Organophosphatic Stem Group Brachiopods - Implications for the Phylogeny of the Subphylum Linguliformea

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Organophosphatic stem group brachiopods - implications for the phylogeny of the Subphylum Linguliformea

Article in Fossils and Strata · February 2008

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OrganophosphaticBlackwell Publishing Ltd stem group brachiopods: implications for the phylogeny of the subphylum Linguliformea

LARS E. HOLMER, LEONID POPOV AND MICHAEL STRENG

Holmer, L.E., Popov, L. & Streng, M. 2008: Organophosphatic stem group brachiopods: implications for the phylogeny of the subphylum Linguliformea. Fossils and Strata, No. 54, pp. 3–11. ISSN 0024-1164

The recognition of potential organophosphatic-shelled stem group brachiopods has important implications for the understanding of brachiopod phylogeny. These groups fall outside the two currently recognized classes of the subphylum Linguliformea – the Lingulata and Paterinata. However, their organophosphatic shell structure and evidence of penetrative setae demonstrate that they are linked phylogenetically with the linguliforms. The proposed Early Cambrian stem-group brachiopods include the problematic, possibly vermiform, organophosphatic sclerite-bearing tannuolinids and the more brachiopod-like Mickwitzia and Heliomedusa. A columnar shell fabric, which was considered previously as a derived feature of acrotretids, is now known also from the tannuolinid Micrina as well as from Mickwitzia, thus indicating that this type of shell structure may be a plesiomorphic character; it was retained in acrotretids and some lingulids, like the Lingulellotretidae, which is here shown to include a wide variety of columnar fabrics. A columnar shell structure (sensu lato) is also identified here from the enigmatic South American Ordovician linguliform brachiopod Bistramia. The shells of Mickwitzia and Heliomedusa also have various types of thicker cylindrical columns (‘tubes’), some of which were clearly open to the exterior surface and can be inferred to have contained setal structures penetrating the shell. Identical perforations are present in Micrina, and a similar function can be inferred for both the columnar fabric of some recently discovered paterinids described here and possibly also of some siphonotretids, indicating that these groups may also be closer to the stem of the brachiopods. Brachiopoda, Lingulellotretidae, Linguliformea, Paterinida, phylogeny, shell structure, Siphonotretida.

Lars E. Holmer [[email protected]] and Michael Streng [[email protected]], Department of Earth Sciences/Palaeobiology, Uppsala University, Norbyvägen 22, 75236 Uppsala, Sweden; Leonid Popov [[email protected]], Department of Geology, National Museum of Wales, Cathays Park, Cardiff CF10 3NP, Wales, UK.

The discovery by Williams & Holmer (2002) that the This paper provides new data on the distribution phosphatic stratiform laminae of the brachiopod- of the columnar shell fabric (sensu lato), including new shaped, two sclerites of Micrina Laurie exhibit a evidence for Micrina–Mickwitzia-type penetrative setae columnar shell fabric (sensu lato) including striated within new groups of organophosphatic brachiopods, apatitic columns (‘tubes’) that, at the surface, must and provisionally explores the implications for the have contained chitinous brachiopod-like setae, and phylogeny of the subphylum Linguliformea. the subsequent identification of identical structures in the stem group brachiopod Mickwitzia (Holmer et al. 2002) re-emphasized the importance of shell Materials and methods structure in phylogenetic studies. Following these findings, a surprisingly rich record of Early Cambrian The organophosphatic-shelled valves examined in organophosphatic-shelled stem group brachiopods is this paper were isolated either by dissolving various beginning to emerge (Holmer et al. 2003, 2004; Peel Cambrian and Ordovician carbonate rocks in 10% 2003; Skovsted & Holmer 2003; Balthasar 2004; Li & acetic acid or, if for material from a non-carbonate Xiao 2004). In particular, the new discoveries confirm unit, mechanically prepared specimens. that the distribution of Micrina–Mickwitzia-type All natural and fractured surfaces where either columnar penetrative setal structures and the acro- coated with gold and examined using high vacuum tretoid columnar shell fabric (see Holmer 1989; scanning electron microscopy (SEM), or in the case Williams & Holmer 1992) occurs both within stem of type museum material, left uncoated and examined and crown group brachiopods. either with a LEO 1530 environmental SEM with

