Reappraisal of Ambulacral Branching Patterns in Blastozoans

Reappraisal of ambulacral branching patterns in blastozoans

E. Nardin & B. David UMR Biogéosciences, Université de Bourgogne, Dijon, France B. Lefebvre UMR CNRS 5125 PEPS, Géode, Université Lyon 1, Villeurbanne cedex, France R. Mooi Department of Invertebrate Zoology & Geology, California Academy of Sciences, San Francisco, CA, USA

ABSTRACT: The type of ambulacral bifurcation is an important diagnostic character in primitive echinoderms. However this character is rarely used in blastozoan phylogenetic studies, whereas the corresponding character is well used in crinoid analyses. The main reason for this gap is the absence of definition of patterns for all blastozoans, resulting in terminological confusion. Although crinoid arms and blastozoan ambulacra are not homologous, the well-defined crinoid terminology has been used to describe the different topologies of ambulacral bifurcation. Unilateral and bilateral patterns have been recognized in blastozoans, and associated with the mineralization of the ambulacra. Four morphoclines are proposed to interpret the morphological relationships between the different patterns.

1 INTRODUCTION character in blastozoans, it has only been used in crinoid analyses and descriptions. This is probably During the last forty years, abundant studies on extant due to absence of definitions for these patterns in all and fossil echinoderms provided numerous clues to blastozoans and hence the terminological confusion. understanding biology and phylogeny of echinoderms. It is clear that the arms of crinoids and the In spite of this progress, the basal relationships among brachiole-supporting ambulacral structures in blasto- the major echinoderm clades (traditionally considered zoans are not homologous (Ubaghs 1967, Sprinkle as “classes”) still constitute some of the greatest chal- 1973, David & Mooi 1999, David et al. 2000). How- lenges in this field. This situation mainly results from: ever, the aforementioned terminology that some have 1. the apparent incompleteness of the fossil record used to describe branching patterns in crinoids can in Cambrian-Ordovician times; 2. the inadequacy of be applied to descriptions of the similar, but inde- clearly identified homologies within and among the pendently derived patterns seen in blastozoans. We clades, and 3. the confusion of terminology inducing propose to apply these terms, but only to the topo- false codings for characters. logical descriptions necessary to describe the patterns Numerous phylogenetic analyses have been per- of ambulacral bifurcation in blastozoans. The termi- formed to understand the evolution of primitive echin- nology will be similar to that for crinozoans, whatever oderms (Paul & Smith 1984, Paul 1988, Smith 1988, their evolution might have been. Sumrall 1997, Ausich 1998, David et al. 2000). Most In the present study,we will elaborate on the branch- of them use approximately the same set of homologies ing patterns observed in blastozoans and consider to describe the stem, theca, ambulacra, and feeding hypotheses about their evolution. appendages (brachioles or arms). These characters mostly concern the plating of each part, the type of respiratory structures, the shape of the theca, and 2 PATTERNS OF ARM BRANCHING IN the position of the openings (mouth, hydropore and CRINOIDS gonopore, and periproct). Although the organization of food-gathering appendages is commonly used in Arm branching is well known in crinoids (Ubaghs crinoid analyses (Ausich 1998), this is rarely the 1953a, Moore & Teichert 1978). Different patterns case in blastozoan analyses (Smith 1988). Ciampaglio of branching have been recognized. The simplest pat- (2002) indicated that the type of arm bifurcation could tern, called atomy, is characterized by the absence of be an ecological character in crinoids and the type arm ramification. The initial pattern of ramification is of ambulacral bifurcation a developmental character isotomy, corresponding to a dichotomous process reg- in blastozoans. Even if the type of bifurcation is an ularly repeated along the arms (Fig. 1A). Heterotomy important systematic character and a developmental could derive from isotomy, defined by the unequal

45 Figure 1. Patterns of arm bifurcation. A, regular dichotomy (isotomy); B, irregular dichotomy (heterotomy); C, bilateral heterotomy; D, endotomous heterotomy; E, exotomous heterotomy; F, holotomy (arms with pinnules); from Ubaghs 1953a.

