Vol. XLII(4): 2010 THE FESTIVUS Page 43

SHELL MICROSTRUCTURES IN EARLY MOLLUSKS

MICHAEL J. VENDRASCO1*, SUSANNAH M. PORTER1, ARTEM V. KOUCHINSKY2, GUOXIANG LI3, and CHRISTINE Z. FERNANDEZ4

1Institute for Crustal Studies, University of California, Santa Barbara, CA, 93106, USA 2Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden 3LPS, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, P.R. China, 414601 Madris Ave., Norwalk, CA 90650, USA

Abstract: Shell microstructures in some of the oldest known mollusk fossils (from the early to middle Cambrian Period; 542 to 510 million years ago) are diverse, strong, and in some cases unusual. We herein review our recent work focused on different aspects of shell microstructures in Cambrian mollusks, briefly summarizing some of the major conclusions from a few of our recent publications and adding some new analysis. Overall, the data suggest that: (1) mollusks rapidly evolved disparate shell microstructures; (2) early mollusks had a complex shell with a different type of shell microstructure in the outer layer than in the inner one; (3) the modern molluscan biomineralization system, with precise control over crystal shapes and arrangements in a mantle cavity bounded by periostracum, was already in place during the Cambrian; (4) shell microstructure data provide a suite of characters useful in phylogenetic analyses of mollusks and mollusk-like Problematica, allowing better determination of the magnitude of disparity during the Cambrian as well as understanding of how the body plans of extant phyla were built through time; (5) calcitic semi-nacre, a type of shell microstructure characteristic of brachiopods and bryozoans, occurred in Cambrian mollusks, suggesting a deeper level of homology in the shells of these lophotrochozoan taxa; and (6) laminar shell microstructures, which are the strongest (most fracture resistant) but most energetically expensive and slowest to build, were common in Cambrian mollusks, suggesting predation was a powerful selective force at that time and providing additional evidence that the origin of mobile predators was a contributing cause of the Cambrian diversification event (Cambrian explosion) and the appearance of mineralized skeletons.

Introduction lineages towards diverse defensive solutions. The evidence is limited but overall is consistent with this The rapid diversification of animals beginning hypothesis, including the following supporting around 542 million years ago was one of the most observations: (1) the earliest signs of predation occur at significant events in the history of life. This event, the base of the Cambrian or just before (Bengtson & known as the “Cambrian Explosion,” is characterized Zhao, 1992); (2) many different types of fossil evidence by the independent appearance and rapid diversification of predation have been recovered from Cambrian rocks, of shells in many animal lineages (Bengtson & Conway including predatory appendages on fossil arthropods Morris, 1992). The Cambrian explosion is the time (Whittington & Briggs, 1985), drill holes (Conway when most fossilizable phyla first appear in the fossil Morris & Bengtson, 1994), bite marks (Conway Morris, record and when most phyla first develop mineralized 1998), ingested prey preserved in the digestive tract of skeletons. Although there have been significant advances predators (Whittington, 1985), and healed shell scars over the past few decades in our understanding of this (Skovsted et al., 2007); and (3) shells, thought by many interval, we are far from knowing the causes of the to be primarily a tool of defense (Vermeij, 1987), event or its detailed pattern. appeared in many different animal lineages during the One of the more prominent hypotheses about the Cambrian explosion (Bengtson & Conway Morris, Cambrian explosion is that it was caused by the onset of 1992) and were made of diverse components and had predation, which likely drove adaptation in various different microstructures (Bengtson & Conway Morris,

*Corresponding author: 14601 Madris Ave., Norwalk, CA 90650, USA. E-mail: [email protected] Page 44 THE FESTIVUS Vol. XLII(4): 2010

