Sirius Passet Lagerstätte

Abstract

This page provides a brief overview of the Sirius Passet lagerstätte, including geological setting, biota, and significance.

Keywords: Sirius Passet, Cambrian, fossil record

Introduction

The Sirius Passet fauna (named after the Sirius sledge patrol that operates in North Greenland) is an exceptionally well-preserved Lower Cambrian assemblage, which derives from the Buen Formation, exposed on the eastern shore of J.P. Koch Fjord in the far north of Greenland (fig. 1). Although less well-known than the Burgess Shale or Chengjiang assemblages, it is probably the oldest of all the major Cambrian lagerstätten.

It was discovered in 1984 by A. Higgins of the Geological Survey of Greenland. A preliminary account was published by Conway Morris et al. (1987), but since then, three expeditions led by J. S. Peel and Simon Conway Morris have returned to the site, in 1989, 1991 and 1994, and a field collection of perhaps 10,000 fossil specimens has been amassed.

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. The fossils are less spectacular, but preserve minute detail, including the ultrastructure of muscle tissue.

"The discoveries in 1984 of the Lower Cambrian soft-bodied faunas of Sirius Passet in North Greenlandand of Chengjiang in southwest China has had a much greater impact on the significance of the BurgessShale fauna - it is no longer the sole major source of our knowledge of Cambrian animal life. The older,Sirius Passet fauna includes animals that look significantly more primitive than those in the Burgess Shale,and seems to have few animals, if any, in common. In contrast, the Chengjiang fauna has so manyanimals in common with the Burgess Shale that it seems to be its geographic extension. Thisdemonstrates that the Cambrian "evolutionary explosion" was over by the time of the Chengjiang fauna.It shows further, that the Burgess Shale/Chengjiang fauna was evolutionarily very stable, and changedlittle over the 15 to 20 million year interval between the two faunas. Thus the pattern for animal evolutionwas set in the early and middle Cambrian : short periods of rapid, significant change, exemplified by theCambrian explosion, alternating with long periods of little change, exemplified by the BurgessShale/Changjiang fauna" (Collins: Geology and Biology of the Burgess Shale).

Geological Setting

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).

Biota

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." (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 and Chengjiang assemblages, arthropods are the most abundant component of the Sirius Passet fauna, comprising about 20 taxa. Although there is only one or two species of trilobite, whereas there are more at Chengjiang and twelve or so in the Burgess Shale, all of the major biramous morphotypes appear to be represented.

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.)

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

A few of the Sirius Passet taxa are described below.

Phylum ?Mollusca

Class Sachitida

Halkieria evangelista

Halkeria is one of a number of the strange armoured "coat of mail" creatures that inhabited the early Cambrian oceans. In addition to calcareous scales it possessed two mollusc-like limpet-shaped shells apparently attached to the dorsal surface, at either end of the animal. Later forms such as the Middle Cambrian Wiwaxia lost the shells and developed long spines instead. It is not certain that these strange creatures are molluscs, although there is some evidence to support the belief. For example, a radula-like structure has been found in Wiwaxia. Jan Bergström suggests that they represent a group of late surviving "Procoelomates", the ancestors of all higher (coelomate body plan) animals. Conway Morris (1998, fig. 83 and associated text) postulates an evolutionary relationship in which first the annelids and then the Sirius Passet halkieriids branch from stem lineage brachiopods.

Phylum Arthropoda Siebold & Stannius 1895

Class Dinocarida Collins 1996

Order ?Radiodonta Collins 1996

Kerygmachela kierkegaardi Budd 1998

Description: The cephalic region is characterised 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.

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)

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Pambdelurion whittingtoni Budd 1997

Another Anomalocaridid-like taxon, but with annulated and flexible spinose frontal appendages. No eyes are known. The ‘Peytoia’ mouthpart is poorly sclerotised. The trunk bears lobopodous paired limbs, probably unconnected to the lateral flaps. The tail is poorly known but apparently consisting of a sub-circular flap.

Subphylum Arachnomorpha

Order Nectaspida Raymond 1920

Genus Buenaspis (?)

Type: Buenaspis forteyi [Authority]

Original Diagnosis: xxx

Description: .

Stratigraphic Range: Lower Cambrian (Sirius Passet)

Discussion: .

Class Trilobita Walch 1771

Order Redlichiida Richter 1932

Suborder Olenellina Walcott 1890

Family Nevadiidae Hupé 1953

Genus Buenellus Blaker 1988

Glabella slightly tapered forward; ocular lobe small, posterior tip about opposite S1 or anterior part of L1; width (tr.) of interocular area equal to or greater than that of extraocular area. Thorax with 17 or 18 segments, maintaining width or widening slightly backward to 8th segment, then tapering posteriorly; posterior segment may be fused with anterior part of simple pygidium; pleural spines short (exsag.); inner pleural regions only slightly wider (tr.) than axis.

The species found in the Sirius Passet assemblage is Buenellus higginsi Blaker 1988.

Kleptothule rasmusseni Budd 1995

A trilobite-like arthropod with ogival cephalic region probably containing five segments, one of which is occipital. Parallel-sided and lobed thoracic region with about 27 segments. Caudal region with at least 20 poorly-defined, ankylosed segments.

This is a small, clearly delimited arthropod known from about 60 fossils. Complete specimens are about 30 mm long and 5-6 mm at their widest point. The ‘cephalon’ comprises probably five segments. The thorax is long and multi-segmented. There is a ‘pygidium’ or caudal plate consisting of a number of fused segments.

Subphylum Crustacea

Pauloterminus spinodorsalis (nomen nudum)

"In 1912, Charles D. Walcott erected the Family Waptiidae to accommodate Waptia fieldensis, a still poorly understood bivalved arthropod from the Middle Cambrian Burgess Shale of British Columbia, Canada. Several other bivalved arthropods seemingly similar to Waptia have since been discovered, such as the Lower Cambrian taxa Pauloterminus spinodorsalis (nomen nudum) from the Sirius Passet fauna of North Greenland and Chuandianella ovata from the Chengjiang fauna of southwest China.

"Despite their overall waptiid-like appearance, however, each of these animals possesses features which suggest their apparent similarity to Waptia fieldensis may be superficial. Variability in segment number, limb number and limb type between these taxa, for example, suggests these animals may not in fact be closely related.

"Other non-waptiid Cambrian arthropods also possess bivalved carapaces, including the Burgess Shale taxa Canadaspis perfecta and Plenocaris plena. This indicates two alternative evolutionary scenarios. First, the relatively common occurrence of bivalved carapaces may indicate a stem-group clade of bivalved arthropods in the Cambrian, united (at least) by their possession of this specialized carapace. A second, perhaps more likely, possibility is the occurrence of evolutionary convergence, resulting in the presence of a bivalved carapace in multiple unrelated Cambrian arthropod taxa." (Taylor 1999)

References

Budd, Graham E. 1995: Kleptothule rasmusseni gen. et sp. nov.: an ?Olenellinid-Like Trilobite from the Sirius Passet Fauna (Buen Formation, Lower Cambrian, North Greenland). Transactions of the Royal Society of Edinburgh: Earth Sciences, 86: 1-12.

Budd, G.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)