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The Anatomy, Affinity, and Phylogenetic Significance of Markuelia

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The Anatomy, Affinity, and Phylogenetic Significance of Markuelia EVOLUTION & DEVELOPMENT 7:5, 468–482 (2005) The anatomy, affinity, and phylogenetic significance of Markuelia Xi-ping Dong,a,Ã Philip C. J. Donoghue,b,Ã John A. Cunningham,b,1 Jian-bo Liu,a andHongChengc aDepartment of Earth and Space Sciences, Peking University, Beijing 100871, China bDepartment of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK cCollege of Life Sciences, Peking University, Beijing 100871, China ÃAuthors for correspondence (email: [email protected], [email protected]) 1Present address: Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK. SUMMARY The fossil record provides a paucity of data on analyses have hitherto suggested assignment to stem- the development of extinct organisms, particularly for their Scalidophora (phyla Kinorhyncha, Loricifera, Priapulida). We embryology. The recovery of fossilized embryos heralds new test this assumption with additional data and through the insight into the evolution of development but advances are inclusion of additional taxa. The available evidence supports limited by an almost complete absence of phylogenetic stem-Scalidophora affinity, leading to the conclusion that sca- constraint. Markuelia is an exception to this, known from lidophorans, cyclonerualians, and ecdysozoans are primitive cleavage and pre-hatchling stages as a vermiform and direct developers, and the likelihood that scalidophorans are profusely annulated direct-developing bilaterian with terminal primitively metameric. circumoral and posterior radial arrays of spines. Phylogenetic INTRODUCTION et al. 2004b). Very early cleavage-stage embryos of presumed metazoans and, possibly, bilaterian metazoans, have been re- The fossil record is largely a record of adult life and, thus, covered from the late Neoproterozoic (Xiao et al. 1998; knowledge of embryological processes and mechanisms in Zhang et al. 1998; Xiao and Knoll 1999, 2000; Xiao 2002; Yin extinct organisms has been restricted to phylogenetic infer- et al. 2004), along with some rather more dubious records enceFcomparative analysis of extant representatives of lin- (Chen et al. 2000; Chen et al. 2002) the veracity of which has eages as a means of inferring the nature of their most recent been seriously questioned (Xiao et al. 2000; Bengtson 2003). common ancestor. As comparisons are made between pro- Early Cambrian cnidarian-like embryos have also been de- gressively more distantly related organisms, so the reliability scribed (Kouchinsky et al. 1999). of the inferences based upon these comparisons become in- However, excitement surrounding the promise of these creasingly tenuous, often indicating incongruent correlations new discoveries has been tempered by frustration with the between traits (Strathmann 1993). Concomitantly, our ability lack of phylogenetic constraint; most of these discoveries pre- to reliably unravel the evolution of development diminishes. serve stages of development that are simply too early to de- One means of testing developmental explanations for ev- termine affinity and, thus resolve evolution or conservation in olutionary events in Deep Time would be through the dis- patterns and processes of development. One exception is covery of a fossil record of early development, and recent Olivooides, for which much of the life cycle has been recon- years have witnessed a shift in expectations, from impossibil- structed from hitherto known but taxonomically discrete ity to plausibility. This began, rather inauspiciously, with the stages, linking cleavage embryos to coronate scyphozoan-like description of some rather poorly, although nevertheless re- cnidarian adults (Bengtson and Yue 1997; Yue and Bengtson markably, preserved cleavage stage embryos, attributed to co- 1999). The other exception is Markuelia, described by Ben- occurring trilobites, from the middle Cambrian (Zhang and gtson and Yue (1997) as an annulated, bilaterally symmetrical Pratt 1994; Pratt and Zhang 1995). These discoveries were vermiform animal represented by only relatively late-stage followed in quick succession by rather more convincing finds, embryos. Indeed, the embryos are preserved at a sufficiently including the complete life cycle of an early Cambrian late stage for viable considerations of affinity. Bengtson and scyphozoan cnidarian (Bengtson and Yue 1997; Yue and Yue (1997) compared Markuelia with arthropods and lobo- Bengtson 1999; Hua et al. 2004; Steiner et al. 2004a), and late- pods (a grade of organisms including the living onychopho- stage embryos of the early Cambrian bilaterian metazoans, rans), and especially the annelids, whereas Conway Morris Markuelia and Pseudooides (Bengtson and