versión On-line ISSN 1851-8044
Ameghiniana v.45 n.2 Buenos Aires abr./jun. 2008
Major developments in conulariid research: problems of interpretation and future perspectives
1Depto. Geologia Sedimentar e Ambiental, IG, USP, Rua do Lago, 562, 05508-900, São Paulo, SP, Brasil. firstname.lastname@example.org
2Depto. Zoologia, IB, UNESP, CP 510, Rubião Júnior, 18618-000, Botucatu, SP, Brasil. email@example.com; firstname.lastname@example.org
3Department of Geology, Hanover College, Hanover, IN 47243, USA. email@example.com
4Depto. de Zoologia, Universidade de São Paulo, Rua do Matão, Travessa 14, 321, 05508-900, São Paulo, SP, Brasil. firstname.lastname@example.org
Abstract. Renewed interest in conulariids has resulted in clarification of problems in the paleobiology of this group. Discoveries of skeletal structures and specimens preserved in situ, coupled with cladistic analyses, have led to a revival of Kiderlen's (1937) hypothesis that conulariids were polypoid scyphozoans or a sister taxon of this class. Until 1979, research on conulariids centered on the description of new species and on the erection of subgroups using phenetic approaches. Few papers addressed the paleobiology and phylogenetic affinities of conulariids, and none employed cladistics. In contrast, the 1980's saw the publication of major papers on the paleoecology of conulariids, and during this decade the hypothesis that conulariids were benthic organisms was corroborated. Also, new ideas concerning the affinities of conulariids, including the proposal that conulariids represent an extinct phylum, were presented. During the 1990's, the problem of conulariid affinities was widely debated, with authors advocating either that conulariids represent a separate phylum or that they were cnidarians. Near the close of that decade, certain advocates of a cnidarian affinity argued that conulariids were most closely related to Cnidaria. Taphonomic evidence indicates that conulariids were benthic animals originally oriented with their aperture opening upward and that they attached to or were embedded in hard and soft substrates. To understand unresolved problems we recommend that (1) conulariid specialists develop a standard morphological nomenclature based on rigorous definitions; and (2) studies on conulariid paleoecology be carried out using a sequence stratigraphy approach.
Resumen. Principales avances en la investigación de conuláridos (Cnidaria): problemas de interpretación y perspectivas futuras. El renovado interés en los conuláridos ha dado como resultado la clarificación de los problemas sobre su paleobiología. Descubrimientos sobre la morfología del peridermo y especímenes preservados in situ, juntamente con la aplicación de la teoría cladística en el análisis de afinidades de los conuláridos, contribuyeron a revivir las ideas de Kiderlen (1937), de que los conuláridos fueron afines a los Scyphozoa. Hasta 1979, las investigaciones sobre los conuláridos estuvieron concentradas en la descripción y propuesta de nuevas especies, fundamentadas en criterios fenéticos. Pocos trabajos abarcaron la paleobiología y afinidades filogenéticas de los conuláridos. En los años 80, el enfoque cambió y se publicaron artículos sobre la paleoecología, siendo ampliamente aceptada la hipótesis de que este grupo fue formado por organismos sésiles. En ese período se presentaron ideas sobre sus afinidades evolutivas, incluyendo la propuesta de que los conuláridos representaban un linaje evolutivo independiente. Durante la década de 90, el problema de las afinidades de los conuláridos fue intensamente debatido. En ese período, muchos autores obtuvieron nuevas evidencias que reforzaron conclusivamente la idea de que los conuláridos son afines a los Cnidaria. Evidencias tafonómicas indicaron que los conuláridos fueron organismos bentónicos fijados o anclados al substrato. Sin embargo, varios problemas permanecen aún no resueltos por lo que recomendamos (1) los especialistas en conuláridos deben aclarar los diversos términos morfológicos y (2) la estratigrafía secuencial debe ser integrada a los estudios sobre la paleoecología de los conuláridos.
Key words. Conulariids; Cnidaria; Scyphozoa; Systematics; Paleoecology; Taphonomy.
Palabras clave. Conuláridos; Cnidaria; Scyphozoa; Sistemática; Paleoecología; Tafonomía.
Students of conulariids have assembled a substantial body of data on this intriguing extinct group of marine metazoans. The last three decades in particular have witnessed a surge in publications on various aspects of conulariid anatomy (e.g., Steul, 1984; Feldmann and Babcock, 1986; Babcock and Feldmann, 1986a, b; Van Iten, 1991a, b, 1992a, b, 1994; Van Iten and Cox, 1992; Jerre, 1993, 1994; Van Iten et al., 1996, 2000, 2005a, b, 2006c, d; Leme et al., 2004), taxonomy (e.g., Bischoff, 1978; Babcock and Feldmann, 1986a, b; Conway Morris and Chen, 1992; Hughes et al., 2000; Leme et al., 2003, 2004, 2006, 2008), paleoecology (e.g., Babcock et al., 1987b; Harland and Pickerill, 1987; Mapes et al., 1989; Van Iten 1991b, c; Van Iten et al., 1996; Rodrigues et al., 2006), paleobiogeography (e.g., Weldon and Shi, 2003; Van Iten and Vhylasova, 2004), and taphonomy (e.g., Van Iten et al., 1996, 2006b; Simões et al., 2000a, 2003; Rodrigues et al., 2003). Renewed interest in conulariids has emerged throughout the world, both in Europe and North America, the traditional centers of conulariid research (Sinclair, 1948), and in other countries including Argentina, Australia, Brazil, and China. Since the publication of the chapter on conulariids in the Treatise on Invertebrate Paleontology (Moore and Harrington, 1956b), so much new information on conulariids has been reported, and new ideas about their paleobiology and systematics proposed, that we think it time to present a critical review of key developments and major problems in conulariid research.
