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Cambrian 2 (Stages 3-4) Small Shelly Fossils from East

By Thomas M. Claybourn

Abstract

An assemblage of Series 2, Stages 3-4 small shelly fossils has been recovered from the Shackleton and Holyoake For- mations of East Antarctica. Small shelly fossils from the early Cam- brian are an important window into the world of Cambrian palaeobiol- ogy, biostratigraphy and biogeography. The aim of this thesis is to add this view the previously under-described fauna from East Antarctica. The molluscs from the Shackleton Limestone prove important in bio- stratigraphic correlation to South , North-East Greenland, North China and South China. Morphometric analysis of these has also yielded insights into inter- and intraspecific variation in the helcionel- loid mollusc Mackinnonia. The remaining fauna contains certain key taxa for biostratigraphic comparison, such as the tommotiid Dailyatia odyssei, the bradoriid arthropod Spinospitella coronata and the brachi- opod Karathele yokensis. This allows for direct correlation with the Stages 3-4 Dailyatia odyssei Zone of South Aus- tralia, further strengthening an already recognised close relationship be- tween the faunas of East Antarctica and South Australia.

Dedication

For Yvonne

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Claybourn, T. M., Holmer, L. E., Skovsted, C. B., Topper, T. P. and Brock, G. A. (2017) Cambrian Series 2, 3–4 micromol- luscs from the Shackleton Limestone, Central , East Antarctica. Manuscript prepared for submission to Journal of Paleontology II Jackson, I. S. C. and Claybourn, T. M. (2017) Morphometric analysis of inter- and intraspecific variation in the Cambrian Se- ries 2, Stages 3-4 helcionelloid mollusc Mackinnonia. Manu- script in review, Palaeontology

Reprints were made with permission from the respective publishers.

Contents

Introduction ...... 7 History of research and geological context ...... 7 Fauna of the Shackleton Limestone and Holyoake Formation ...... 11 Poriferans ...... 11 Coeloslerotophorans ...... 12 Hyoliths ...... 14 Arthropods ...... 14 ...... 16 Molluscs: Systematics, Size and Form ...... 19 Size bias in helcionelloid steinkerns ...... 20 Biomineralisation and shell structure in helcionelloids ...... 21 References ...... 23

Introduction

Metazoan life at the beginning of the Cambrian was characterised by the orig- ination of a host of new biological innovations. Of interest here are an informal group of fossils known as ‘small shelly fossils’ or ‘small shelly fauna’ (SSFs), which comprises taxa from across the eumetazoan phyla and includes numer- ous problematica (Matthews & Missarzhevsky 1975; Bengtson 2004). As Bengtson (2004, p. 67) pointed out, they are often neither small nor shelly. The earliest SSFs appear in the uppermost and include enigmatic calcareous and likely secondarily phosphatised tubular fossils such as Cloudina Germs 1973 and the possible lophophorate Namacalathus Grotzinger et al. 2000, which represent some of the earliest biomineralising taxa in the fossil record. The advent of biomineralisation plays a key role in understanding biological innovation in the latest Ediacaran and Cambri- anMany invertebrate taxa had evolved biomineralised skeletons by Cambrian Series 2, Stage 3 (Kouchinsky et al. 2012).

History of research and geological context Cambrian fossils from Antarctica were first reported and figured from erratic blocks of the Shackleton Limestone at the base of the Beardmore Glacier from samples collected by Ernst Shackleton’s 1907-09 Expedition. These in- clude sections containing archaeocyath steinkerns (Taylor 1914, pl. 76-79 figs. 1-4) and a ‘tuning-fork’ that resembles a spicule from the calcitic Dodecaactinella Reif 1968 (pl. 79, fig.5). The Shackleton Limestone itself was only named almost half a century later by Laird (1963) for archae- ocyath-bearing of the Holyoake Range and Mt. Nares. Laird esti- mated the unit may have a stratigraphic thickness “in the order of 20,000 ft” (c. 6000m) (Laird 1963, pg. 477). These units and those of the overlying Douglas Conglomerate and Starshot Formation comprised the Byrd Group. Laird (1963) also outlined the stratigraphy and named the conformably over- lying “Holyoake Gabbro” (Holyoake Formation of Myrow et al. 2002) and Starshot Formation. Palaeontological work was not conducted until the 1970s with publications formally describing the archaeocyaths (Debrenne & Kruse 1986), (Palmer & Gatehouse 1972; Palmer & Rowell 1995) and some small components of the SSF faunas (Evans & Rowell 1990;