Yue 1997; Steiner (1998a) plumbed for an affinity with the extinct halkieriids, a 468 & BLACKWELL PUBLISHING, INC. Dong et al. The anatomy and af¢nity of Markuelia 469 putative grade of lophotrochozoans including members of the not discernable (for developmental or taphonomic reasons) mollusc, annelid and brachiopod total groups, including stem but the surface layers are nevertheless organized into a paired members of clades comprising combinations of these phylum ‘‘S’’ loop (Fig. 1C). In later stages, features of the adult, in- level total groups (Conway Morris and Peel 1995). More re- cluding a homonomously annulated, or possibly segmented, cently we published a preliminary report on new collections of trunk are present (Fig. 1D). The condition of the head and tail Markuelia from the Middle and late Cambrian of Hunan, is not revealed in these specimens, although they may have south China, which include a greater range of developmental developed by this stage. The full volume of the sphere is not, stages than were known hitherto (Dong et al. 2004). These however, occupied by the embryo, adjacent portions of the provide further constraint over the affinity of Markuelia and characteristically ‘‘S’’-coiled trunk separated by intervening we argued for assignment to Scalidiophora, a clade composed undifferentiated fields (Fig. 1D), which we interpret as yolk. of the phyla Kinorhyncha, Loricifera, and Priapulida, and constituting a sister clade to Panarthropoda, possibly includ- ing Nematoidea (Nematoda plus Nematomorpha). Here we Prehatching provide a more complete description of Markuelia, including The vast majority of embryos represent very late develop- data from recently recovered material, and a further consid- mental stages in which there are no undifferentiated fields eration of the affinity of Markuelia and its significance in present in the embryo. These embryos range in diameter from uncovering the evolution of development. 340 to 490 mm (not including specimens that have obviously undergone post mortem shrinkage or other deformation, pre- sumably as a result of dehydration, e.g., Fig. 1E). The em- MATERIALS AND METHODS bryos are sufficiently tightly coiled into an ‘‘S’’-shaped loop that the lateral margins of the trunk are directly juxtaposed All of the described material is from the middle and late Cambrian of Wangcun, Hunan Province, south China. The embryos are pre- (Fig. 1F). The trunk varies in width from 150 to 240 mmin served in calcium phosphate and were recovered from limestone well-preserved specimens, to less than 100 mm in specimens using 10% buffered acetic acid. Several hundred embryos have so that have clearly undergone post mortem shrinkage. Frac- far been recovered from approximately 8000 kg of rock, together tured specimens reveal that the trunk was compressed with an with several thousand more phosphatic spheres of approximately ovoid cross-sectional profile (Fig. 1, G and H). However, it is the same size that are identical to the preserved membrane envel- possible that the trunk had a circular cross-sectional profile in oping many of the specimens that are verifiably embryos. The vivo and that the ovoid outline is an artifact of close packing. specimens were studied under a scanning electron microscope. The Transverse annulations (Fig. 1, D and F–I) range in an- figured specimens are deposited in the Geological Museum of terior–posterior length from approximately 10 to 25 mm, de- Peking University, Beijing, China (GMPKU) and the Swedish pending upon the degree of curvature of the trunk. The outer Museum of Natural History, Stockholm, Sweden (NRM). surfaces of the annuli are ornamented with 0.3–0.5-mmanas- tomosing ribs, which are aligned to the anterior–posterior axis FOSSILIZED DEVELOPMENTAL STAGES OF of the animal (Fig. 1I). Annuli exhibit varying degrees of MARKUELIA HUNANENSIS inflation, reflecting differing states of preservation, but frac- tured specimens indicate that the surface divisions between Cleavage embryos the successive annulae extend internally as shallow septa with The earliest developmental stages present in our collections a finite depth of approximately 5 mm (Fig. 1G). The constant are cleavage embryos that preserve the surface boundaries depth of these divisions suggests that their extent is a reflec- between adjacent blastomeres (Fig. 1A) or the blastomeres tion of original anatomy rather than preservational artifact. themselves (Fig. 1B). These embryos are approximately the In some specimens small spines (approximately 1-mmdi- same size as the late-stage embryos of Markuelia and, given ameter, 3–5-mm length) emerge from circular pores in the that rapidly developing embryos of metazoans invariably trunk with perpendicular aspect (Fig. 2A). The spines have maintain a constant volume of cytoplasm and yolk through-
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