1992), and so likely evolved independently in many of these are likely members of extinct phyla, as Gould clades. Nevertheless, in spite of this preliminary suggested, many of these still problematic fossils are support, the hypothesis that predation was a major probably members of a stem lineage of an extant driving force of the Cambrian explosion is still disputed. phylum, and as such are critical in understanding the The Cambrian Period is also characterized by many early evolution of animal morphology (cf. Budd, 2003). problematic taxa (“Problematica” or “Incertae sedis”, The body plans of extant phyla were built in a piecemeal class uncertain) that cannot easily be classified into manner through time, and understanding which fossils modern groups of animals. Gould (1989) emphasized are members of which extinct stem lineages can help us such taxa in his popular book Wonderful Life: The understand how the body plans of extant taxa originated Burgess Shale and the Nature of History, wherein he (Figure 1). Moreover, understanding which of these inferred much higher morphological disparity (i.e., fossils are not members of stem lineages of extant phyla many more phyla, more basic anatomical designs) of will help us better assess the extent of phylum-level animals in the Cambrian than later in Earth history. By disparity in the Cambrian, allowing improved testing of this logic, many of the problematic fossils in the Gould’s (1989) hypothesis of higher Cambrian disparity. Cambrian are members of phyla that went extinct by the end of the Cambrian. The title of the book was a reference to the 1946 movie, It's a Wonderful Life, because Gould (1989) envisioned contingency in the history of life: by his reasoning, if you re-ran the tape of life (i.e., a “do over” in schoolyard speech), different groups of animals would go extinct during the Cambrian, and our planet would have a very different fauna, with perhaps no vertebrates and probably no humans. Conway Morris (1998) strongly objected to Gould’s reasoning, pointing out the commonality of convergent evolution throughout the history of life and suggesting that lineages will be pulled towards certain forms by natural selection. Convergent evolution is the phenomenon where distantly related lineages independently evolve similarities in form as a result of adaptation to similar environments (e.g. torpedo shape with fins in dolphins, sharks, and ichthyosaurs — an extinct group of reptiles). Gould (2001) was unconvinced by Conway Morris’ argument, seeing no inevitability in evolution or the survival of vertebrates or humans, and maintained his opinion on contingency. Meanwhile, Gould’s (1989, 2001) claim of higher disparity during the Cambrian was disputed by researchers who used multivariate morphospace analyses Figure 1. The character states that define the crown group of a taxon to try to quantify disparity in arthropods and priapulids, are likely to have arisen in a piecemeal manner. The crown group is defined as the last common ancestor of all living members of a taxon noting that disparity by these measures was about the of interest (e.g., Mollusca) and all of its descendants. The stem same in the Cambrian as today (Wills, 2001). lineage is comprised of any taxa outside the crown group that are However, Gould (1989) emphasized disparity among, more closely related to the crown group than they are to the most not within, phyla, so it is unclear to what extent the proximal extant outgroup (i.e., the sister group). The basal taxa of a stem lineage would therefore be expected to share only some of the results reported by Wills (2001) bear on Gould’s characters uniting the crown group. They would lack any hypothesis. apomorphies (derived character states) arising in any members more Since the publication of Gould’s Wonderful Life, a closely related to the crown group. (Diagram modified from Budd, number of previously problematic fossils have been 1998.) assigned to the stem lineages (Figure 1) of modern phyla, although numerous fossils from the earliest Mollusks occur in many of the early animal fossil assemblages of animals remain problematic. While some assemblages and clearly began to diversify in the early Vol. XLII(4): 2010 THE FESTIVUS Page 45

Cambrian. Most of the earliest (early Cambrian) shelled shell microstructure (the shape and arrangement of mollusks were univalves with slight coiling but some mineral crystals in the shell) in sub-micrometer detail. had shells with greater coiling and others were bivalves Runnegar showed that coatings on the inner shell surface (Figure 2). The relationships among these earliest of secondary calcium phosphate – probably precipitated mollusks remain controversial because there are so few as a result of bacterial decay shortly after the death of characters that are well known from their fossil shells. the animal (Lucas & Prévôt, 1991) – could preserve Pioneering work by Bruce Runnegar (1985) revealed imprints of shell microstructures in both inner and outer that phosphatic molds of Cambrian mollusks (internal or shell layers (Figure 3). The nature of the shell external coatings of the shell) preserve details of their microstructure contributes to the beauty and strength of

Figure 2. Some Cambrian mollusks (1-8) and a mollusk-like problematic fossil (9). All specimens are calcium phosphate internal molds. 1, Aldanella, early Cambrian, Siberia. 2, Pojetaia, early Cambrian, Australia. 3, Watsonella, early Cambrian, Siberia. 4, Mellopegma, middle Cambrian, Australia. 5, Parailsanella, early Cambrian, Australia. 6, Oelandiella, basal Cambrian, Siberia. 7, Mackinnonia, early Cambrian, Australia. 8, Anabarella, early Cambrian, Siberia. 9, Emarginoconus, early Cambrian, Siberia. All scale bars 200 :m. the mollusk shell, and provides information about the control crystal shapes and arrangements compared with underlying process of biomineralization. Shell other skeletonized phyla. Runnegar (1985) documented microstructure is rarely preserved in Early Paleozoic that some of the more common varieties of shell fossils, and so Runnegar’s work revealed a new suite of microstructure in modern mollusks (e.g. crossed potential characters that could be used to better lamellar, prismatic, nacre, foliated calcite) also occurred understand the degree of diversification, phylogeny, and in Cambrian forms. His results suggested the shells of shell strength of the early mollusks. In spite of the great mollusks by the middle Cambrian had already potential, little analysis has been done since on shell diversified at a microstructural level, although the microstructures in Cambrian mollusks outside of major detailed pattern of evolution of shell microstructures in contributions made by Kouchinsky (1999, 2000). the earliest mollusks is still unclear. Modern mollusks have remarkably variable shell The continuing goal of our work is to better microstructures (Figure 4), with a greater number of document the distribution of various shell types of shell microstructure than in any other animal microstructures in Cambrian mollusks (Vendrasco et al., phylum (Carter, 1990). Moreover, Carter and Clark 2010), so that details of the earliest evolution of (1985) inferred that mollusks have a greater ability to molluscan biomineralization can be deciphered. The Page 46 THE FESTIVUS Vol. XLII(4): 2010