Conulariids range from the Cambrian (Furongian Series), and possibly the Ediacaran (Vendian), to the Upper Triassic, and have been found in epeiric marine formations on all continents except Antarctica. They achieved their greatest generic diversity during the Ordovician Period (Van Iten and Vhylasova, 2004), but remained relatively diverse and widespread during the Silurian and Devonian periods (figure 1). Well-preserved specimens consist of a steeply pyramidal, generally four-sided phosphatic skeleton that commonly ranges between two and ten centimeters long, though exceptionally large specimens originally exceeded 30 centimeters in length (Babcock and Feldmann, 1986a; Van Iten, 1991a) (figure 2). Scanning electron microscopy (Van Iten, 1992b; Van Iten et al., 2005a) has revealed that conulariid skeletons are built of numerous parallel lamellae that may be less than one micron thick. Conulariid skeletons exhibit a distinctive ornament consisting of transverse ridges (ribs), longitudinal ridges, or fine nodes, and the inner surface of the skeleton commonly shows longitudinal carinae or septa located at the corners and/or midlines. In nearly all species described thus far, the corners are marked by a sulcus, and the midline of the faces may also exhibit a sulcus or a low ridge. The apical (narrow) end may be pointed or closed by a more or less smooth, generally outwardly convex transverse wall, called the schott or apical wall.
Figure 1. Spindle diagram of conulariid diversity, based on described genera in the literature. Data are from Sowerby (1821); Boucek (1928, 1939); Foerste (1928); Sinclair (1940, 1942, 1943, 1948, 1952); Sugiyama (1942); Moore and Harrington (1956b); Thomas (1969); Qian (1977); Bischoff (1978); Xu and Li (1979); Chen Mengue (1982); Parfrey (1982); Hergarten (1985, 1994); Zhu (1985); Babcock and Feldmann (1986a, 1986b); He and Yang (1986); Waterhouse (1979,1986); Babcock et al., 1987a; Qian et al., 1997; Hughes et al., 2000; Ivantsov and Fedonkin 2002; Leme et al. 2003; Van Iten et al., 2005b / diagrama de la diversidad de conuláridos, basado en los géneros descritos en la literatura. Los datos provienen de Sowerby (1821); Bou?ek (1928, 1939); Foerste (1928); Sinclair (1940, 1942, 1943, 1948, 1952); Sugiyama (1942); Moore y Harrington (1956b); Thomas (1969); Qian (1977); Bischoff (1978); Xu y Li (1979); Chen Mengue (1982); Parfrey (1982); Hergarten (1985, 1994); Zhu (1985); Babcock y Feldmann (1986a, 1986b); He y Yang (1986); Waterhouse (1979,1986); Babcock et al., 1987a; Qian et al., 1997; Hughes et al., 2000; Ivantsov y Fedonkin 2002; Leme et al. 2003; Van Iten et al., 2005b.
Figure 2. Conulariid thecal morphology, showing the main anatomical terms applied to the group (Modified from Leme et al., 2004). / morfología tecal de conuláridos, enseñando los principales términos anatómicos aplicados al grupo (modificado de Leme et al., 2004).
During the nineteenth and early twentieth centuries, conulariids were generally classified as mollusks (figure 3). Based in part on the presence of one or more septum-like apical walls (schotts) in some conulariid specimens, Eichwald (1840) and Vanuxem (1842) assigned conulariids to the class Cephalopoda. Later, Barrande (1867) and Lindström (1884) classified conulariids as pteropods (figure 3), based on gross similarities in form between conulariid skeletons and pteropod shells. The hypothesis of a cephalopod affinity for conulariids has since been rejected (e.g., Kiderlen, 1937; Kozlowski, 1968) on the grounds that the similarities between conulariid skeletons and nautiloid (or other cephalopod) shells are superficial, with conulariids lacking any structure that can be homologized with the cephalopod siphuncle, and with conulariid specimens whose apical end is pointed generally lacking transverse internal walls comparable to cephalopod septa (e.g., Van Iten, 1991b). Due in large part to the critique of Pelseneer (1889), the hypothesis of a pteropod affinity for conulariids likewise has been rejected.
Figure 3. History of phylogenetic interpretations of conulariids, showing principal publications discussed in the text / historia de las interpretaciones filogenéticas de los conuláridos, mostrando las principales publicaciones discutidas en el texto.
Ruedemann (1896a, b) referred conulariids to the phylum Annelida, primarily on the basis of interpretations of Sphenothallus Hall, 1847 from the Upper Ordovician of New York (figure 3). This taxon is similar to certain tubiculous annelids in having a tubular skeleton and a sessile benthic mode of life (see discussion below). Although Sphenothallus was assigned to the conulariids by Moore and Harrington (1956b), it has since been removed from the group (Feldmann et al., 1986) (figure 3), and except for Moore et al. (1952) the hypothesis of an annelid affinity for conulariids has not been seconded.
More recently, conulariids were compared with pterobranchs (Termier and Termier, 1949, 1953) and vertebrate chordates (Steul, 1984). Termier and Termier (1949, 1953) proposed that the divaricate ornament (transverse ridges) exhibited by many conulariids is a reflection of the presence of fusellary tissues homologous to those of pterobranchs, a group now assigned to the phylum Hemichordata (Clarkson, 1998). Later, Steul (1984) examined material interpreted by her as Conularia and collected from the Hunsrück Slate (Lower Devonian) of Germany. Based on interpretations of x-ray photographs of these specimens, Steul concluded that some of them preserved relic soft parts or other structures homologous with internal structures and organs of vertebrates (Steul, 1984) (figure 3). Steul's interpretations now appear to be untenable, as at least some of the specimens she examined, including the single specimen purportedly preserving a backbonelike "axial element," are siphuncle-bearing, orthocone nautiloids misidentified as conulariids (Hergarten, 1994). Similarly, recent work on the microstructure of conulariid skeletons (e.g., Van Iten, 1992b) failed to reveal any evidence of structures comparable to the fusellar half rings of graptolites or pterobranchs.
