Kerygmachela kierkegaardi

Abstract

This page provides a brief taxonomic description of the Cambrian ?arthropod fossil, Kerygmachela kierkegaardi. The primary source reference is Budd 1998, from which most of this page is derived. Any errors, however, are mine.

Keywords: Kerygmachela kierkegaardi, Sirius Passet, Cambrian, fossil record

Introduction

The Sirius Passet fauna (named after the Sirius sledge patrol that operates in North Greenland) derives from the Buen Formation, exposed on the eastern shore of J.P. Koch Fjord in the far north of Greenland.

The fauna is inevitably compared to that of the Burgess Shale, although it is probably ten to fifteen million years older – 518 vs. 505 Ma (Martin et. al. 2000) – and more closely contemporaneous with that from Chengjiang.

Geological Setting

Stratigraphy

The Sirius Passet soft-body fossils are found in rocks of the Lower Cambrian Buen Formation, in mud shales, representing a rather deeper water facies than the Burgess Shale, formed on the outer continental shelf, off-shore from a carbonate escarpment. "Large chunks from the edge of the carbonate platform occasionally fell or slid into the adjacent basin, where the Sirius Passet fauna lived" (Conway Morris 1998, p. 117).

Preservation

In general, preservation of the Sirius Passet fossils is not spectacular.

Systematics

Discussion of early metazoan evolution has for many years been dominated by fossil evidence from the Middle Cambrian Burgess Shale and, in particular, by its famous problematic arthropods – Anomalocaris, Leancoila, Opabinia, and so on. "No one would dispute that these fossils are problematic, in the sense that they are difficult to understand. However, that methodological difficulty should not be confused with the possibility that these fossils have only remote affinities with all living groups" (Budd 1997, p.125).

The Sirius Passet fossils are approximately ten to fifteen Ma older than those of the Burgess Shale, presenting us with an even earlier window upon metazoan evolution; a glimpse of forms which, if anything, are even more challenging to interpret.

As in the Burgess Shale, arthropods are the most abundant component of the Sirius Passet fauna although there is only one or two species of trilobite whereas, in the Burgess Shale, there are twelve or so. In fact there are generally few taxa having shelly skeletons; the trilobites, "rare hyoliths, a number of sponges with prominent spicules, a few small brachiopods, and no echinoderms or molluscs" (Conway Morris 1998, pp. 120-121). Of the arthropods lacking calcified exoskeletons, some are somewhat – but not markedly – similar to Burgess Shale species. Many are large, reaching 50 cm or more in length. In addition there are a number of polychaete annelids and large priapulids (ibid.)

Phylum Arthropoda Siebold & Stannius 1895

Euarthropoda

Class Dinocarida Collins 1996, p. 291

Discussion: As defined by Collins, Dinocarids are bilaterally symmetrical arthropods with a body divided into two principal tagmata, recalling the prosoma and opisthosoma of chelicerates, and a non-mineralised cuticle. The front part shows no external segmentation, bears one or more pre-oral claws, one or more pairs of prominent eyes, and a ventral mouth; differing from other arthropod classes in possessing no antennae and only one appendage or pair of pre-oral appendages on the prosoma, and in bearing gilled lateral lobes on the metameric trunk. The jaws vary from none to forms with both radiating teeth and teeth in rows.

Collins included within the group the Anomalocaride (Anomalocaris and Laggania), Opabiniidae (Opabinia), Hurdia, Proboscicaris, Cassubia, and "three, possibly five, unnamed genera from China" within the Dinocarida, but was unconvinced of any close relationship between Anomalocaris and Kerygmachela. However, it is retained here for the present on account of the gill-bearing lateral lobes of the trunk.

cf. Order Radiodonta Collins 1996, p. 291

Discussion: Collins restricted the Radiodonta to exclude the Opabiniidae and, although he does not explicitly say so, certainly Kerygmachela also.

Kerygmachela kierkegaardi Budd 1998

Description: The cephalic region is characterized by a pair of stout unsegmented appendages each bearing long spinose processes, and an anterior mouth. The trunk shows alternating rows of tubercles and transverse annulations along the axis, to which are attached 11 pairs of gill-bearing lateral lobes and lobopodous limbs. The caudal region is small, and bears two long tail spines. There is some evidence for circular musculature arranged around the trunk and a dorsal, longitudinal sinus, and several details of the muscular pharynx have been preserved.

Occurrence: To date known only from the Buen Formation at Sirius Passet.

Discussion: The combination of characters found in Kerygmachela allows it to be allied with the lobopods, represented in the extant fauna by the onychophorans, tardigrades, and possibly the pentastomids, and in the Cambrian fossil record by a morphologically diverse set of taxa, some of which are not assignable to the extant groupings. It also shares important characters with the problematic Burgess Shale forms Opabinia regalis Walcott and Anomalocaris Whiteaves, and another Sirius Passet form, Pambdelurion Budd (see below). These taxa together form a paraphyletic group at the base of the clade of biramous arthropods.

"In the Sirius Passet, the putative lobopod Kerygmachela shows marked similarities to Burgess Shale Leanchoilia, suggesting an alternative interpretation for its axial structure. I argue here that most three-dimensionally preserved axial structures in Burgess Shale-type arthropods can be interpreted as well developed midgut diverticula. Such a condition is characteristic of living carnivorous arthropods and provides a key to interpreting the ecology of ancient forms" (Butterfield 1999).

References

Budd, Graham E. (1997): Stem Group Arthropods from the Lower Cambrian Sirius Passet Fauna of North Greenland. In Fortey, R.A.; Thomas R.H. (eds.): Arthropod Relationships. Systematics Association Special Volume Series 55.

Budd, Graham E. (1999): The morphology and phylogenetic significance of Kerygmachela kierkegaardi Budd (Buen Formation, Lower Cambrian, N Greenland). Transactions of the Royal Society of Edinburgh: Earth Sciences, 89, 249-290.

Butterfield, N.J. (1999): Interpreting Axial Structures in Burgess Shale-Type Fossils. Palaeontological Association 44th Annual Meeting, University of Edinburgh, 17-20 December 1999 (Oral Presentation)

Conway Morris, Simon (1998): The Crucible of Creation. Oxford.

Conway Morris, Simon; Peel, J.S.; Higgins, A.K.; Soper, N.J.; Davis, N.C. (1987): A Burgess Shale-Like Fauna From the Lower Cambrian of North Greenland.   Nature, 345: 802-805.

Martin, M.W.; Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L. (2000): Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution.  Science v.288: 841-845.

Taylor, Rod S. (1999): 'Waptiid' Arthropods and the Significance of Bivalved Carapaces in the Lower Cambrian. Palaeontological Association 44th Annual Meeting, University of Edinburgh, 17-20 December 1999 (Oral Presentation)