7 Rowell et al. 1988; Rode et al. 2003; Evans 1992). More recently, a strati- graphic scheme has been created and the tectonics of the eastern margin of East Antarctica has been better clarified. The scheme of Myrow et al. (2002) first identified the lateral relationship between the Douglas Conglomerate as cutting through the the Starshot Formation, Holyoake Formation and Shack- leton Limestone. This stratigraphical relationship is followed herein. More recent estimations of the thickness of the fault-repeated Shackleton Lime- stone indicate a thickness of around 200om thick (Laird et al. 1971; Burgess & Lammerink 1979) The Shackleton Limestone is interpreted to have been deposited as a high-energy carbonate platform, on the unconformably underlying Neoprote- rozoic Goldie Formation (Rees et al. 1989). It was deposited as a carbonate shelf bordering the palaeo-Pacific Ocean, on the eastern margin of East Gondwana (Laird & Waterhouse 1962; Rees et al. 1989). The top of the for- mation is characterised by phosphate hardgrounds and an abrupt transition into the Holyoake Formation, a carbonate-nodule bearing shale (Myrow et al. 2002). The molassic siliciclastic Starshot Formation and Douglas Con- glomerate follow as the tectonic system transitioned to a subduction regime from a passive margin and marked the onset of the Ross Orogeny (Boger & Miller 2004; Goodge, Myrow, et al. 2004; Goodge, Williams, et al. 2004). Initial subsidence drowned the carbonate platform, leading to the formation of phosphate hardgrounds, which are often associated with marine transgres- sive events (Southgate 1986; Southgate 1988; Myrow et al. 2002). East Antarctica was, during Cambrian Series 2, sutured between South Australia and the modern eastern coastline of southern Africa, together form- ing the province of East Gondwana. The palaeogeographic position of East Gondwana has historically been a contentious matter, but modern palaeoge- ography resolves East Antarctica bisected by the equator and the coastline facing east, with Australia to the north (Torsvik & Cocks 2013).

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Figure 1 Map of Antarctica and extent of the Central Transantarctic Mountains and the study area, including the Holyoake Range, where the main fossil-bearing sec- tions were made. Adapted from paper I.

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Figure 2 Simplified stratigraphy of the Shackleton Limestone from three sections of the Holyoake Range with occurrences of fossils significant to biostratigraphic correlation and molluscs.

10 Fauna of the Shackleton Limestone and Holyoake Formation

The Shackleton Limestone and Holyoake Formation contains a relatively di- verse array of SSFs, including archaeocyaths, brachiopods, bradoriids cam- broclavid sclerites, chancelloriid sclerites hyoliths, molluscs, sponge spic- ules, tommotiids and eodiscid trilobites. Much of the fauna suggests a strong faunal connection to the other continents of East Gondwana, particularly South Australia and the North and South Chinese blocks. This section pro- vides a brief overview of the undescribed portion of the fauna, which ex- cludes the molluscan component which is dealt with later and in papers I and II.