H. Kiderlen and B. Werner
Kiderlen (1937) departed substantially from the opinions of previous authors by classifying conulariids as a group of scyphozoan cnidarians (figure 3). Kiderlen's hypothesis was based in large part on Wiman's (1895) illustrations of serial transverse cross sections through Eoconularia loculata (Wiman), a Silurian conulariid from Gotland that exhibits a prominent, Y-shaped (transversely) midline septum on the inside of each of its four faces (see also Van Iten, 1992a; Jerre, 1994). Kiderlen compared these structures with the four endodermal (1937) septa of stauromedusans, a medusozoan order traditionally classified as scyphozoans (Marques and Collins, 2004; Van Iten et al., 2006a), and proposed that the four septa of E. loculata originally were covered by soft part structures homologous to the scyphozoan septa. Based on the presence in other conulariids of geometrically regular, lappet-like infoldings of the apertural region, Kiderlen (1937), Moore and Harrington (1956a), and Bishoff (1978), Sinclair, (1940, 1942, 1948, 1952) concluded that the midlines were also sites of a longitudinal retractor muscle, homologous to the ectodermal retractor muscle in scyphozoan septa. In his reconstruction of a living conulariid, Kiderlen (1937, figs. 46, 47) depicted the animal as having circumoral tentacles and a biphasic life cycle consisting of an attached polypoid phase and a free-swimming medusa. The hypothesis that conulariids had a free-living medusoid life stage was based in part on the existence of conulariid specimens whose apical end is covered by a schott, interpreted by Kiderlen as a detachment scar. In the same year, Knight (1937) documented the presence of relic tentacles in Conchopeltis Walcott, 1876 (Late Ordovician, New York), a probable medusoid cnidarian assigned by Knight to the conulariids. However, Oliver (1984) argued that Conchopeltis probably is not a conulariid or even closely related to conulariids, and it has been removed from that group.
Since the publication Kiderlen's paper, a number of subsequent authors (e.g., Moore and Harrington, 1956a, 1956b; Chapman, 1966; Werner, 1966, 1967, 1969, 1973; Bischoff, 1978; Bouillon, 1981; Van Iten, 1991a, b; 1992a, b; Van Iten and Cox, 1992; Jerre 1993, 1994; Bergström, 1995; McKinney et al., 1995; Van Iten et al., 1996, 2000, 2006a; Hughes et al., 2000; Collins et al., 2000, 2006; Marques and Collins, 2000, 2004; Leme et al., 2004, 2008; Babcock et al., 2005; Rodrigues et al., 2006) have argued in support of a scyphozoan affinity for conulariids, based in some cases on new anatomical or paleoecological evidence (figure 3). Werner (1966, 1967), a zoologist and cnidarian specialist whose work on conulariid affinities and evolution has been no less influential than that of Kiderlen, proposed that conulariid skeletons are homologous (1937) to the steeply conical, finely lamellar chitinous periderm of polypoid scyphozoans of the order Coronatae. Werner argued further that conulariids were the ancestors (1966, 1967) of the coronatids, and thus that the origin of the latter group involved the loss of phosphatic mineralization and rounding of the periderm. Subsequently, Van Iten (1991b, 1992a, 1992b) made additional, gross morphological and microstructural comparisons between conulariid skeletons and the coronatid periderm. Van Iten (1992b) documented three new types of internal skeletal structures in conulariids, including seriated corner and midline ridges similar to the seriated internal projections of certain coronatids, and concluded that no other currently known skeleton or theca is more similar to the skeleton of conulariids than is the coronatid periderm.
Kozlowski (1968), best known for his work on graptolites, addressed the problem of the affinities of conulariids in a study of fragmentary specimens identified by him as remains of node-bearing conulariid skeletons. Using transmitted light microscopy, R. Kozlowski discovered new and intriguing microstructural features, including microscopic circular "pores" and "vermiform canals." In his discussion of the paleobiology of conulariids, R. Kozlowski placed particular emphasis on specimens exhibiting "perforate nodes." Kozlowski (1968) interpreted the perforations as primary anatomical features, and proposed that they were sites of an external sensory organ. Calling the perforated nodes "choanophymes," Kozlowski (1968) also argued that they were covered late in life by lamellar skeletal material laid down on the exterior surface of the skeleton. Based in part on these interpretations, Kozlowski (1968, p. 525) rejected the hypothesis of a scyphozoan affinity for conulariids, and concluded that conulariids "represent a separate branch having no direct phylogenetic affinity with other known branches."
Kozlowski's choanophyme hypothesis was challenged by Bischoff (1978, p. 298), who reported that"sections through nodes [of circonulariids; see discussion below] showed that the lamellae of the external layer are continuous over the whole of the nodes, thus excluding the possibility of the existence of a passageway between the interior and the surrounding medium." One might also note that many conulariids, for example all members of the genera Climacoconus, Conularina, and Eoconularia, lack nodes. Somewhat later, Jerre (1993) published scanning electron photomicrographs of node-bearing conulariid shell fragments ("microfossils") from the Silurian of Gotland. Interestingly, none of the nearly 100 illustrated nodes, some of which appear to be broken, exhibits evidence of perforation. Van Iten et al. (2006b) documented perforate nodes in conulariid fragments from the Late Ordovician of Iowa, but they did not address the question of the origin of the perforations. If, as suggested by Bischoff (1978), the perforate nodes observed by Kozlowski and other investigators are preservational or taphonomic artifacts, then closely spaced serial sections through node-bearing conulariids whose nodes are intact (whose tips have not been broken) should consistently show that the core of the nodes is composed of phosphatic skeletal material.
M.F. Glaessner and G.C.O. Bischoff
Glaessner (1971, 1984), a leading student of the Ediacaran Biota, presented detailed critiques of the interpretations of Kiderlen (1937) and Werner (1966, 1967). Although Glaessner (1971) rejected the hypotheses that conulariids were scyphozoans and directly ancestral to coronatids, he argued nevertheless that conulariids were closely related to scyphozoans. Moreover, Glaessner (1971) proposed that conulariids, Conchopeltis, and scyphozoans were all derived from the Ediacaran genus, Conomedusites Glaessner and Wade. Later, Glaessner (1984) modified his earlier proposal by placing conulariids in Scyphozoa, although he again derived them (directly) from Conomedusites (see Glaessner, 1984, fig. 3.2).
Bischoff (1978) erected the conulariid suborder Circonulariina to receive fragmentary tubules collected from Siluro-Devonian limestones of Australia and exhibiting a more or less circular, as opposed to rectangular, transverse cross section. Bischoff showed not only that circonulariids are most similar to pyramidal conulariids in their gross morphology, but also that circonulariids exhibit microscopic circular "pores" similar to those previously documented by Kozlowski (1968). Bischoff advocated a modified version of Glaessner's (1971, 1984) phylogenetic hypotheses, deriving (as did Glaessner) conulariids and Conchopeltis from Conomedusites, and the Conulata (= conulariids + Conchopeltis + Conomedusites) and Scyphomedusae from a hypothetical, scyphozoan common ancestor. Incidentally, Bischoff (1978) classified conodonts as conulariids, an error that he later seems to have corrected.