Figure 3. Two different ways that the outer layer of shell microstructure can be preserved in an internal mold. 1, side view of a radial section of a typical mollusk shell that thins out at the distal (lower right) margin. External surface of shell facing down, inner surface and internal mold of calcium phosphate facing up. 2, cross- sectional view of a mollusk with pronounced inner ridges — the phosphatic inner mold in this case replicates the outer layer of shell microstructure in the troughs on the inner part of the shell (corresponding to “ridges” on the internal mold). Such preservation is seen in Cambrian mollusks such as Mackinnonia. Key: i.l.=region of internal mold where the inner layer of shell microstructure can be replicated; o.l.=region of internal mold where outer layer of shell microstructure can be replicated. Gray line shows boundary between outer and inner shell layers. shell microstructure data can serve as a source of Figure 4. Some of the more common varieties of shell microstructure phylogenetically useful information to help determine that occur in extant mollusks. 1, nacre, Haliotis cracherodii, horizontal section (parallel to shell surface), image shows numerous affinities of early animal Problematica (Vendrasco et al., thin laminae, scale bar = 20 :m. 2, nacre tablets of aragonite in 2009), work that has implications for the magnitude of Haliotis cracherodii; grooves bordering tablets represent spaces the Cambrian explosion as well as the macroevolution of where organic molecules had been. Scale bar = 5 :m. 3, crossed modern animal groups. Finally, the data can provide lamellar shell microstructure, Lottia digitalis, horizontal section, scale bar = 20 :m. 4, prismatic shell microstructure near valve evidence to test the hypothesis that predation drove the margin in Donax gouldii, shell treated briefly with dilute acetic acid, evolution of early animal skeletons. horizontal section, scale bar = 20 :m. 5, simple calcitic prismatic, Mytilus californianus, horizontal section, scale bar = 5 :m. 6, Materials and Methods spherulitic (aragonitic) prismatic, tangential section (perpendicular to shell surface), Donax gouldii, scale bar = 100 :m New cases of shell microstructure in Cambrian mollusks were documented from newly and previously middle Cambrian mollusks (Vendrasco et al., 2009, 2010, collected rocks from the early Cambrian of Australia, personal observations). In addition, we have re-assessed Siberia, and China, and the middle Cambrian of cases of shell microstructure described and/or Australia. Detailed descriptions of the localities and photographed in the literature in order to improve processing of specimens are provided in Vendrasco et estimates of temporal and phylogenetic patterns of shell al. (2009, 2010). The fossils were isolated from the microstructures in Cambrian mollusks. We are continuing enclosing limestone matrix by ~10% buffered acetic and expanding this work in a broad synthesis paper to acid. The fossils were then sorted and placed on Scanning Electron Microscope (SEM) stubs. These come, but our initial results and interpretations are stubs were gold coated and examined on Scanning summarized below. Electron Microscopes at the Santa Barbara Museum of Our studies have added to the known diversity of Natural History, the Swedish Museum of Natural types of shell microstructures in Cambrian mollusks History, and the Nanjing Institute of Science and (Figure 5; Table 1). In addition to the varieties that Technology. Runnegar (1985) listed, we also found numerous mollusks with calcitic semi-nacre (Figure 5.2, 5.4) and Results and Discussion others with lamello-fibrillar (Figure 6.2) shell We found many new occurrences of preserved shell microstructures. More recent unpublished work (MJV microstructure in over twenty species of early and and Antonio Checa) has revealed the occurrence of Vol. XLII(4): 2010 THE FESTIVUS Page 47