growth of each new secondary branch or the deletion of few authors have used its classification. Sprinkle & the bifurcation process at different, randomly chosen Collins (2006) recently described a new eocrinoid points (Fig. 1B).Arms could afford to switch from iso- Lyracystis using the crinoid terminology in a sense tomous to heterotomous branching after the first two different to that of Ubaghs (1953a, 1967). (or, rarely three) bifurcations. When ramuli (this term is generally used for secondary arms formed by het- 3.1 Description and designation of the blastozoan erotomous branching) occur alternately on both sides food groove bifurcation of the main groove, the heterotomous process is bilateral (Fig. 1C).At the beginning of the arm in a bilateral Contrary to the situation in crinoids, ambulacral bifur- heterotomous pattern, the smaller branch could be on cations occur at two levels in blastozoans. The first the outside – that is to say on the side away from the one concerns the brachiolar connection to the ambu- ray axis, the larger (ramus) being on the inside. At lacral food grooves. The second one refers to the level the next point of bifurcation, ramuli can be on the of division of the main ambulacral food groove. The inside of the main branch, the rami being on the out- designation of the pattern of ambulacral branching side. When ramuli occur in only one side of the main in blastozoans is based on the crinoid arm branching groove, the heterotomous process is unilateral. The classification. appearance of ramuli only on the internal side is named The simplest pattern, named atomy, is character- endotomy, and growth restricted to the external side, ized by one brachiole growing at the end of one exotomy (Fig. 1D–E, respectively). Ubaghs (1953a) food groove (Figs. 2A, 3A). This pattern occurs in suggested that holotomy could be the most derived few rhombiferans (pleurocystitids, hemicosmitids and process, characterized by equal branches or pinnules fistuliporites; Parsley 1970), the eocrinoids Ampheris- alternately disposed along the main groove (Fig. 1F). tocystis and Nolichuckia (Sprinkle 1973, Frest 2005), The pinnules are attached in an alternate fashion on some diploporans (e.g.Aristocystites and Lepidocalix; either side of the arm whereas unilateral heterotomous Chauvel 1978), cinctans (e.g.Trochocystites; Barrande ramuli branch only from the inner or outer side of 1887), and solutes (e.g. Coleicarpus and Minervaecys- the major arm. tis; Caster 1967). Most diploporan genera have short (restricted around the peristome) and highly bifurcated food grooves lying directly on their thecal plates 3 AMBULACRAL BIFURCATIONS IN (isotomy, Figs. 2B, 3B1). This pattern also occurs in BLASTOZOANS rhipidocystid eocrinoids. It is mostly retained by the organisms in which the food grooves lie directly over The type of ambulacral bifurcation is a diagnostic char- the thecal plates, without any mineralized ambulacral acter in blastozoans, in spite of the absence of clear def- plates (except in Rhombifera and Tholocystis). initions. Kesling (1967a) identified different patterns In crinoids, endotomy and exotomy are easy to dis- of ambulacral bifurcation in diploporans and rhomb- tinguish because of the numerous subdivisions of the iferans. Similar patterns were observed in blastoids arms. Each new branch can be located on the outside and paracrinoids, without being named (Beaver et al. of the food groove (on the side away from the ray 1967, Kesling 1967b, Frest et al. 1980). Ubaghs (1967) axis) or on the inside of the food groove. In blasto- was the first to define clearly ambulacral branching zoans, brachioles often branch on only one side of a patterns in eocrinoids, based on the classification elab- single ambulacral food groove, without pronounced orated for crinoids (Ubaghs 1953a). However, these bifurcation (Fig. 2C). When it is not possible to say if denominations were incomplete: few other patterns are brachioles are branched away from or towards the ray realized in the other blastozoan classes. In addition, axis, the pattern is called unilateral (Figs. 2C–D, 3F).

46 Figure 2. Typical brachiolar connections with ambulacral food grooves in some blastozoan genera. A, atomous ambulacral groove bearing each one brachiole (Echinosphaerites); B, isotomous food groove, of which each new food groove bears one brachiole (Codiacystis); C, unilateral ambulacral food groove (Gomphocystites); D, endotomous and unilateral ambulacral food groove (Lyracystis); E, holotomous ambulacral groove (Mesocystis); F, holotomous food groove and three ambulacra having two main food grooves each (Callocystites). Representations A–C and E–F are modified from Kesling (1967a).