4 Holmer et al. FOSSILS AND STRATA 54 (2008) field emission (EBC, Uppsala University) or a provisional, and as noted by Holmer et al. (2002, CamScan MaXim 2040S SEM with variable vacuum p. 880), ‘the phylogenetic range of columnar and (National Museum of Wales and Cardiff University). apatitic tubes has yet to be fully determined among The details of the specimens used to illustrate the the Brachiopoda and Problematica’. shell structures and other features are as follows: Acrotretoid columnar structure • Micrina etheridgei (Tate) – Lower Cambrian Wilkawillina Limestone, Wilkawillina Gorge, Flinders As defined originally (Holmer 1989, p. 31) and used Range, South Australia (Williams & Holmer 2002). later by Williams & Holmer (1992), this term referred • Mickwitzia cf. occidens Walcott – Lower Cambrian only to ‘acrotretoid shell fabric of discrete apatitic (Botomian) Ella Island Formation, northeast Greenland lamellae connected by microscopic perpendicular (Holmer et al. 2002; Skovsted & Holmer 2003). columns’ (Williams et al. 1997, p. 427), and was • Cryptotreta? undosa (Moberg) – Lower Cambrian generally considered to represent a synapomorphic Kalmarsund Sandstone, Kalmarsund, Sweden character for the order Acrotretida (Holmer 1989). (Åhman & Martinsson 1965). In general, the typical acrotretoid columns are solid • Lingulellotreta malongensis (Rong) – Lower structures, around 1.5–5 μm in diameter, with thin Cambrian (Botomian–Toyonian), Ushbaspis limbata axial canals that penetrate the successive sets of and Redlichia chinensis–Kootenia gimmelfarbi compact laminae. Cusack et al. (1999) and Williams biozones, Ushbas River, Malyi Karatau, Kazakhstan & Cusack (1999) identified an acrotretoid columnar (Holmer et al. 2001). structure in the Lower Cambrian lingulid Lingulel- • Aboriginella denudata Koneva – Upper Cambrian, lotreta, indicating for the first time that this fabric Eolotagnostus scrobicularis beds (sample 1413-1), was more widespread than previously suspected. The Shabakty River, Malyi Karatau, Kazakhstan (Holmer cylindrical structures described by Holmer et al. et al. 2001). (2002) and Skovsted & Holmer (2003) from Mickwitzia • Vaculina obscura Koneva – Middle Cambrian, are identical with the acrotretoid columns (Fig. 1E, F). Glyptagnostus stolidotus Biozone (sample 1352- III), Kyrshabakty River, Malyi Karatau, Kazakhstan Micrina–Mickwitzia columnar structure (Holmer et al. 2001). • Mirilingula postuma Holmer, Popov, Koneva and Williams & Holmer (2002) identified a different, but Bassett – Lower Ordovician (Tremadocian; basically columnar fabric (referred to as ‘canals’) in sample 382 m), Batyrbai, Malyi Karatau, Kazakhstan the Lower Cambrian stem group brachiopod Micrina (Holmer et al. 2001). (Fig. 1A). In Micrina, the columns (generally around • Bistramia elegans von Hoek – Ordovician (Caradoc), 10 μm in diameter, but increasing in width with Cochabamba, El Cristo de la Concordia, Bolivia growth) are generally orthogonal to the shell surface, (Holmer & Popov 2000). with a well-defined, wide central canal penetrating successive compact laminae; the columns sometimes The illustrated specimens are deposited in the Museum open to the outer shell surface through a pore. An of Evolution – Palaeontology, Uppsala (PMU); identical type of columnar shell structure is also National Museum of Wales, Cardiff (NMW); found in species of the Lower Cambrian stem group Geological Museum of Copenhagen (MGUH); South brachiopod Mickwitzia, and although its shell has Australian Museum, Adelaide (SAM). been described generally as ‘punctate’ (e.g. Laurie 2000), Holmer et al. (2002) and Skovsted & Holmer (2003) showed that the ‘punctae’ of Mickwitzia actually constitute thicker columns (‘tubes’) that are Terminology of the columnar generally disposed orthogonally to the laminar shell, (sensu lato) shell fabric and some of them were open to the exterior through pores (Fig. 1D, E), from which it was hypothesized The main types of shell fabrics within extinct that setal structures may have emerged. Balthasar linguliform brachiopods have been described and (2004) proved that some of the orthogonal columns summarized mainly by Holmer (1989), Williams & (‘tubes’) of Mickwitzia indeed must have contained Holmer (1992), Williams et al. (1997, 1998, 2004), setal structures, since the pores at the surface are Cusack et al. (1999), Williams & Cusack (1999), and provided with groove-like depressions from an Williams (2003). emerging cylindrical object; he also showed that The terminology adopted below for the various there is a wider range of variation of orthogonal types of ‘columnar’ fabrics should be considered as columnar (‘tubular’) shell structures within Mickwitzia