(2) conchiferans very quickly diversified in their mechanisms of biomineralization soon after the shell originated in one or a few groups of mollusks. We favor the latter hypothesis because of the similarities in the shell and its basic structure among conchiferans (e.g., all have periostracum, and many have prismatic shell microstructure in the outer layer). Prismatic shell microstructure may be the most primitive type of shell microstructure in mollusks (Runnegar, 1985); new evidence in support of this hypothesis includes its occurrence in the earliest known mollusk Oelandiella and that it is the most commonly preserved variety of shell microstructure in early Cambrian mollusks (personal observations). Prismatic shell microstructure is common in the outer shell layer of modern mollusks, where the polygonal network of the periostracum serves as a template for isotropic nucleation of crystals that grow to merge together and then grow inward, forming the prismatic pattern (Checa, 2000). Other early Cambrian mollusks, younger than Oelandiella, show more diverse types of shell microstructure, including crossed lamellar (Runnegar in Bengtson et al., 1990), calcitic semi-nacre (Vendrasco et al., 2010), lamello- fibrillar (Feng & Sun, 2004; Vendrasco et al., 2009), foliated calcite (Runnegar, 1985), foliated aragonite (unpublished observations, MJV and Antonio Checa), Figure 5. Shell microstructures in Cambrian mollusks. 1, Aldanella, and many types of unusual shell microstructures that are early Cambrian, Siberia. Prismatic shell microstructure. Scale bar = difficult to classify (Kouchinsky, 1999; Carter, 2001; 100 :m. 2, Mellopegma, middle Cambrian, Australia. Calcitic semi- nacre. Arrow shows location of a complete calcite rhomb. Scale bar Vendrasco et al., 2010). This rapid diversification of = 10 :m. 3, Mackinnonia, early Cambrian, Australia. Prismatic. shell microstructures and hence specific Scale bar = 50 :m. 4, Anabarella, middle Cambrian, Australia. biomineralization methods suggests that: (1) mollusks Calcitic semi-nacre. Arrow shows location of a complete calcite may have been experiencing selective pressure to rhomb. Scale bar = 20 :m. 5, Pojetaia, early Cambrian, Australia. Prismatic (Runnegar, 1985) or large tablet nacre (Carter, 2001). modify their types of shell microstructure; and (2) the Scale bar = 50 :m. 6, Ribeiria, middle Cambrian, Australia. very earliest mollusks had the genetic framework to Laminar microstructure, possibly calcitic semi-nacre. Arrow shows rapidly evolve different microstructures that would imprint of the ends of crystal tablets. Scale bar = 10 :m. 7, better suit them to their environment (although after the Eotebenna, middle Cambrian, Australia. Foliated calcite. Scale bar = 20 :m. initial diversification, shell microstructures appear to have become more entrenched — see discussion below foliated aragonite in the earliest bivalves. Our results on utility of shell characters in phylogenetic analyses). add support to the idea that even though molluscan Our research has also shown that many of the early shells may not have been very diverse in overall form or molluscan shells were complex, with each shell external ornament, these similarities belie the containing multiple types of shell microstructure. pronounced underlying variation at a microstructural Runnegar (1985) demonstrated that this is true for level. Mellopegma georginensis from the middle Cambrian of The diversity of shell microstructures in the earliest Australia, and we have discovered many more cases mollusks (Table 1) suggests one of two scenarios: (1) from the middle Cambrian (Vendrasco et al., 2010) and many groups of mollusks originated shells independently early Cambrian (personal observations). In internal of each other, and the precise method of molds the imprints of the outer layer of shell biomineralization evolved differently between groups; or microstructure are preserved at the shell margin or in Page 48 THE FESTIVUS Vol. XLII(4): 2010