In few cases, ambulacral grooves are bifurcated at Some sphaeronitids (e.g. Eucystis & Haplosphaero- a high level. For example, the genus Lyracystis has nis) have an irregular isotomous ambulacral branch- 6 ambulacra interpreted as the A ray – which is subdi- ing, the ramuli being of unequal length (Bockélie vided in two main ambulacral grooves – and B–C and 1976). In this case, the pattern can be called anisotomy D–E rays (Sprinkle & Collins 2006). Ambulacra B–C (Fig. 3B2). and D–E show a unilateral branching pattern, whereas Four genera (Fungocystites, Glyptosphaerites, Pro- ambulacrum A shows an endotomous pattern, notably tocrinites, and Quadrocystis) present a pattern mixed because brachioles are mounted on the inside of both between unilateral and bilateral topologies (Regnéll main ambulacral grooves (Fig. 2D). 1945, Sprinkle 1982). Most ramuli appear on one side The last and most common pattern is bilateral of the food groove except a few that occur on the other (Fig. 2E–F). Holotomy is defined when the brachioles side at the beginning and at the end of the groove are regularly and alternately branched on both sides of (Fig. 3E). As the branches are the consequence the main food groove, often supported by mineralized of a random branching, this pattern can be called ambulacral plates (Fig. 3C–D). Unilateral and bilat- heterotomy. eral seem to occur with true ambulacra (having their The other type of exception results from subdivi- own mineralized plates), except in Gomphocystites sions of the food grooves at different scales in both and Schizocystis. unilateral and bilateral patterns. The first modification is the division of the major food groove into two or more main ambulacral grooves, each show- 3.2 Exceptions ing a common pattern (Fig. 3G). It occurs in some Few exceptions to the common patterns exist in the rhombiferans (e.g. Callocystites & Sphaerocystites, major classes of blastozoans. Several rhombiferan Fig. 2F). The second modification corresponds to genera (e.g. Macrocystella & Glyptocystella) and one the degree of the subdivisions of the ramuli. The diploporan genus (Eumorphocystis) possess a partic- eocrinoid Ascocystites has an irregular unilateral pat- ular holotomy (Parsley 1982, Sprinkle & Guensburg tern with an additional bifurcation of secondary food 2001). In a normal holotomy, all ramuli occur alter- grooves (Fig. 3F2; Régnault 1990). The rhombifer- nately on both sides of the main food groove. The ans Hadrocystis and Hesperocystis show simple and shifted holotomy is characterized by the same alterna- double additional subdivisions of each branch result- tion, except for the two first adoral ramuli occurring ing from a holotomous pattern, respectively (Fig. 3D2; on the same side of the food groove (Fig. 3D1). Sprinkle 1982).

47 Figure 3. Diagrams representing the different ambulacral branching types. At the end of each food groove, one brachiole is erected. A, atomous pattern; B, isotomy (B1) and anisotomy (B2); C, holotomy; D, shifted holotomy, with a double ramification in D2; E, heterotomous pattern; F, unilateral pattern simple (F1) and with a double ramification (F2); G, holotomy with a high number of main food groove ramifications.

3.3 Stratigraphic and phylogenetic distributions of the patterns Lepidocystids and gogiids are frequently considered as primitive blastozoans, at the base of the clade con- taining the other classes (Sprinkle 1973, Paul 1988, Smith 1988, David et al. 2000). Some eocrinoid gogiids could be at the origin of the rhombiferan glyptocystitids, themselves related to the blastoid- like classes. Moreover, the eocrinoid Nolichuckia could be closely related to the rhombiferan fistuliporites (e.g. Echinosphaerites) (Paul 1988, Smith Figure 4. Topological sequence proposed to relate the 1988) and Lichenoides to the diploporans sphaeroni- morphology of each branching pattern, seen in Figure 3. tids (e.g. Glyptosphaerites, Haplosphaeronis) (Paul 1988). Cinctans and solutes could have evolved from The last genera to have the unilateral pattern lived dur- the eocrinoid gogiids (David et al. 2000). ing the Middle Ordovician and are found in eocrinoids Holotomy was developed by the lepidocystids and (e.g. Mandalacystis; Lewis et al. 1987), and in the the gogiids in the Lower Cambrian (Barrande 1887, Lower Silurian in both rhombiferans and diploporans Sprinkle 1973, Ubaghs &Vizcaïno 1990) and occurred (e.g. Schizocystis; Kesling 1967a, Gomphocystites; until the Late Permian in blastoids (e.g. Rhopaloblas- Foerste 1920). The modified pattern of shifted holo- tus, Horowitz et al. 1985). The eocrinoids Lichenoides tomy is documented from the Lower Ordovician (e.g. and Nolichuckia have both anisotomous and atomous Macrocystella) to the Middle Silurian (e.g. Callocys- patterns in the Middle and the Late Cambrian (Ubaghs tites; Frest & Paul 1971). Heterotomy was developed 1953b, Sprinkle 1973). These patterns were well from the late Early Ordovician to the Upper Ordovician developed during the Middle and Upper Ordovician, (e.g. Glyptosphaerites). Patterns showing varieties occurring in numerous diploporans (e.g. Phlyctocystis, of subdivisions all occurred during the late Middle Sinocystis) and rhombiferan fistuliporites (e.g. Echi- Ordovician to the Middle Silurian (e.g. Ascocystites, nosphaerites, pleurocystitids) (Chauvel 1978, Parsley Sphaerocystites). 1970, 1998). They occurred in these two classes until the Middle Silurian and the Middle Devonian, 4 DISCUSSION respectively (Paul 1971, Prokop & Petr 2004). The unilateral pattern appeared during the late Mid- Branching patterns can be organized along morpho- dle Cambrian (Lyracystis; Sprinkle & Collins 2006). clines based only on their topologies (Fig. 4). Again,