FOSSILS AND STRATA 54 (2008) Organophosphatic stem group brachiopods phylogeny 5

Fig. 1. A–C. Micrina etheridgei (Tate). A. Detail of interior margin of mitral sclerite showing a penetrative striated setal column and a Micrina–mickwitziid column (opening marked by arrow), SAM P42035. B. Detail of A, showing striated interior canal of setal column. C. Oblique lateral view of juvenile mitral sclerite showing numerous external pores (of penetrative setal columns), SAM P42036, Lower Cambrian Wilkawillina Limestone, Wilkawillina Gorge, Flinders Range, South Australia (Williams & Holmer 2002). D–H. Mickwitzia cf. occidens Walcott. D. Detail of external opening of a Micrina–Mickwitzia column, scale bar = 10 μm. E. Section through primary and secondary layer with a Micrina–Mickwitzia column as well as numerous acrotretoid columns. F. Detail of section through secondary layer with compact laminae separated by slit and acrotretoid columns, scale bar = 5 μm. G. Detail of two penetrative striated setal columns on the ventral pseudointerarea, scale bar = 10 μm. H. Detail of pseudointerarea showing openings of penetrative setal columns on the ventral pseudointerarea. Lower Cambrian (Botomian) Ella Island Formation, northeast Greenland (Skovsted & Holmer 2003).

(sensu lato) as compared with those described by Penetrative striated setal columns (‘tubes’) Holmer et al. (2002), and Skovsted & Holmer (2003). Holmer et al. (2003) have shown that the enigmatic The striated columnar shell structure of Micrina Lower Cambrian Heliomedusa (Chengjiang Lagerstätte, (Fig. 1A–C) has been known for some time (e.g. South China) is also a stem group brachiopod, which Laurie 1986, Conway Morris & Chen 1990). Williams has a Mickwitzia-like orthogonal columnar shell, with & Holmer (2002) concluded that the striae indicate emerging pyritized setae (Holmer & Popov 2007). that microvillous setoblasts must have been involved