regions of internal troughs in the shell, where the shell appears to have helped them survive the extinctions of thins out (Figure 3). Commonly the outer shell layer is the Paleozoic and evidence from the fossil record prismatic and the inner layer is laminar, a typical pattern suggests this ability gave them an advantage over other for modern mollusks, in particular extant mono- prey when larger shell crushing predators became placophorans (McLean, 1979; Bouchet et al., 1983; common during the Mesozoic Era (250 to 65 million Warén & Bouchet, 1990; Warén & Hain, 1992; Warén years ago) (Vermeij, 1987). & Gofas, 1996). The commonality of a complex shell Our work has also shown that laminar shell with different types of shell microstructure in outer and microstructures were common in early mollusks inner layers suggests that the modern method of (Vendrasco et al., 2010). The most famous type of molluscan biomineralization was present already in the laminar shell microstructure is nacre, a type of earliest representatives. In particular, the precipitation microstructure composed exclusively of the mineral of the outer shell layer was likely controlled by the aragonite. Nacre is widely considered to be the most periostracum as in modern mollusks, with crystals fracture-resistant type of shell microstructure, many nucleating at equally spaced locations on the inner times stronger than bone (Currey, 1990). Nacre surface of the periostracum and growing together to consistently outperforms other types of shell form a prismatic structure, and the underlying shell microstructures in strength tests, especially in tensile, layer was precipitated within the mantle cavity (Checa, compressive, and bending strength (Taylor & Layman, 2000). Preservation of the periostracum is rare in 1972; Currey, 1988). The great strength of nacre allows Paleozoic mollusks, but a few early and middle Nautilus, with gas-filled internal shell chambers, to Cambrian mollusk specimens contain a layer replicated inhabit depths of over 500 meters with just a thin shell by phosphate that covered the shell (e.g., Gubanov et (Runnegar, 1990). This shell microstructure appears to al., 2004, figs. 5n, 6s, 9g-h), and we interpret those owe its great resistance to breakage in part to the fact structures as replaced periostracum. that the crystal elements are largely embedded in Our work has also revealed a remarkable level of organic matrices, making it more difficult for cracks to control over the orientation of crystals within the shell, propagate through the shell (Taylor & Layman, 1972; with a consistent orientation of crystal tablets throughout Jackson et al., 1988; Currey, 1990, 1999; Smith et al., shell layers of many Cambrian mollusks (Figure 5). In 1999). For a crack to propagate through nacre, it must these mollusks, the crystal tablets in the shell are be transmitted from layer to layer; the organic sheaths oriented in the same way throughout the layer (Figure surrounding nacre tablets help to prevent this by 5.2, 5.4-7), with particular crystal faces consistently absorbing much of the energy as they shear (Currey, aligned in the same direction. This is true from 1990). Nacre is a slow and energetically expensive type specimen to specimen of the same species (Vendrasco et of shell microstructure to form (Palmer, 1983, 1992; al., 2010). Rather than the haphazard arrangement of Currey, 1988), with each thin horizontal lamina crystals that one might expect given that these are requiring the molecular construction of an interlamellar among the earliest shell-forming mollusks, the evidence organic membrane via chitin molecules which then suggests that quickly in the evolution of the molluscan become covered by proteins secreted into the extrapallial shell these organisms evolved precision in control over space, followed by aragonite nucleation and crystal which mineral was deposited in the shell, how the growth to fill out the layer (Cartwright & Checa, 2007). nascent crystals were oriented, and which crystal faces This process gets repeated as the nacre is built upward, grew at what rates. lamina by lamina. In fact, Currey (1977) argued that While the presence of highly diverse, evolutionarily nacre is not suitable for shells that must be built quickly, flexible, precisely controlled shell microstructures may as much of its strength comes from the precise at first seem to explain why mollusks came to dominate geometric arrangement of tablets within the organic many marine environments, it should be remembered framework. This slow, energetically expensive type of that brachiopods, with their apparently simpler method shell microstructure is not one that would be expected to of biomineralization (sensu Carter & Clark, 1985), were evolve unless there is a strong selective advantage for it more common than mollusks through most of the (Palmer, 1983, 1992). With shell-crushing predators Paleozoic Era (540 to 250 million years ago). In any present in an environment there is such an advantage. case this refined ability in mollusks to control Some of the early Cambrian mollusks appear to biomineralization and to evolve a more suitable type of have had foliated aragonite, a likely precursor to nacre, shell microstructure for a particular environment while others had the very similar form, calcitic semi- Vol. XLII(4): 2010 THE FESTIVUS Page 49