Poriferans Sponge spicules, particularly heteractinids, form an abundant portion of fos- sils from the Shackleton Limestone. Two distinctive species can be identi- fied: the calcitic Dodecaactinella cynodontota Bengtson & Runnegar in Bengtson et al. 1990 (Figure 3B) and the heteractinid Eiffelia araniformis Missarzhevsky & Mambetov 1981 (Figure C). The Shackleton specimens of D. cynodontota typically are not as well preserved as those from South Aus- tralia, from which they were originally described and typically only retain the central parts of the large, complex spicules (Bengtson & Runnegar in Bengtson et al 1990, fig 11). Dodecaactinella cynodontota spicules have also been described from glacial erratics of King George Island (KGI), Ant- arctica (Wrona 2004) Cambrian Series 2, Stage 3-4 of South Australia (Bengtson et al. 1990; Topper et al. 2009) and Marianian (Cambrian ?Stage 3–4) of eastern Germany (Elicki 1994). Spicules of E. araniformis have also been described from and Series 2, Stage 3-4 of South Aus- tralia (Demidenko in Gravestock et al 2001, Bengtson in Bengtson et al 1990) and KGI glacial erratics (Wrona 2004). Outside East Gondwana, E. araniformis is also present in the Cambrian Series 2, of north-east Greenland (Skovsted 2006), North China (Yun et al 2016), Jordan (Elicki 2011) and Kazakhstan (Missarzhevsky & Mambetov 1981).

11 Figure 3 Coelosclerotophorans and poriferans from the Shackleton Limestone. A: Chancelloriella sp.; B: Dodecaactinella cynodontata; C: Eiffelia araniformis; D: Cambroclavus absonus; E: heteractinid sponge spicule; F-G: Archaeocyath stein- kerns; H: Cambroclavus absonus.

Coeloscleritophorans The problematic group of fossil metazoan sclerites referred to as the coelo- scleritophorans are represented in the Shackleton Limestone by chancelloriid and cambroclave sclerites. Sachitiids are missing from the recovered re- mains. Recent treatments of these often disarticulated sclerites, from the ‘sponge grade’ chancelloriids (Moore et al. 2014) and putative bilaterian cambroclavids (Li et al. 2015) have synonymised many morphospecies into simpler taxonomic schemes. Many disarticulated sclerites have been recov- ered from the Shackleton limestone, including Allonia sp., Archiasterella sp., and Chancelloriella sp. (Figure 3A) Disarticulated Cambroclavus ab- sonus Conway Morris in Bengtson et al. 1990 sclerites (Figure 3D, H) have also been recovered from the Shackleton Limestone. The stratigraphic and biogeographic range for Cambroclavus Mambetov & Repina 1979 has led to it being postulated as being a potentially useful fossil for defining the base of across East and peri-Gondwana, due to its large geo- graphic range (Li et al. 2015). It is known from the Cambrian of South Aus- tralia (Demidenko in Gravestock et al 2001, Topper et al 2009) and North China (Li et al 2016).

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Figure 4: Hyolithellus and arthropods from the Shackleton Limestone and Holyoake Formation. A-C: Hyolithellus sp. from the Holyoake Formation; D-F: Mongolitubu- lus cf. squamifer from the Shackleton Limestone; G-J compatulinid brodoriid frag- ments from the Holyoake Formation; H-L: Spinospitella coronata fragments from the Holyoake Formation; M-P: eodiscid Neocobboldia sp. from the Holyoake For- mation, M-N: cephala, O-P pygidia; Q-U: Pagetides sp. from the Holyoake For- mation.

13 Hyoliths Hyoliths are represented by both hyolithid and orthothecid forms and the more problematic Cupitheca holocyclata, all of which demonstrate markedly different . The orthothecid Conotheca cf. australiensis (Figure 5 A-B, M-L, N-O) appears to preserve the intracrystalline matrix as a phos- phatic mould preserving the shell morphology (Figure 5 E). The original pre- sumably aragonitic laths are marked by spaces in the shell wall. The hy- olithid Microcornus sp. (Figure 5, C-D, G-K) preserved as phosphatic stein- kerns, with imprinted, potentially aragonitic lath moulds on the exterior (Fig- ure 5, J-K). Specimens assigned to Conotheca holocyclata are preserved as steinkerns, often including an external mould, but with no information on the shell structure (Figure 5 Q-T). The external moulds of Cupitheca have re- cently been interpreted to be phosphatised periostracum, with small tubules connecting the steinkern with the phosphatised exterior, which was com- pared to the periostracum of brachiopods and other lophophorates (Vendrasco et al. 2017). No associated operculae or helens have been recov- ered for any of the hyolith specimens. A fourth, problematic form of hyolith- like fossil has also been recovered, with a unique morphology (Figure 5 N- P). The apex and is similar in shape to species of Conotheca, with a decol- lement, but is flared towards the apex, creating a distinct pinch. C. holocy- clata is the only useful fossil for correlation amongst these specimens and is known from the Cambrian Series 2 South Australia (Skovsted et al 2016, Bengtson in Bengtson et al 1990), KGI glacial erratics, (Wrona 2003), North China (Skovsted et al. 2016) and North-East Greenland (Malinky & Skovsted 2004).