L. E. Babcock and R. Feldmann
In a series of papers published during the mid 1980's and 1990's, L. E. Babcock and R. Feldmann reinterpreted the anatomy, systematics, and phylogenetic affinities of conulariids (figure 3). Echoing Kozlowski (1968), Feldmann and Babcock (1986) and Babcock and Feldmann (1986a) proposed the new invertebrate phylum, Conulariida. These authors argued that most of the morphological similarities shared by conulariids and scyphozoans are homoplastic, and that apertural lappets and Wiman's (1895) Y-shaped septa, two major bases of the scyphozoan hypothesis, were taphonomic artifacts. Based in part on the observation that the transverse cross section of many conulariids is rectangular, Feldmann and Babcock concluded that conulariids were bilaterally symmetrical organisms, and that the phylum Conulariida was more closely related to bilaterians than it was to cnidarians or other diploblastic metazoans. In addition, Babcock and Feldmann (1986b, fig. 2.1) proposed that conulariids possessed an alimentary canal, though they did not specify whether it was blind or terminated in an anus. Feldmann and Babcock also reinterpreted the basic architecture of conulariid skeletons, which had previously (e.g. Barrande, 1867) been described as consisting of numerous, thin, parallel lamellae. According to Feldmann and Babcock (1986), conulariid skeletons consist of discrete, solid, transverse"rods" (in some cases bearing nodes and "spines") embedded within a finely lamellar "integument."
Babcock and Feldmann (1986a, c) concluded that the phylum Conulariida consisted of approximately 40 genera, distinguishable on the basis of (a) the spacing of the rods (transverse ribs); (b) the proportion of rods that abut at the facial midline to those that alternate at this site; (c) the apical angles; and (d) the presence or absence of nodes and spines. Based on the apparent absence of rods in Conularina triangulata (Raymond) (a species with three sides instead of four) and Metaconularia Foerste, 1928, Feldmann and Babcock (1986) and Babcock et al. (1987b) removed these two taxa from Conulariida. Later, Babcock (1991) presented a cluster analysis of taxa previously classified as conulariids. Included in this analysis were the three suborders of Miller and Gurley (1896), Conchopeltis, the suborders Conulariina and Circonulariina, and three genera of early Cambrian small shelly fossils (Carinachites Qian, 1977, Hexaconularia He and Yang, 1986, and Hexangulaconularia He, 1987). Sphenothallus Hall, 1847 was also included in the analysis because it had historically been considered to be a conulariid or closely related to them. Babcock (1991, p. 137) concluded that the results of his cluster analysis showed that some of these taxa are highly dissimilar to fossils considered "authentic conulariids." On this basis, Babcock removed from Conulariida the suborders Conchopeltina (Conchopeltis), Conulariina, and Circonulariina, the three small shelly taxa, Sphenothallus, and the genera Conulariella Boucek, 1928 and Conulariopsis Sugiyama, 1942.
Although the erection of the phylum Conulariida has received support from some authors (e.g., Brood, 1995a, b), subsequent work has shown that the interpretations of conulariid anatomy on which this new taxon was based are problematical [see in particular the critiques of Van Iten (1991a, 1992a, 1992b), Bergström (1995), Van Iten et al. (1996), and Hughes et al. (2000)]. Van Iten (1991a, 1992b), who conducted scanning electron imaging of sectioned conulariid skeletons, showed that features interpreted by Feldmann and Babcock as discrete, spine-bearing rods are parts of a single, continuously laminated structure exhibiting transverse and/or longitudinal corrugation. Van Iten (1992a) pointed out that the apparent bilateral symmetry of conulariid skeletons is consistent with a cnidarian affinity for this group, as it can also be interpreted as a kind of biradial symmetry. As noted above, Van Iten (1992a) argued that conulariid skeletons are most similar to the periderm of polypoid coronatids, a major group of scyphozoans. Jerre's (1994) study of the anatomy of Eoconularia loculata showed that the Y-shaped midline septa discovered by Wiman (1895) are not taphonomic artifacts, but actual skeletal structures. Likewise, several authors (e.g., Reed, 1933; Kowalski, 1935) have together documented cases of highly regular infolding (plicated closure; Moore and Harrington, 1956b) of the oral end of the periderm. These specimens contradict Feldmann and Babcock's claims that apertural lappets can only be interpreted as taphonomic artifacts, for it is at least as likely that they functioned to close the oral end of the periderm in life.
Interestingly, while Babcock (2005) reiterated the hypothesis that conulariids represent an independent bilaterian phylum, Babcock et al. (2005), in their study of the Late Proterozoic genus Corumbella, appear to have abandoned it. Specifically, Babcock et al. (2005, p. 16) assigned Corumbella to the scyphozoans, and stated that "[s]everal features of Corumbella suggest a possible sister-group affinity with the conulariids." These features include a "radial symmetry similar to that of conulariids" (though with conulariids having a "bilateral pattern of symmetry on the four faces" that may be "secondary to development of an underlying radial symmetry"), and the presence of transverse ridges or annulations, with the transverse ridges (ribs) of conulariids consisting of "thickenings of the phosphatic layers that comprise the skeleton," which is how the architecture/ microstructure of conulariid skeletons has traditionally been characterized (Van Iten, 1992b).
Cambrian and Precambrian conulariids
Previous discussions of the anatomy and phylogenetic affinities of conulariids have been based almost exclusively on material from rocks of Ordovician or younger age. Although conulariids have been reported from the Cambrian System since the late nineteenth century (Walcott, 1890; see also reviews in Conway Morris and Chen 1992, Hughes et al. 2000, and Van Iten et al. 2006c, it was not until the beginning of the present century that their presence in Cambrian strata was definitively demonstrated by Hughes et al. (2000), who erected the genus Baccaconularia to receive two new species of nodose conulariids lacking a corner sulcus and discovered in the Furongian Series of the Upper Mississippi Valley, USA. Van Iten et al. (2006c) reviewed the gross morphology and microstructure of Baccaconularia, showing that some specimens exhibit microscopic lamellae similar to those of post-Cambrian conulariids as well as microscopic circular pores. Reviewing possible interpretations of the pores, these authors concluded that the most likely hypotheses were that they were primary anatomical features (a view previously advocated by Van Iten et al., 2005a) or fungal or bacterial microborings.