nacre (Vendrasco et al., 2010), and still others had a its tip and inconsistent with mechanical damage that plywood-type microstructure called lamello-fibrillar would produce more variation in wound patterns; (4) where the fibers in successive layers had different coprolites (fossil feces) from predators are preserved in orientations (Vendrasco et al., 2010), providing greater these beds; (5) predation is the predominant cause of resistance to breakage than if the fibers were oriented in shell injury in modern mollusks, inferred in part by the same direction throughout the shell. In the case of observations of mollusks in turbulent waters without calcitic semi-nacre, a type of shell microstructure that predators suffering much lower rates of shell injuries has not to our knowledge been tested for fracture than those in calm waters with predators (Vermeij, resistance, the overall characteristics are similar to nacre 1987); and (6) similar tiny mollusks from the early in that they consist of roughly equidimensional tablets Cambrian have healed shell damage in a low-energy that merge together to form thin laminae. The environment, suggesting predation was also the cause of abundance of organic material in the shell, which gives that damage (Skovsted et al., 2007). These observations nacre much of its strength, is difficult to accurately support the idea that the earliest mollusks were under estimate in these fossils, but some signs point to its high strong predatory pressures, consistent with the abundance: (1) organic molecules are common in all hypothesis that the onset of predation helped drive the modern mollusks examined, making up to ten percent of Cambrian explosion. the weight of the calcified layers of the shell (Watabe, 1988); (2) many early mollusks appear to have had a large organic component to their shells, in some cases consisting of a thick organic layer in which isolated crystals were embedded (Figure 6.2, 6.4; Kouchinsky, 2000); (3) some Cambrian mollusks show a thick organic periostracum that covered the calcareous parts of the shell; and (4) the earliest mollusks prior to the origin of the shell probably had a tough organic coat that later served as template for mineral precipitation. Thus calcitic semi-nacre and the other types of shell microstructures in Cambrian mollusks probably had a high abundance of organics in the shell, and hence probably had relatively fracture resistant shells. In addition to the evidence for strong, fracture resistant shell microstructures in the earliest mollusks, we also observed numerous cases of healed shell scars in Mellopegma from the middle Cambrian of Australia (Figure 6.5-6). Most of these cases of damage are preserved as indentations on the internal molds. Most signs of damage, both scars and missing regions of shell, have smooth borders, suggesting the breaks occurred and were healed during the lifetime of the Figure 6. 1-4, phosphate-replaced thick organic sheets in the shells animals. It is difficult to absolutely rule out a of Parailsanella. 5-6, Mellopegma, showing imprint of shell damage. 1, view of sub-apical shelf of Parailsanella, same specimen as in mechanical source of this damage, but a number of Figure 2.5, arrow shows location of 2, scale bar = 50 :m. 2, close- observations suggest that predation is the likely cause of up of replaced organic matrix, scale bar = 10 :m. 3, side view of this damage: (1) the damage occurred at highest another specimen, arrow shows location of 4, scale bar = 100 :m. : frequency in the region of the shell with the largest 4, close-up of replaced organic matrix, scale bar = 5 m. 5, Mellopegma from the middle Cambrian of Australia, arrow shows aperture width and hence easiest access to the animal’s imprint of damage on shell, magnified in 6, scale bar = 200 :m. 6, flesh; (2) most cases of damage occur over broad marking on internal mold caused by a portion of the shell that was regions, a pattern consistent with being caused by a caved in, note smooth margin around damage near the base, crushing appendage, not impact from debris; (3) in suggesting this wound was healed, scale bar = 100 :m. many cases (Figure 6.5-6.6) the wound tapers upward toward the apex of the shell, consistent with an We also observed that many of the Cambrian appendage whose force would be most concentrated at mollusks had shells composed at least in part of calcitic Page 50 THE FESTIVUS Vol. XLII(4): 2010

