Introduction

Calcareous tubeworms are common fossils throughout the Phanerozoic, but only recently have their systematics been addressed in detail (Vinn and Mutvei 2009). Studies of skeletal microstructure were used in the early 1990s to begin sorting out the serpulids, tentaculitids, cornulitids, trypanoporids, and other fossil tubeworms (Weedon 1990, 1991, 1994). The Order Microconchida Weedon, 1991, in particular has emerged as a diverse group found from the Upper Ordovician to the Middle Jurassic around the world (Vinn 2006, 2010a; Vinn and Mutvei 2009; Zatoń and Taylor 2009). Long misidentified as serpulid worms (especially “Spirorbis”) (Taylor and Vinn 2006), the microconchids are now seen as likely lophophorates (Taylor et al. 2010). In addition to normal marine environments, microconchids also colonized freshwater, brackish and hypersaline environments during the Devonian to Triassic (Taylor and Vinn 2006). However, in general, microconchids occupy a similar niche in the Upper Paleozoic to that of the oyster/serpulids association of the Mesozoic and Cenozoic (Burchette and Riding 1977; Wright and Wright 1981; Weedon 1990). This paper describes an unusual microconchid (Helicoconchus elongatus gen. et sp. nov.) from the Lower Permian of Texas that provides significant new information on the morphology and paleoecology of the order.

These microconchids from the Wichita-Albany Group had previously been described as “Serpula” and “serpulid worm colonies” (Walsh 2002). They are locally known as “spaghetti corals” (Peter Holterhoff, personal communication 2010). Late Paleozoic to Triassic microconchids often occur as secondary frame builders in algal/microconchid buildups (Peryt 1974; Burchette and Riding 1977; Toomey and Cys 1977; Wright and Wright 1981). In contrast, Helicoconchus forms relatively large independent buildups (Fig. 1), which is also less common in microconchids than solitary growth, biostromes or small aggregations (a few cm in diameter). Serpulid buildups of similar size (fossil and modern) have commonly been termed “reefs” (e.g., Leeder 1973; Hove and Hurk 1993; Bianchi and Morri 2001; Moore et al. 2009), so we prefer to use this term for the aggregations of Helicoconchus as well because it emphasizes their large size for microconchids and their integrated skeletons. The earliest known microconchid reefs are probably Early Devonian (Vinn 2010a). In contrast to the Carboniferous (Barrois 1904; Leeder 1973; Burchette and Riding 1977; Wright and Wright 1981; Weedon 1990) and Triassic (Brönnimann and Zaninetti 1972; Peryt 1974; Ball 1980; Warth 1982; Weedon 1990) microconchid faunas, little attention has paid to Permian microconchids (Toomey and Cys 1977). Permian microconchids are important for reconstructing the evolutionary history of Microconchida, especially for understanding the survival strategies of microconchids in the end-Permian mass extinction.

Institutional abbreviations.—C/W, The College of Wooster Geology Department, Wooster, USA; NHM, Natural History Museum, London, UK; NPL, Non-vertebrate Paleontology Laboratory, Texas Natural Science Center, The University of Texas at Austin, USA.

Geological setting

Sedimentary deposits containing microconchid tubeworm aggregates of Helicoconchus occur in strata of ArtinskianKungurian (Early Permian) age on the Eastern Shelf of the Midland Basin in central Texas, within an outcrop trend extending north from the Colorado River Valley to the Brazos River Valley. Helicoconchus occurs in strata of the Elm Creek, Valera, Bead Mountain and Leuders Formations of the Wichita-Albany Group, an interval of thick limestone and shale, with some beds of evaporite sediments present near the top of the Valera Formation (Moore 1949). These sediments were deposited on a flat shelf surface, gently tilted to the west and located between highlands to the east and the deep center of the Midland Basin (Brown et al. 1987). Clastic sediments in this area were derived from mountainous highlands of the Ouachita trend, a upland region that was eroding during the Permian and later subsided to great depths during late Triassic rifting and opening of the Gulf of Mexico by detachment of the Yucatan block (Bird et al. 2005). Artinskian sedimentation on the Eastern Shelf is the beginning of dominantly autochthonous carbonate deposition in an area formerly dominated by siliciclastic sediments derived from the eastern uplands.

Strata of the Wichita-Albany Group are characterized by low diversity, mollusk-dominated marine biotas, in contrast to the high diversity marine biotas present in the underlying siliciclastic-dominated Cisco Group. This biotic change is associated with a change to drier climates and occurrence of intermittent evaporite sediment deposition (Moore 1949). In the Colorado River Valley exposures, microconchid tubeworm aggregates are common in the base of the Bead Mountain Formation within an interval at the top of a transition from deposits of bedded gypsum evaporites (top of Valera Formation) to beds of argillaceous-to-silty limestone (base of Bead Mountain Formation) This interval of common Helicoconchus aggregates is traceable to the Brazos River Valley (Peter Holterhoff, personal communication 2010).