6 Holmer et al. FOSSILS AND STRATA 54 (2008) in secretion of the columns. Moreover, since the concluded that the shell consisted of stratiform pores include surface depressions (Fig. 1C), it is most laminae and lacked all signs of columnar shell. It has likely that chitinous brachiopod-like setae emerged recently been shown (Holmer et al. 2006) that the from the openings. Holmer et al. (2002) and Skovsted cryptotretid Askepasma (Lower Cambrian, South & Holmer (2003) later identified identical striated Australia) has a stratiform lamellose shell that is columns, oriented almost parallel to the lamellae, and penetrated by phosphatic columns (15–30 μm in emerging from the ventral pseudointerarea of Mick- diameter), with a well-defined outer wall and an witzia cf. occidens (Fig. 1G, H). Penetrative striated open central canal. The columns in Askepasma are setal columns also occur in the exceptionally oriented subparallel to the shell lamination, and are preserved material of the stem group brachiopod open to the exterior; although there is no trace of Mickwitzia occidens Walcott from the Early Cam- striation, they most likely contained setae that would brian of Nevada (Holmer & Popov 2007). have emerged between lamellae. Evidence for penetrative setal structures, but Non-penetrative columnar lacking preserved striation, is also present in the (‘orthogonal baculate’) Lower Cambrian cryptotretid Cryptotreta? undosa (Fig. 2). As noted by Williams et al. (1998), the This type of fabric, which is here first described from periostracum of all available material of this taxon is the family Lingulellotretidae, can also be termed seemingly strongly phosphatized, which obscures columnar sensu lato. These columns are similar in some of the finer structure. However, the extremely size and morphology to the acrotretoid columnar strongly developed nick-points show evidence of fabric discussed above, but differ in that the compact openings, and the surrounding exterior shell has laminae enclosing the columnar sets remain imperfo- impressions of cylindrical emerging objects (Fig. 2B, rated, and a defined central canal is lacking (Fig. 3C–E). C, E, F). The secondary shell structure of Cryp- At the same time, the lingulellotretid columns are totreta? undosa was also described by Williams et al. not typical linguloid baculi, which were defined by (1998, p. 232), who noted circular shallow pits in the Williams et al. (1997, p. 424), as ‘microscopic apatitic stratified laminae, which were interpreted as either rods forming a criss-cross array in an organic matrix ‘diagenetic solution hollows’ or ‘original depressions’. in linguloid and discinoid shells’. However, this type Further examination of similar structures in new of non-penetrative columnar fabric can be compared material of the same species indicates that most of with that illustrated briefly from Dysoristus (Cusack the circular structures (around 10–15 μm in diameter) et al. 1999, pl. 9, fig. 4, appendix 1 ‘character state 22’), have a well-defined wall with an open central canal and it is possible that the columnar fabric recently (Fig. 2G–I) and are quite similar to the Micrina– described in the family Curticiidae by Streng & Mickwitzia columnar fabric, but further studies are Holmer (2005) may prove to belong to this category. required to confirm this notion.

Siphonotretid setal columnar perforations Family Lingulellotretidae Williams et al. (2004) discovered that the shells of The small family Lingulellotretidae is among the Helmersenia and Gorchakovia lack siphonotretid hol- oldest known members of the order Lingulida. The low spines, but are perforated by canals with external first representative of the family, Lingulellotreta, appears depressions (antechambers) that in life contained in the late Atdabanian–early Botomian of South chitinous structures – possibly some kind of tubercles. Kazakhstan and South China. During the Mid- Similar perforations can be observed also in the Cambrian–Early Ordovician interval, lingulellotretids umbonal areas of both valves of Siphonotreta and seemed to have survived only in Kazakhstan, where other siphonotretids, but the distribution of this character they are represented by the three genera Vaculina, has not yet been fully investigated. This shell fabric Aboriginella and Mirilingula (Holmer et al. 2001). can only be termed ‘columnar’ in the widest sense. Williams & Cusack (1999) and Cusack et al. (1999) revealed that Lingulellotreta has a basically acrotretoid columnar shell structure, comprising sets of orthogonal New records of columnar shell fabric columns (up to 3.5 μm in diameter), extending between pairs of compact laminae, up to 2 μm thick (Fig. 3A, B). Order Paterinida Columns and compact laminae are perforated by canals around 1μm in diameter or less (Fig. 3B). The basic shell structure of the paterinate brachiopods The Mid-Cambrian Vaculina and the Late was described in detail by Williams et al. (1998), who Cambrian Aboriginella exhibit larger variations in FOSSILS AND STRATA 54 (2008) Organophosphatic stem group brachiopods phylogeny 7