semi-nacre (see above), a type of microstructure previously unknown in mollusks but common in brachiopods (Williams & Wright, 1970; Williams, 1997; Pérez-Huerta & Cusack, 2008) and bryozoans (Weedon & Taylor, 1995; Taylor & Weedon, 2000). This type of shell microstructure was confirmed in Cambrian mollusks by making measurements such as the interfacial angles of crystallite imprints. The interfacial angles in most cases were much closer to what would be expected for calcite rhombs instead of aragonitic tablets (Vendrasco et al., 2010), and so these imprints were not likely formed by nacre, which by definition is aragonitic in composition (Carter, 1990). The results suggest that calcitic semi-nacre occurred in at least two species of middle Cambrian mollusks, and probably in at least five others (Vendrasco et al., 2010), and likely occurred in early Cambrian mollusks as well. The more extensively coiled Cambrian mollusks Figure 7. Lamello-fibrillar shell microstructure in a mollusk, Aldanella, early Cambrian, Siberia (1-2) and in a problematic fossil, (probable gastropods) and Ocruranus, possibly the likely a mollusk and possibly a chiton, Ocruranus, early Cambrian, earliest known chiton (Vendrasco et al. 2009), shared a China (3-4). type of laminar microstructure that is uncommon in modern mollusks: lamello-fibrillar (Vendrasco et al., ancestor of brachiopods and mollusks probably did not 2009, 2010) (Figure 7). This is a plywood type of shell have a shell, as many soft-bodied taxa appear to be microstructure where the orientation of fibers differs more closely related to mollusks and brachiopods than between adjacent horizontal layers. The type of the latter two are to each other (Dunn et al., 2008). microstructure in these fossils is similar to crossed-bladed, However, the most recent common ancestor of mollusks which is the only variety found in brachiopods but poorly and brachiopods likely possessed an outer organic coat developed in mollusks (Carter, 1979). Moreover, in likely formed by mechanisms that later became contrast to the other, highly ordered types of shell independently co-opted for formation of a mineralized microstructure in Cambrian mollusks (see above), shell when the origin of mobile predators drove such lamello-fibrillar shell microstructure represents a looser evolution in the early Cambrian. control over biomineralization, similar to what Carter Similarities in shell microstructure among Cambrian (1979) inferred as a commonality between early mollusks mollusks that otherwise look very similar have been and brachiopods. noted in a few cases, including bivalves (Runnegar & Brachiopods and mollusks were for many decades Pojeta, 1992) and rostroconchs (Kouchinsky, 1999; thought to be distantly related, and so similarities in their rostroconchs are an extinct class of mollusk with a shells like those highlighted above were interpreted as univalved larval shell and a bivalved adult shell). Our convergent. Recently, however, both brachiopods and new data have provided many more examples of mollusks have been shown to belong to the major Cambrian mollusks with similar overall form and similar metazoan clade Lophotrochozoa (or Spiralia), and in fact shell microstructure. Some examples include: (1) highly are fairly closely related within that clade (Dunn et al., coiled forms sharing lamello-fibrillar microstructure; (2) 2008). This prompts a re-examination of the degree of species of Mellopegma and similar forms such as homology between the shells of these two taxa. The shells Ribeiria sharing calcitic semi-nacre; (3) bivalved forms of brachiopods and early mollusks share a number of sharing an unusual shell microstructure that is similar to similarities, including: (1) pores in the shell; (2) a high the foliated aragonite of modern monoplacophorans component of organics in the shell; (3) a periostracum (Checa et al., 2009); and (4) tall, bivalve-like forms (outer organic layer of shell); and (4) similar shell sharing foliated calcite (Vendrasco et al., 2010). These microstructures—all types of microstructures seen in observations are consistent with preliminary cladistic brachiopods – including calcitic semi-nacre as discussed analyses of Cambrian mollusks using only micro- above – also occur in mollusks (Carter & Clark, 1985; structural characters, which are highly congruent with Vendrasco et al., 2010). The most recent common those using other characters (Vendrasco et al., 2006). Vol. XLII(4): 2010 THE FESTIVUS Page 51

Altogether, these results provide evidence that shell (University of California at Los Angeles, UCLA) for microstructures in the early mollusks had a strong providing samples and for helpful discussions. S. phylogenetic signal, with similar patterns of shell Bengtson also allowed use of the SEM at the SMNH. microstructure in closely related species. Therefore shell Henry Chaney (Santa Barbara Museum of Natural microstructures may be particularly useful in History, SBMNH) allowed use of facilities at SBMNH, determining relationships among the early mollusks, including the SEM. Daniel Geiger (SBMNH) provided which otherwise often differ little. In addition, advice that improved the quality of our SEM images. microstructural data can be used to help determine Douglas Eernisse improved the clarity and accuracy of relationships of some of the many still problematic this paper through a thoughtful review; the more precise Cambrian taxa, helping to decipher the stem lineages of wording in the caption to Figure 1 was provided by him. modern animal phyla and thereby allowing better MJV thanks Carole Hertz for inviting this paper and understanding of how the body plans of major animal improving it. This research was funded by a grant from taxa were built. NASA Exobiology (EXB04-0000-0117) to SMP. AK received support from the NordCEE (Nordic Center for Concluding Remarks Earth Evolution) project (Danish National Research The Cambrian explosion is unusual because body Foundation (Danmarks Grundforskningsfond) grant to plans became diverse very quickly, and since then D. Canfield. GL’s work was funded by the National relatively few new body plans have appeared. The same Science Foundation of China. pattern can be seen in shell microstructures in Cambrian mollusks. Many Cambrian fossils look familiar and Literature Cited seem relatively easy to classify into modern phyla, but there are also many unusual forms (Problematica) that BENGTSON, STEFAN & SIMON CONWAY MORRIS 1992. Early radiation of biomineralizing phyla. Pp. 447-481, seem unusual by modern standards and cannot in: J. H. Lipps & P. W. Signor (eds.), Origin and Early comfortably be placed in modern groups. Likewise, Evolution of the Metazoa. Plenum Press, New York. many Cambrian mollusks had shell microstructures or 578 pp. patterns that are common today (e.g. prismatic outer BENGTSON, STEFAN & YUE ZHAO shell layer, laminar inner one), but other cases of shell 1992. Predatorial borings in Late Precambrian mineralized exoskeletons. Science, 257: 367-369. microstructures in Cambrian mollusks are more unusual BENGTSON, STEFAN, SIMON CONWAY MORRIS, BARRY J. and difficult to classify. Additional fossils and SEM COOPER, PETER A. JELL & BRUCE N. RUNNEGAR work will help us better assess both problematic taxa 1990. Early Cambrian fossils from South Australia. and shell microstructures from this important time Association of Australasian Palaeontologists, Brisbane. period. Memoir 9 of the Association of Australasian We are fortunate that shell microstructural data is Palaeontologists. 364 pp. BOUCHET, PH., JAMES H. McLEAN & ANDERS WARÉN common in phosphatic molds of mollusks in many 1983. Monoplacophorans in the North Atlantic. Oceanologica Cambrian sedimentary deposits, and that phosphatized Acta, 6(2):117-118. fossils, rare from most other time periods, are so BUDD, GRAHAM E. abundant in Cambrian rocks (Porter, 2004b). This good 1998. Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle. Lethaia, 31(3): fortune has allowed researchers like us to learn much 197-210. about how and why the earliest mollusks formed their 2003. The Cambrian fossil record and the origin of the phyla. shells, and has improved our understanding of the Integrative and Comparative Biology, 43(1): 157-165. relationships among modern and fossil groups of CARTER, JOSEPH G. animals. Many species of Cambrian mollusk remain to 1979. Comparative shell microstructure of the Mollusca, Brachiopoda and Bryozoa. Pp. 439-446 & 456, in O. be closely examined for traces of shell microstructure. Johari (director). Scanning Electron Microscopy/1979, Thus our continued work on this subject promises II. Chicago Press Corporation, Chicago. 910 pp. improved understanding of the Cambrian explosion, the 1980. Environmental and biological controls on bivalve shell most dramatic evolutionary radiation in the history of mineralogy and microstructure. Pp. 69-113, in D. C. life on Earth. Rhoads, D. C. & R. A. Lutz (eds). Skeletal growth of aquatic organisms: biological records of environmental change. Plenum Press, New York. 750 pp. Acknowledgments 1990. Skeletal biomineralization: patterns, processes and We thank Stefan Bengtson (Swedish Museum of evolutionary trends, Volume 1. Van Nostrand Natural History, SMNH) and Bruce Runnegar Reinhold, New York. 832 pp. Page 52 THE FESTIVUS Vol. XLII(4): 2010