The possible presence of conulariids in the Lower Cambrian was addressed extensively by Conway Morris and Chen (1992), who presented a comprehensive review of the anatomy and taxonomy of three genera of conulariid-like small shelly fossils (Arthrochites Chen Mengue, 1982, Carinachites Qian, 1977, and Hexaconularia He and Yang, 1986) from lowermost Cambrian strata of South China. Based on similarities in gross morphology, these authors concluded that the conulariid-like small shelly fossils probably were closely related to conulariids. Later, Van Iten et al. (2005a) compared the microstructure of numerous post- Cambrian conulariids with that of Arthrochites and carinachitiids (Carinachites + Emeiconularia Qian et al., 1997). These authors observed extremely fine skeletal lamination and microscopic circular pores in many post-Cambrian conulariid specimens, but they found no such features in specimens of the small shelly taxa. Nevertheless, Van Iten et al. (2005a) concluded that carinachitiids probably were closely related to conulariids, and provided an expanded list of six gross morphological similarities uniquely shared by these fossils.
The hypothesis that conulariids traced their cnidarian ancestry to Precambrian times (Glaessner 1971, 1984; Bischoff, 1978) was revived by Ivantsov and Fedonkin (2002), who erected the new taxon, Vendoconularia triradiata, to receive a single specimen from Upper Vendian strata of European Russia (White Sea coast). Ivantsov and Fedonkin interpreted this specimen as a six-sided conulariid and assigned it to the Scyphozoa. Later, Van Iten et al. (2005b) reinterpreted the anatomy of Vendoconularia, proposing that features thought by Ivantsov and Fedonkin to be homologous to the corners of Paleozoic conulariids were homologous to their midlines.
Conulariid systematics-from phenetics to cladistics
Early taxonomic studies
Historically, research on the taxonomy and systematics of conulariids has been dominated by studies of lower level taxa. Not surprisingly, most publications in the period from 1821 to 1979 are descriptions of new species and genera based on phenetic (overall) similarity (figure 3). However, a number of authors, namely Kiderlen (1937), Termier and Termier (1949, 1953), Finks (1955), Moore and Harrington (1956a, b), Kozlowski (1968), Salvini-Plawen (1978, 1987), and Bischoff (1978), addressed broader paleobiological and phylogenetic issues, including the problems of the phylogenetic affinities of conulariids in general and the phylogenetic relationships among conulariids (figure 3). None of these studies, including those published after Hennig (1966), was based on cladistic theory.
Boucek (1939) proposed the first classification of conulariids, recognizing three families (Conulariidae Walcott, Conulariellidae Kiderlen, and Serpulitidae Boucêk), five genera, and four subgenera within the order Conulariida Miller and Gurley (1896) (table 1). Later, Sugiyama (1942) emended Boucêk's classification, erecting a new family, Conulariopsidae Sugiyama (table 1).
Table 1. List of suprageneric conulariid names according to Boucek (1939), Sugiyama (1942), Sinclair (1952), Moore and Harrington (1956b), and Bischoff (1978) / lista de los nombres supragenéricos de los conuláridos de acuerdo con Boucek (1939), Sugiyama (1942), Sinclair (1952), Moore y Harrington (1956b) y Bischoff (1978).
Sinclair (1940) described a number of species of Metaconularia from North America, Scotland, and Germany using a combination of qualitative external (e.g., geometry of the transverse ribs) and internal (e.g., presence or absence of carinae) characters. In subsequent years, G.W. Sinclair erected several new genera, including Paraconularia Sinclair 1940, Climacoconus Sinclair, 1942, Conularina Sinclair, 1942, Eoconularia Sinclair, 1943, Glyptoconularia Sinclair, 1948, Anaconularia Sinclair, 1952, Calloconularia Sinclair, 1952, Ctenoconularia Sinclair, 1952, Diconularia Sinclair, 1952, and Exoconularia Sinclair, 1952.
Sinclair (1952) proposed a new taxonomic arrangement of the family Conulariidae Walcott, erecting two new subfamilies (Paraconulariinae Sinclair, and Ctenoconulariinae Sinclair) and retaining most of the families of Boucek (1939) (rejecting only Serpulitidae Boucek) and the subfamily Conulariinae Walcott (table 1). It is interesting that in classification schemes published prior to1952, the relationships among the conulariid families are not discussed.
Based on the work of Kiderlen (1937) and Knight (1937), Moore and Harrington (1956a, b) placed conulariids in the class Scyphozoa of the phylum Coelenterata, and erected the subclass Conulata (table 1). These authors also recognized two suborders of conulariids, Conchopeltina Moore and Harrington and Conulariina Miller and Gurley, and a new conulariid family, Conchopeltidae Moore and Harrington (table 1). The families (Conulariidae Walcott and Conulariellidae Kiderlen) previously recognized by Sinclair (1952) were retained.
As noted above, Bischoff (1978) described a new group, the suborder Circonulariina, from the Silurian and Devonian of Australia. Bischoff erected three new circonulariid genera, which he placed in two new subfamilies, Austraconulariina and Circonulariinae (table 1). Bischoff (1978) also presented detailed descriptions of internal characters of circonulariid skeletons, recognizing several types of internal septa (midline thickenings). Following Kiderlen (1937), Bischoff (1978) suggested that the midlines were sites of a longitudinal retractor muscle similar to that of living stauromedusans.
Additional genera and species of pyramidal conulariids were erected by a number of other authors, including Thomas (1969), Mariñelarena (1970), Méndez-Alzola and Sprechmann (1973), Waterhouse (1979, 1986), Lammers and Young (1984), Hergarten (1985, 1994), Conway Morris and Chen (1992), Parfrey (1982), Jerre (1993), Van Iten et al. (1996), Qian et al. (1997), Hughes et al. (2000), Ivantsov and Fedonkin (2002), and Leme et al. (2003).