Fig. 2. Cryptotreta? undosa (Moberg), PMU Sm1. A. Ventral valve exterior, with locations of B, D, E, G, H indicated. B. Detail of A, showing a partly exfoliated phosphatized periostracum with epithelial cellular imprints. C. Detail of B, showing cylindrical external openings of possible penetrative setal columns causing impressions in the shell. D. Detail of A, showing exfoliated shell surface with tubular opening of possible penetrative setal column. E. Detail of A, showing mature ornamentation with nick points and drapes, which are indented by groove-like impressions of possible setal structures. F. Detail of E, showing nick point and impressions of possible setae. G. Detail of A, showing exfoliated phosphatized periostracum with epithelial cellular imprints and sections through possible Micrina– Mickwitzia columns. H. Detail of A, showing completely exfoliated shell exposing secondary layer with two possible Micrina–Mickwitzia columns. I. Detail of H. Lower Cambrian Kalmarsund Sandstone, Kalmarsund, Sweden (Åhman & Martinsson 1965). characters of their secondary shell structure. In these and its secondary shell comprises only various types two taxa, the posterior part of both valves shows sets of baculate sets (Fig. 3F). of non-penetrative columns (‘orthogonal baculi’), connecting a pair of imperforated compact laminae Genus Bistramia (Fig. 3C–E). The non-penetrative columns are up to 2.5 μm in diameter and composed possibly of The systematic position of the enigmatic and poorly aggregates of pinacoids (Fig. 3C, D). known Ordovician Bistramia elegans van Hoek has The Early Ordovician Mirilingula retains all remained elusive and it was placed provisionally in characteristic lingulellotretid features of morphology, the superfamily Linguloidea both by Rowell (1965, but it completely lacks any type of columnar fabric, p. 269) and by Holmer & Popov (2000, p. 77, fig. 37: 3). 8 Holmer et al. FOSSILS AND STRATA 54 (2008)

Fig. 3. A, B. Lingulellotreta malongensis (Rong), NMW 2006.18G.2. A. Detail of shell fracture through secondary shell, showing acrotretoid columns. B. Detail of shell fracture through secondary shell, showing compact laminae, perforated by canals. Lower Cambrian (Botomian–Toyonian), Malyi Karatau, Kazakhstan. C. Aboriginella denudata Koneva, NMW 98.61G.1111, detail of shell fracture through secondary shell showing non-penetrative columns. Upper Cambrian, Shabakty River, Malyi Karatau, Kazakhstan. D, E. Vaculina obscura Koneva, NMW 98.61G.1052. D. Detail of shell fracture showing secondary shell with short non-penetrative columns. E. Detail of shell fracture showing secondary shell with numerous compact laminae connected by non-penetrative columns. Middle Cambrian, Glyptagnostus stolidotus Biozone, Kyrshabakty River, Malyi Karatau, Kazakhstan. F. Mirilingula postuma Holmer et al. (2001), NMW 98.61G.1110. Detail of shell fracture showing baculate shell structure. Lower Ordovician, Tremadocian, Batyrbai, Malyi Karatau, Kazakhstan. FOSSILS AND STRATA 54 (2008) Organophosphatic stem group brachiopods phylogeny 9

Fig. 4. Bistramia elegans von Hoek, NMW 2006.18G.1. A. Detail of partly exfoliated surface of ventral valve, location of B indicated. B. Detail of A, showing Micrina–Mickwitzia columns in the secondary layer, location of C and E indicated. C. Detail of B, showing Micrina–Mickwitzia column surrounded by microgranular apatite. D. Detail of C, showing column with central canal. E. Detail of B, showing column with well defined wall and central canal. F. Detail of exterior surface of A, showing faint fila and circular openings (marked by arrow). Ordovician (Caradoc), Bolivia.