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Table 1. Varieties of shell microstructure seen in Cambrian mollusks, and their descriptions.

Descriptions of the types of shell microstructure taken from the Glossary of Skeletal Biomineralization in Carter (1990), except for foliated aragonite, which was described after that publication. Abundance defined as follows: Rare, fewer than five cases; Common, five to twenty cases; Abundant, more than twenty cases. There is uncertainty in the abundance of calcitic semi-nacreous structure because of the numerous cases of uncertain types of laminar shell microstructures in Cambrian mollusks. Laminar microstructure is defined as consisting of “rods, laths, blades or tablets comprise sheets which are oriented parallel or nearly parallel to the depositional surface” (Carter, 1990, p. 611); laminar microstructures include nacre, calcitic semi-nacre, and foliated calcite. Many Cambrian fossils have a laminar shell microstructure that shares similarities with calcitic semi-nacre as well as other types of laminar microstructures (Vendrasco et al., 2010); hence the uncertainty indicated here.

Type of shell Description from Carter (1990, pp. 610-612) Occurrence microstructure in Cambrian

Prismatic "Mutually parallel, elongate, adjacent structural units that do not interditate strongly Abundant along their mutual boundaries"

Semi-nacreous "Laminae consisting of polygonal tablets which show more abundant screw dislocations Rare to and less lateral continuity of the laminae than in typical nacreous structure. Generally common calcitic, rarely aragonitic"

Lamello- "The horizontal fibers in successive laminae differ in orientation by irregularly varying Common fibrillar angles."

Foliated "More or less mutually parallel calcitic blades or laths arranged in laminae dipping at a Common calcite generally uniform angle and in the same general direction over large portions of the depositional surface."

Crossed "Numerous mutually parallel rods, laths or blades aggregated into variably shaped but Rare lamellar commonly lensatic to braching first order lamellae. Each first-order lamella is oriented with its height axis more or less perpendicular or uniformly oblique to the depositional surface, and with its length axis usually parallel or perpendicular to a major morphologic axis. The second-order lamellae in adjacent first-order lamellae show two predominant dip directions, and these dip directions regularly alternate between adjacent first-order lamellae."

Foliated N/A; defined in Checa et al. (2009) for a microstructure consisting of laminae Rare aragonite comprising elongate aragonite laths.