Beginning in the early 1990's, the problems of the phylogenetic affinities and within group systematics of conulariids were addressed again, this time in the framework of phylogenetic systematics (Van Iten, 1991a, 1992a, 1992b; Bergström, 1995; Van Iten et al., 1996; Hughes et al., 2000; Collins et al., 2000, Marques and Collins, 2000, 2004, and Van Iten et al., 2006a; Leme et al., 2008). Collins et al. (2000), Marques and Collins (2000, 2004), and Van Iten et al. (2006a) argued that conulariids were scyphozoan or medusozoan cnidarians (figure 3). Using both paleontological and neontological data, Collins et al. (2000) and Marques and Collins (2000, 2004) concluded that conulariids are the sister taxon of Stauromedusae, a group of sessile, progenetic medusoid cnidarians traditionally classified as scyphozoans but recently removed from this group by Collins et al. (2000). These authors concluded that the group consisting of stauromedusans and conulariids diverged early in the history of Cnidaria.
Van Iten et al. (2006a) rescored most of the character states for conulariids in the data matrix of Marques and Collins (2004), and repeated their phylogenetic analysis. The most parsimonious hypothesis of phylogenetic relationships yielded by the new analysis was that conulariids are the sister group of the scyphozoan order Coronatae rather than Stauromedusae, though the latter taxon again emerged as the earliest diverging lineage of Medusozoa. Among the putative synapomorphies supporting the new hypothesis are the presence in conulariids and coronates of a periderm that completely covers the polyp. Also, strobilation emerged as a synapomorphy uniting conulariids, Coronatae, Rhizostomeae, and Semaeostomeae. This interesting result supports the controversial interpretation (Van Iten, 1991a) of one exceptionally preserved conulariid that potentially shows that these animals produced ephyrae by strobilation.
In spite of recent refinements of our understanding of conulariid affinities (e.g. Collins et al., 2000; Marques and Collins, 2000, 2004; Van Iten et al., 2006a), the phylogenetic relationships of genera and species within Conulata, as well as the taxonomic scope of this group, are still poorly understood. Leme (2002) presented a preliminary cladistic analysis using the taxa and characters presented by Moore and Harrington (1956b). The strict goal of this analysis was to test the consistency of the traditional suprageneric groups (families and subfamilies) recognized by these authors. The results of this analysis indicated that, with the exception of Ctenoconulariinae, none of the groups recognized by Moore and Harrington (1956b) is monophyletic. Most recently, Leme (2006) and Leme et al. (2008) presented a cladistic analysis of the phylogenetic relationships among 16 conulariid genera. Their work produced a single most parsimonious cladogram containing several new, monophyletic groups, including a group (Climacoconus, Notoconularia, Paraconularia, Reticulaconularia) whose existence had previously been predicted by Van Iten et al. (2000). Their results suggested that all previously recognized suprageneric taxa within the Conulata are either paraphyletic or polyphyletic.
Investigations of the paleoecology of conulariids may yield additional information bearing on the problem of their phylogenetic affinities. Based in part on prior interpretations of their functional morphology and/or phylogenetic affinities, conulariids have variously been interpreted as planktonic, pseudoplanktonic (e.g., Ruedemann, 1934; Mendes, 1979; Babcock and Feldmann, 1986a, 1986b, 1986c), nektonic (e.g., Kiderlen, 1937; Moore and Harrington, 1956a), or sessile benthic organisms (e.g., Ruedemann, 1898; Babcock and Feldmann, 1986a; Babcock et al., 1987a, 1987b; Harland and Pickerill, 1987; Van Iten, 1991a, 1991b; McKinney et al., 1995; Simões et al. 1999, 2000a, 2000b; Rodrigues et al., 2003). Together with the broad geographical distribution of conulariids, their occurrence in widely dissimilar lithofacies prompted some authors to argue in favor of a planktonic or pseudoplanktonic mode of life. Van Iten et al. (1996) pointed out that even though planktonic taxa generally do exhibit broad geographical ranges and/or occur in strongly dissimilar lithofacies, these observations alone do not constitute sufficient evidence in favor of a planktonic or pseudoplanktonic lifestyle. The hypothesis that adult conulariids were nektonic animals was based in part on Knight's (1937) discovery of relic tentacles in Conchopeltis Walcott, which as noted above was once classified as a conulariid. Several subsequent authors (Werner, 1966, 1967; Van Iten 1991b; Brood, 1995a, 1995b) rejected this hypothesis, based on comparisons with scyphozoan polyps (Werner, 1966, 1967), consideration of the hydrodynamics of conulariid skeletons (Brood, 1995a, b), or analysis of the stratigraphical distribution of conulariids (Van Iten, 1991b).
The hypothesis that conulariids were sessile benthic is based on several lines of evidence. Finks (1955, 1960), and later Van Iten (1991c), documented the occurrence of molds of Paraconularia in lithistid sponges from the Permian of Texas. Finks (1955) and Van Iten (1991c) argued that the most likely interpretation of the mode of life of these conulariids is that they were sessile, and that at least some of these specimens were attached to the sponges at their apex. Other authors have documented evidence that conulariids were embedded, in life, in bryozoans (Sinclair, 1948), or that they attached themselves, at their apex, to bivalve shells (Sinclair, 1948), Sphenothallus (Van Iten et al., 1996), nautiloids (Van Iten, 1991c; Van Iten et al., 1996), or hyolithids (Babcock et al., 1987a). Other authors (Rooke and Carew, 1983; Harland and Pickerill, 1987; Simões et al., 2000a; Rodrigues et al., 2003) have reported or documented evidence that conulariids attached themselves to or were embedded in bottom sediments. Probably the most compelling evidence for such a mode of life is provided by the discovery of numerous Conularia quichua (Devonian, Paraná Basin) preserved with their long axis oriented at high angles or perpendicular to bedding, and with their aperture consistently facing upwards (Petri and Fúlfaro, 1983; Simões et al., 2000a; Rodrigues et al., 2003).
In spite of the aforementioned evidence that conulariids were sessile benthic animals, exactly how conulariids attached or anchored themselves remains unknown (Rodrigues et al., 2006). Several authors (e.g. Ruedemann, 1898; Slater, 1907; Kozlowski, 1968; Bischoff, 1978; Hemish, 1986) have reported evidence of basal attachment discs, rootlets, or other anchoring systems. Although most of these reports (e.g., Ruedemann, 1898) are spurious or doubtful, Kozlowski (1968) documented rootlet-like structures, possibly used for attachment, in conulariid apices etched from Ordovician limestones of Poland. Based on examination of certain specimens of Paraconularia from North America, Babcock and Feldmann (1986b, 1986c) proposed that conulariids attached to substrates by means of a slender, flexible, possibly non-mineralized apical stalk. Van Iten and Vhylasova (2004) suggested that relatively large conulariids were not attached but rather were recumbent, lying on the seafloor on one or two of their faces.