It has never been studied in detail, and the internal structure demonstrates that its secondary shell is entirely and external morphological characters remain extremely of the Micrina–Mickwitzia columnar type (Fig. 4). poorly known. New topotypic material from the The columns in Bistramia are up to 15 μm in Ordovician at Cochabamba, Bolivia, has recently diameter, with a well-defined outer wall (5 μm thick), become available, and an initial study of its shell apparently composed of acicular apatite crystallites, 10 Holmer et al. FOSSILS AND STRATA 54 (2008) and a central canal (5 μm in diameter; Fig. 4B–E). found within members of this stem group, and the The columns are invariably orthogonal to the shell Micrina–Mickwitzia columnar fabric and penetrative lamination and the spaces between columns are striated setal columns are retained. either empty or filled completely by a microgranular 3. The inclusion of the extinct class Paterinata apatitic matrix (Fig. 4C, D). The exterior shell surface within the Linguliformea has always been prob- is invariably strongly phosphatized and it is impossible lematical. The work by Williams et al. (1998) to determine if the columnar structure penetrates to indicates that they had fused mantle lobes in the exterior, but the surface of the shell occasionally combination with an attachment to the substrate exhibits poorly visible circular depressions (Fig. 4F). by means of a cuticular pad (Williams et al. 1998). Williams et al. (1998) proposed that some of these features possibly represent plesiomorphic Discussion and conclusions characters retained from the stem group. To the list of potential retained plesiomorphic charac- Recent studies of shell fabrics of living and extinct ters can now be added the penetrative columnar linguliforms by Williams et al. (1998, 2004), Cusack structures described above from members of et al. (1999), Williams & Cusack (1999), and Williams the family Cryptotretidae. The penetrative setae (2003), have revealed that the organophosphatic were evidently lost in members of the family skeletal systems are complex. There is little doubt Paterinidae. that mapping the distribution of shell structure is of 4. The phylogenetic position of the family Lingulel- great importance for understanding the phylogeny of lotretidae within the Linguliformea remains both the Linguliformea and the Brachiopoda as a problematical. The earliest members exhibit a whole. However, the significance of homoplasy in typical acrotretoid columnar shell structure, but the evolution of organophosphatic brachiopod shell the characters of changes in the shell structure lamination, especially in the lingulids, remains far observed here from later lingulellotretids seemingly from clear. As a result, the published cladograms have suggest that the primitive acrotretoid columnar so far been inconclusive, with highly varying topologies shell structure is modified through the non- depending on a large degree on how characters were penetrative columnar (‘orthogonal baculi’) sets to coded and which groups were included in the analysis. a secondary shell that is completely built of The new types and new records of columnar shell variable baculate sets in later lingulellotretids. fabrics described above indicate that there is still The possibility that the baculate shell structure in insufficient understanding of the basic shell structure lingulellotretids evolved independently from of many key stem and crown group brachiopods. that found in other lingulids requires further Although no new cladistic analysis is attempted here, examination. it is possible to draw the following phylogenetic 5. The systematic position of the family Curticiidae conclusions from the distribution of the various within the order Acrotretida should be re- columnar shell fabrics. examined following the discovery that Curticia exhibits both baculate secondary shell as well as a 1. The organophosphatic-shelled Micrina-like stem columnar fabric in the tertiary layer (Streng & group of the Brachiopoda is characterized by the Holmer 2005). Micrina–Mickwitzia columnar shell fabric as well 6. The phylogenetic position of the order Sipho- as penetrative striated setal columns, both of notretida within the Linguliformea also needs to which are plesiomorphic for the subphylum be investigated further. As noted by Williams Linguliformea. et al. (2004), the stratiform shell structure is most 2. The family Mickwitziidae, as used by Laurie similar to that of the Paterinata. It is possible that (2000) and others (Skovsted & Holmer 2003), is the canals with external depressions and their inferred probably not a monophyletic group, but seem- external chitinous structures are homologous ingly represents a paraphyletic stem group. It with the setigerous penetrative setal columns remains preferable at present to treat this as an found within the stem group of the Brachiopoda informal grouping of mickwitziid-like stem group as well as some paterinates, and represent a retained brachiopods, pending further study. As noted by plesiomorphic character also for the Siphonotretida. Balthasar (2004), the great variation in morphology 7. The phylogenetic significance of the Micrina– and shell structure between the species presently Mickwitzia columnar shell structure in the enigmatic placed within Mickwitzia (sensu lato) requires further linguliform Bistramia is difficult to evaluate at investigation, pending re-study of the type species. present, but it may represent a surviving member The acrotretoid columnar shell structure is first of a basal linguliform stem group. FOSSILS AND STRATA 54 (2008) Organophosphatic stem group brachiopods phylogeny 11

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