Arthropods Both bradoriids and eodiscid trilobites have been recovered from acid resi- dues of the Holyoake Formation. Bradoriids carapaces are fragmentary, so identification has relied on micro-ornamentation or spines. They are repre- sented by Monogolitubulus cf. squamifer (Missarzhevsky 1977) in the Shackleton Limestone (Figure 4 D-F) and Spinospitella coronata Topper et al. 2011 in the Holyoake formation (Figure 4 H-I, K-L). Both these species are identifiable based on their distinctive ornaments. S. coronata has only been described from a single formation- the Cambrian Series 2, Stage 3 Mernmerna Formation of the Arrowie basin (Skovsted et al 2006). The range of M. squamifer is broader, occurring in the Cambrian Series 2, Stages 3-4 of South Australia (Demidenko in Gravestock et al 2001, Topper et al 2009) North East Greenland (Skovsted & Peel 2001), New South Wales, Australia (Brock & Percival 2005), Mongolia (Missarzhevsky 1977) and Kazakhstan (Missarzhevsky & Mambetov 1981). Broken compatulid fragments have

14 Figure 5 Hyoliths from the Shackleton Limestone. A-B, M-L, N-O: Conotheca cf. australiensis, C-D, G-K, Microcornus sp.; N-P: Hyolith-like fossils; Q-T Cupitheca holocyclata. E: Magnified view of the apeture of F; H-J progressively greater magni- fication of the shell microstructure of G; M: oblique apetural view of L.

also been recovered from the Holyoake Formation (Figure 4 G-J). Few eodiscid remnants have been recovered from the Holyoake Formation. Iden- tification of these has proven difficult, as they have likely been effaced dur- ing the acid-etching process, unlike crack-out specimens. They are repre- sented by the Neocobboldia sp. (Figure 4 M-P) and Pagetides sp.(Figure 4 Q-U).

15 Figure 6 Brachiopods and tommotiids from the Shackleton Limestone and Holyoake Formation. A-E Karathele yorkensis from the Holyoake Formation; F-G Acrotretid indet. from the Shackleton Limestone; H-I Shetlandia multiplicata from the Shack- leton Limestone; J-K: Eoobolus priscus from the Shackleton Limestone; L-N: Dai- lyatia odyssei from the Shackleton Limestone.

Brachiopods

16 Lingulate brachiopods are common in acid residues of the Shackleton Lime- stone, particularly acrotretids, with minor numbers of Eoobolus sp., Eoobo- lus priscus Poulsen 1932 (Figure 6 J-K). The botsfordiid Karathele yorkensis Ushatinskaya & Holmer in Gravestock et al. 2001 (Figure 6 A-E) is present in the Holyoake Formation. A large number of both ventral and dorsal valves of a simple acrotretid, appear to be of an early ontogenetic stage and are ef- faced, hindering generic designation for these fossils (Figure 6 F-G). Kara- thele yorkensis has only been described from the Parara limestone of the Yorke Peninsula, South Australia (Ushatinskaya & Holmer in Gravestock 2001). Eoobolus. priscus has a broader range, known from north-east Green- land(Skovsted & Holmer 2005). Skovsted & Holmer (2005) also recognised specimens of Eoobolus aff. E. elatus as E. priscus from the Taconic alloch- thon, New York State, USA (Landing & Bartowski 1996), KGI, Antarctica (Holmer et al 1996) and South Australia (Ushantiskaya & Holmer in Grave- stock et al 2001) as E. priscus.

