As exemplified by Aegirocassis and Pahvantia, there is no obvious relationships between size and feeding strategy in this group. Raptorial predators are represented in red, sediment sifters in purple, and suspension feeders in blue. Lerosey-Aubril & Pates (2018). 

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