48 it is important to emphasize that this is not a character heterotomy (E). They are first recorded in the Middle transformation series, but merely a manner by which to Cambrian eocrinoid Lyracystis, whereas heterotomy compare increasing complexities. Ultimately, phylo- occurred in Lower Ordovician diploporans. Until the genetic analyses could reveal evolutionary events that phylogenetic position of Lyracystis is better known, the occurred in very different orders and/or polarities. correlations of the unilateral cline cannot be further Sequences stem from the simplest pattern, discussed. atomy (A). The cline A–B1/B2 reflects the topological derivation of the aniso- and isotomy from atomy. The bilateral sequence A–C–D1–D2 could reflect the 5 CONCLUSION construction of the holotomous pattern from atomy by regular and alternate brachiolar bifurcation. Shifted The evolution of the patterns of ambulacral bifurcation holotomy (D1) seems to be a topological derivation seems to be different in blastozoans and in crinoids. of holotomy (C) by the branching of the last ramu- This is not surprising, since crinoid arms and blas- lus off the same side as the previous one (Fig. 4). The tozoan ambulacra are not homologous (Ubaghs 1967, occurrence of both patterns on the same specimen (e.g. Sprinkle 1973, Mooi & David 1997, David et al. 2000). Macrocystella, Cystoblastus; Kesling 1967a) suggests Underscoring this point is the fact that arms are evolu- a strong relationship between them. In the morpho- tionarily stable structures with persistent mineralized cline A–C–G, (G) could be produced by a division components, whereas ambulacra can be mineralized of main ambulacral grooves in three major grooves, or unmineralized, and reduced to the food grooves. each showing a holotomous pattern (C). The unilat- Moreover, the expected topological clines seem not to eral sequence A–C–E–F1–F2 seems to best interpret be supported by the stratigraphic occurrences of the as the presence of the unilateral patterns (F1 &F2)by patterns. Blastozoans seem to have a more confused a disturbance of holotomy (C) through irregular bra- history with evolutionary phases allowing the ambu- chiolar bifurcations (E). This last pattern (heterotomy) lacral bifurcation to be simplified or to become more is characterized by the apparent random branching of complex. However, these sequences cannot really be ramuli along the main food groove. Therefore, a new compared without mapping bifurcation patterns onto ramulus might be able to appear on the same side as more fully developed phylogenetic hypotheses for both the previous one or on the opposite side, with a sim- major clades. Doing so will shed light on how the ilar probability. The last stage of both unilateral and branching patterns themselves evolved. This approach bilateral clines results from an additional subdivision will also be fruitful in revealing the interplay between of the secondary branches (D2 &F2). function and ecology – each of which is influenced by Combining phylogenetic and stratigraphic occur- the evolutionary events represented by changes in the rences of each pattern, we can examine whether there branching patterns. are correlations between these occurrences and the topological clines (Fig. 4). The bilateral cline A–C– D1–D2 begins with atomy (A) and passes to holotomy ACKNOWLEDGEMENTS (C) and associated patterns (D1 &D2). Atomy is first reported in cinctans during the Middle Cambrian and This paper is a contribution of the UMR CNRS in the eocrinoid Nolichuckia during the Upper Cam- Biogéosciences. We also thank the Doctoral School brian. 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