Tommotiids The tommotiid Dailyatia odyssei Evans & Rowell 1990 (Figure 6 L-N) and putative tommotiid Sheylandia mulitplicata Wrona 2004 (Figure 6 H-I) have been recovered from the Shackleton Limestone. Dailyatia odyssei is a key taxon for biostratigraphy in the Cambrian Series 2, allowing for direct com- parison to the D. odyssei Zone in South Australia (Betts et al. 2016; Betts et al. 2017). Shetlandia multiplicata has only been recovered from the King George Island glacial erratics (Wrona 2004), which aids in sourcing the gla- cial erratics to the Shackleton Limestone. The affinity of these broken and disarticulated sclerites is uncertain, but they are at least superficially similar to tommotiid sclerites.

Notes on Biostratigraphy Paper I includes a section detailing the biostratigraphic correlation of the molluscan component of the Shackleton Limestone and Holyoake For- mations. This can be made chiefly by bracketing the stratigraphy between the occurrence of the fossil species Dailyatia odyssei and Spinospitella coro- nata, the eponym and an important accessory taxon of the newly defined Dailyatia odyssei Zone respectively (Figure 2, Betts et al. 2017). Although S. coronata ranges into their underlying Micrina etheridgei Zone (Betts et al. 2016; Betts et al. 2017), its presence only in the Holyoake Formation and D.

17 odyssei only very low in the Shackleton Limestone, indicates that the com- plete fossiliferous interval of the sections measured correlate with the D. odyssei Zone of South Australia (Figure 2).

18 Molluscs: Systematics, Size and Form

Lower Cambrian univalved molluscs are a ubiquitous part of SSF assem- blages (Bengtson 2004; Kouchinsky et al. 2012). Their high-rank taxonomy and relationship to the rest of the univalved mollusc classes has historically been problematic, with some authors associating them with the mono- placophora (Knight 1952), some as basal gastropods (Parkhaev 2017). Two taxa have been erected to include some forms: the Paragastropoda Linsley & Kier 1984 and the Helcionelloidea Peel 1991 (See introduction in paper I for a full discussion on Cambrian univalved mollusc higher taxonomy). These attempts have been hindered in particular by their preservation. Phos- phate precipitation or secondary phosphatisation was by far the most com- mon mode of preservation in the and Series 2 Stage 3 of the Cambrian (Porter 2004). During periods of high phosphate precipitation (Jacquet et al. 2016) calcitic and aragonitic shells formed a nucleation point for phosphate to precipitate and form internal (steinkern) and external moulds (Creveling et al. 2014). The fidelity of steinkerns’ representation of the original shell is problematic in that it does not necessarily preserve the entirety of the shell, forming an incomplete ‘teilsteinkern’ (Dattilo et al. 2016). Evidence also suggests that at least some taxa represent juvenile shells of much larger organisms (Martí-Mus et al. 2008; Jacquet & Brock 2016). The preservation of phosphatic steinkerns is also highly facies de- pendent (Landing 1992; Mount & Signor 1992; Steiner et al. 2004; Steiner et al. 2007; Jacquet et al. 2016). These issues filter down to cause problems with generic and specific level taxonomy of steinkern-based taxa, exemplified by univalved molluscs. The different information preserved by steinkerns, which is controlled by the rate of dissolution of the aragonitic or calcitic shell, rate of phosphate precip- itation and inclusion of foreign objects can lead to a strong facies-dependent bias in the size and shape of preserved steinkerns (Creveling et al. 2014; 2013).

19 1600

1400

1200

1000 m) μ 800

Height ( Stansbury Basin, 600 South Australia

Shackleton 400 Limestone

Ajax Formation, 200 Arrowie Basin

0 0 500 1000 1500 2000 2500 Length(μm)

Figure 7 Height and length measurements of Mackinnonia rostrata (Zhou and Xiao 1984) steinkerns from Australia and Antarctica. Measurements of Stansbury Basin M. rostrata specimens are from Parkhaev in Gravestock et al. (2001, p. 177).