Closely related to the foregoing problem is the nature of conulariid specimens that exhibit an apical wall, or schott. As discussed by Van Iten (1991b), schott-bearing conulariid specimens have been interpreted as (1) individuals that were severed, in life, by currents (Werner, 1966, 1967; Van Iten, 1991b); (2) free-swimming medusae (e.g., Kiderlen, 1937; Boucek, 1939; Moore and Harrington, 1956a; Chapman, 1966; Grasshoff, 1984); or (3) non-injured, sessile individuals that retracted the apical part of their soft body toward the oral end, as part of their normal life history (e.g., Sinclair, 1948; Babcock and Feldmann, 1986a). In order to test these hypotheses, Van Iten (1991b) investigated the anatomy and patterns of occurrence of conulariid schotts and schott-bearing specimens. He reported that conulariid specimens whose apical end is more or less pointed lack internal schotts, and that schott-bearing conulariids occur in significantly higher proportions in samples from high-energy deposits than in samples from lowenergy deposits. Van Iten concluded that these observations are difficult to reconcile with hypotheses 2 and 3, but are consistent with the hypothesis that schott-bearing specimens represent originally sessile individuals that survived severance of the apical region.
Several authors have documented monospecific clusters of conulariid specimens that converge adapically on a common center (e.g., Slater, 1907; Ruedemann, 1925; Sinclair, 1940; Babcock and Feldmann, 1986a, b; Van Iten et al., 1996; Sabattini and Hlebszevitsch, 2004). Such clusters, called radial clusters (Van Iten and Cox, 1992), have been interpreted as evidence of clonal budding or as evidence of gregarious behavior. In most documented radial clusters (e.g., Van Iten et al. 1996, pl. 1, fig. 6), which contain anywhere from two to over twenty specimens, the specimens are oriented more or less parallel to bedding. In others (e.g. Van Iten and Cox, 1992, fig. 2), the component specimens are inclined to bedding, at relatively high angles. As noted by Van Iten and Cox (1992), who compared radial conulariid clusters with colonial scyphozoan polyps, resolving the problems of the anatomy of the conulariid apex and the nature of radial clusters may have implications for the problem of conulariid affinities.
Interestingly, one of the specimens in an in situ cluster of C. quichua, documented by Leme et al. (2004) and Rodrigues et al. (2006), has five faces instead of four. The occurrence of conulariids having an abnormal number of faces had been observed in other conulariids (Conularina triangulata, three faces; Paraconularia missouriensis and Vendoconularia triradiata, six faces) (Babcock et al., 1987b; Ivantsov and Fedonkin, 2002; Van Iten et al., 2005b), but never in Conularia. It is well known that aberrant adults of certain extant scyphozoans (e.g., Aurelia aurita, A. labiata, Chrysaora fuscescens, Pelagia colorata and Phacellophora camtschatica), which normally exhibit tetramerous radial symmetry, show bimerous to heptamerous symmetry (Gershwin, 1999). The nature of developmental variation in medusozoan symmetry is currently unclear, and it may be caused by genetic, environmental, or ontogenetic factors or some combination of these (Gershwin, 1999). The medusozoan ability to produce alternative phenotypes raises the possibility that pre-existing plasticity might have been co-opted as grist for diversification among lineages (West- Eberhard, 1986, 1989; Gershwin, 1999).
Finally, only a few authors have devoted attention to the paleosynecology of conulariids. Conulariid thecae with epibionts such as inarticulate brachiopods (orbiculoids), echinoderms, and encrusting bryozoans were discussed by Hall (1876, 1879), Babcock and Feldmann (1986a, 1986c), Finks (1955), Barrande (1867), and Moore and Harrington (1956a). The colonization of conulariid thecae by encrusters has variously been interpreted as occurring during life or after death (Babcock and Feldmann, 1986a). Establishing the timing of encrustation could have important implications for the origin and growth of the conulariid skeleton, which has variously been interpreted as an ectodermal derivative (e.g., Van Iten, 1992b) or as an internal skeleton with soft tissue originally covering both its internal and external surfaces (Kozlowski, 1968).
This relatively new area of investigation, which includes conulariid biostratinomy and diagenesis, has already yielded important results. The first studies were published near the end of the 1980's (Feldmann and Babcock, 1986; Harland and Pickerill, 1987), and additional taphonomic studies appeared in the 1990's (Van Iten et al., 1996) and in the present century (Simões et al., 2000a, b; Rodrigues et al., 2003; Van Iten et al., 2006b). Van Iten et al. (1996) documented the distribution of eight conulariid species in the Elgin Member of the Late Ordovician (Richmondian) Maquoketa Formation of northeastern Iowa and southeastern Minnesota, USA. These authors showed that Elgin Member conulariids collectively occur in both oxic shelf and dysoxic shelf slope and shale basin facies, and that the distribution of Elgin Member conulariids is facies dependent, with particular species or genera occurring preferentially in certain facies. Van Iten et al. (1996) also showed that radial conulariid clusters in the Elgin Member occur predominantly in the shelf slope facies. Based on these observations, Van Iten et al. (1996) concluded that the Elgin Member conulariids were benthonic and eurytopic organisms that originally inhabited all of the depositional environments in which their remains now occur.