Size bias in helcionelloid steinkerns Paper II deals with variation in shape of specimens of the helcionelloid ge- nus Mackinnonia (Zhou & Xiao 1984). Simpler methodologies and observa- tions can be used to determine potential size bias in the genus also. Figure 3 shows simple height and length measurements of specimens of Mackinnonia rostrata (Zhou and Xiao 1984). Although smaller, less well preserved speci- mens are not necessarily documented in the literature for the purposes of systematics, the South Australian specimens measured by Parkhaev in Gravestock et al. (2001) and those from the Ajax Formation of South Aus- tralia show that at least the maximum size of M. rostrata steinkerns is larger than the steinkerns from the Shackleton Limestone. Incomplete steinkern formation can also lead to a loss of information on the aperture and apertural characteristics (e.g. the parietal train) of helcionelloids. An example of this may have been identified in fig. 6 of paper I, in steinkerns of Stenotheca sp.,

20 which have apex, ribs, protoconch and in one case external mould of orna- ment very similar to those of Stenotheca drepanoida (He et al. 1984). The transverse ribs of these specimens are truncated at the base of the steinkern, indicating the base is not the original aperture of the shell, and the parietal gape present in stenothecids may be missing.

Figure 8 Distriubution of shell microstructure on different ontogenetic stages of Mackinnonia rostrata from the Shackleton Limestone. A-C: Specimens of Mackin- nonia rostrata steinkerns, ordered youngest (A) to oldest (C); D-F: Diagrams of the distribution of each of the four crytsal types.

Future directions: Biomineralisation and shell structure in helcionelloids A redeeming feature of steinkerns is the often fine scale imprints of various mineral structures on their exterior (Runnegar 1985). Recognition of differ- ent crystal morphs was achieved through comparison to modern mollusc in- ternal shell wall structure and formalised by Vendrasco et al. (2010).Based on these microstructures, four types have been recognised in steinkerns from the Shackleton Limestone of Antarctica as well as South Australian speci- mens: laminar, polygonal, pitted and smooth. Shell microstructure is often

21 not uniformly distributed across the steinkern. In the case of Mackinnonia rostrata, for example, up three of the four types can be observed in a single specimen (Figure 8). The smooth and laminar structures are concentrated at the protoconch and concave areas of the steinkern, whereas the pitted is only present on the parietal train of smaller specimens. This indicates a transition in biomineralising processes in the mantle of Mackinnonia. Vendrasco et al. (2010) speculated a number of explanations for the pitted structure on the helcionelloid Figurina Parkhaev in Gravestock et al. 2001. These included phosphatic replacement of organic matrix, a mould of rapidly dissolved pris- matic microstructure, a tubule-bearing inner surface of the shell wall and early diagenetic alteration of the shell. As these pitted structures are typically concentrated around the youngest part of the shell in Mackinnonia rostrata, an ontogenetic explanation for the growth of prismatic calcite may be estab- lished. According to this hypothesis the narrow pits of the ‘pitted’ micro- structure may be underdeveloped prismatic columns, allowing for broader ridges between the pits, representing phosphatised intercrystalline matrix (Figure 9).

Figure 9 Model of the growth of calcite prisms in molluscs (top row) and hypothet- ical relationship to pitted and prismatic microstructure present on the exterior of steinkerns, in cross section (bottom row).

22 Acknowledgements

A team of supervisors were vital for getting me started with this pro- ject and the production of this thesis. Thanks for this go to Glenn Brock, Lars Holmer, Christian Skovsted and Graham Budd. Thanks also to Tim Topper for always being an email away for answering silly questions. Michael Streng and Milos Bartol are thanked for their help with the SEM, Michael especially for his endless pateince in the acid lab. Illiam Jackson and Giannis Kesidis never buy the coffees for fika but at least they’ve usually been available for some R&R.

23 References

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