Still more recently, Simões et al. (2000a, b) and Rodrigues et al. (2003) studied the distribution and taphonomy of Conularia and Paraconularia in the Devonian Ponta Grossa Formation of the Paraná Basin, Brazil. Their results, which included discoveries of clusters of Conularia preserved in their original life orientation (Rodrigues et al., 2006), revealed that the distribution and preservation of conulariids correlate with rates of sedimentation and levels of physical (current) energy. Conulariid-bearing strata of the Ponta Grossa Formation were deposited in platformal or shallow water settings, in a muddy epeiric sea subjected to storm events. Taphonomic data indicate that the conulariids Conularia quichua Ulrich in Steinmann and Doderlein, 1890 and Paraconularia africana (Sharpe, 1856), fall into three distinct taphonomic classes defined by four criteria: a) the spatial distribution of the conulariids in the rock matrix; b) their occurrence as isolated specimens or in clusters; c) the degree and type of deformation of the conulariids, and d) the degree of matrix bioturbation. Taphonomic class 1 includes isolated and clustered conulariids that are preserved with their long axis oriented vertical or nearly vertical to bedding. Commonly, these specimens are inflated (non-compacted), non-fragmented, and preserved in massive or laminated siltstones, in some cases containing discrete and scattered ichnofossils. Taphonomic class 2 encompasses isolated or clustered conulariids that are inclined to bedding. Specimens assigned to this class are preserved in extensively bioturbated siltstones or in siltstones containing traces fossils such as Zoophycus. Taphonomic class 3 is represented by isolated conulariids that are preserved with their long axis parallel to bedding. This class consists of four subclasses (I-IV), each of which is characterized by a distinct and complex taphonomic history. Vertically oriented conulariids (class 1) represent in situ (autochthonous) specimens that were buried, suddenly and while alive, by mud clouds generated during storm events. Inclined (class 2) and horizontally oriented specimens preserved in extensively bioturbated rocks (class 3-I) represent autochthonous or parautochthonous occurrences.
Recent efforts towards the integration of the sequence stratigraphy and taphonomy of the Paleozoic sequences of the Paraná Basin revealed that conulariid taphonomy can be a useful tool in parasequence recognition. This fact is important because the parasequence concept is not fully applicable in offshore facies successions (Simões et al., 2002; Rodrigues et al., 2003) such as the Ponta Grossa Formation. This unit encompasses five third order sequences that were deposited in offshore to lower shoreface settings (Simões et al., 2002; Rodrigues et al., 2003). Each sequence consists if a lithologically monotonous succession of extensively bioturbated shales, mudstones, siltstones, and minor sandstones. In this and similar units, delineating parasequence boundaries using lithological criteria alone is very difficult, especially where geochemical data are lacking or of dubious value. In the Ponta Grossa Formation, these problems have been overcome by the use of abundant, high resolution taphonomic data for trilobites, brachiopods, mollusks, and conulariids (Simões et al., 2002). These data show that in situ specimens of solitary or clustered conulariids (Conularia), and co-supportive clusters of articulate brachiopods (Australospirifer), provide consistent indications of changes in relative water depth (minor flooding events), thus enabling the delineation of parasequences (Simões et al., 2002; Rodrigues et al., 2003).
Taphonomic data obtained from the Ponta Grossa Formation also show that caution must be exercised when using biometric characters to diagnose conulariid species and genera. This is because some of the biometric characters (e.g., the spacing of the transverse ribs and the apical angle) that have previously been used to diagnose conulariid species are entirely or partly a product of a particular process of fossilization or weathering (taphotaxa sensu Lucas, 2001). For instance, in C. quichua the spacing of the transverse ribs is highly susceptible to modification by compaction of the theca parallel to its long axis (longitudinal deformation; Simões et al., 2003). In specimens preserved with the aperture facing upward, this character generally has been modified by compaction, especially in fine-grained lithotypes. By contrast, specimens preserved parallel to bedding do not exhibit this type of taphonomic modification. Similarly, the value of the apical angle varies considerably between compressed (both longitudinally and laterally) and uncompressed specimens. It even varies between the different faces of a given complete specimen. It should be noted, however, that we are not arguing against the use of biometric characters or morphometric methods. Rather, we recommend that descriptions of new conulariid genera and species be based on collections encompassing the full spectrum of preservational patterns. Also, the erection of new taxa should be based as much as possible on complete material, and morphometric comparisons should be made using specimens that are "isotaphonomic."
Finally, Van Iten et al. (2006b), building on the studies of Kozlowski (1968), Jerre (1993), and Richardson and Babcock (2002) on conulariid microfossils, discovered submicroscopic conulariid fragments in acid digestion residues from the Upper Ordovician Brainard Shale (Upper Mississippi Valley, USA), a rock unit from which conulariids had not previously been reported. Van Iten et al. (2006b) argued that the single lime packstone bed from which the conulariid fragments were extracted was a distal storm deposit, and hypothesized that the fragmentary state of the conulariid remains was the result of breakage by bottom currents. They went on to suggest that the apparent rarity of conulariid remains in many other shallow shelf deposits may be (partly) the result of preferential break up of conulariid remains by currents. Some of the fragments exhibited broken nodes similar to the perforate nodes documented by Kozlowski (1968), and this again raises the possibility that Kozlowski's "choanophymes" are preservational/taphonomic artifacts. Of course another possible interpretation of the Brainard Shale conulariid fragments is that they are artifacts of the acid digestion process, which does pose some risk of fragmentation and therefore must be controlled for when doing this kind of work.
Paleontologists have made substantial progress toward resolving fundamental problems in the interpretation of conulariids, but some problems remain unresolved or little explored and additional light may still be shed on problems (e.g., the phylogenetic affinities of conulariids) that are now generally thought to be resolved. Unresolved or little studied problems include (a) the phylogenetic affinities of conulariid-like small shelly taxa; (b) aspects of the life history of conulariids, including their manner of (presumed) attachment and the nature of monospecific (radial) clusters; (c) the origin(s) of microscopic circular pores; and (d) details of conulariid soft-part anatomy. To guide future research on these and other questions, we make the following recommendations. First, conulariid specialists should develop a standard morphological nomenclature based on rigorous definitions of characters and character states for conulariid thecae. We also recommend that the use of certain biometric characters be avoided or used with caution, mainly because of complications associated with post-mortem modification. Finally, investigations on the paleoecology of conulariids should focus on patterns of preservation at a variety of scales and on processes of transport and sedimentation in the paleoenvironments in which conulariids lived and/or were preserved. In this connection the use of sequence stratigraphical approaches promises to be very fruitful.
We thank Rodney M. Feldmann and one anonymous reviewer for their constructive comments on an earlier version of this paper. Financial support for this research was provided by grants (99/10823-5, 99/10824-1, 00/14903-2, 00/14904-9, 01/12835-2, 02/12534-5) from the FAPESP (The State of São Paulo Research Foundation) and the CNPq (301023/94-8; 302596/2003-8) to MGS and ACM, and by a grant from the Hanover College Faculty Development Committee to HVI. The authors are also indebted to undergraduate and graduate students from IBB/UNESP for field assistance at collecting localities in southern Brazil.
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Recibido: 20 de marzo de 2007.
Aceptado: 2 de junio de 2008.