The common occurrence of Helicoconchus in the late stages of a transition from evaporite sediment deposition suggests it was tolerant of fluctuating salinities in a shallow marine environment. Strata in this interval have nearly planar bedding, consistent with deposition on a shallow gently sloping surface with minor seafloor relief. Bioclastic strata contain fragmental fossils and indicate episodic higher energy conditions, but these beds also contain substantial amounts of siliciclastic silt and mud and are poorly winnowed.

Material and methods

This material was studied with both light microscopes (dissecting and petrographic) and an environmental scanning electron microscope in the Department of Geology at the University of Akron. A micrometer system was used in the light microscopes for the measurements. All of the figured specimens, holotypes, and paratypes have been registered into the fossil worm collections of NHM. Additional topotype material is deposited in NPL.

Microconchids and polychaete tubeworms

The order Microconchida is distinguished from tubicolous polychaetes by its lamellar skeletal microstructure and bulblike (rather than open) tube origin (Weedon 1991; Taylor and Vinn 2006). It is these features that have been used to classify all pre-Cretaceous specimens of the ubiquitous encruster “Spirorbis” as microconchids and not serpulids like the Cretaceous to Recent Spirorbis (Taylor and Vinn 2006).

Systematic paleontology

Fig. 1.

Microconchid tubeworm Helicoconchus elongatus gen. et sp. nov. aggregation, NHM PI AN 1183 (holotype) in top (A), basal (B), and side (C) views.

Fig. 2.

Isolated origin of microconchid tubeworm Helicoconchus elongatus gen. et sp. nov. (scanning electron micrograph), NHM PI AN 1184.

Fig. 3.

Microconchid tubeworm Helicoconchus elongatus gen. et sp. nov. reef in the lower portion of the E unit of the Bead Mountain Formation (Lellis and Holterhoff, 2010) and more specifically the 3B cycle set of Lellis (2010) at location N latitude 32° 41′ 39.31″, W longitude 99° 22′ 58.37″. Viewed from above. The radiating surficial pattern is due to weathering. The scale is numbered in tenths of meters. Photograph courtesy of Peter Holterhoff.

Fig. 4.

Thin-section of tube origin in microconchid tubeworm Helicoconchus elongatus gen. et sp. nov., NHM PI AN 1185.

Discussion

Helicoconchus elongatus extends our knowledge of microconchid morphology and paleoecology, and hence what we can deduce about the evolution of the group. The initially helical and greatly extended erect impunctate tube, centrally pitted diaphragms, and two styles of budding of H. elongatus are distinctive in the order. Its lateral style of budding, shell structure and growth lines show similarities with the hederelloids (Taylor and Wilson 2008), and its centrally pitted diaphragms resemble those of some microconchids and other tentaculitoids (Weedon 1990).

The H. elongatus reefs are in the “loosely coiled helical aggregative tubes” tentaculitoid morphotype of Vinn (2010a: 214), which he interpreted as an adaptation to limited hard substrates and as a protection against burial by sediment. As such, these microconchid reefs are in the same ecological niche space as serpulid reefs and bioherms of the Upper Jurassic and above (Palma and Angeleri 1992; Hove and Hurk 1993; Kiessling et al. 2006).

Reef building.—Because of its asexual reproduction of daughter tubes, Helicoconchus could be the most advanced reef builder among microconchids. Forming a colony by budding is probably energetically cheaper than forming a colony via larvae with gregarious behavior. It could also offer better control of the growth of the “colony”, increasing its mechanical strength and probably offering more effective feeding opportunities for individuals. Helicoconchus aggregations probably evolved from helical gregarious (not budding) reef-building microconchids that first appeared in Early Devonian (Vinn 2010a). Budding could be considered the last innovation in the evolution of microconchids, which otherwise had not changed much since the Early Devonian (Vinn 2010b). Microconchid aggregations and reefs are not known from the earliest Triassic, probably due to the end-Permian ecosystem collapse and extinction of Helicoconchus-like forms. Microconchid aggregations and small reefs evolved in the Middle Triassic, but they did not have asexual reproduction (budding), and were similar to the microconchid buildups of their earliest evolutionary stage (Devonian—Carboniferous).

Phylogenetic affinities.—Asexual reproduction is widespread among many tubicolous members of various invertebrate phyla (i.e., cnidarians, bryozoans, annelids, phoronids) and so does not on its own support any particular suggested biological affinity of microconchids (i.e., tentaculitoid tube-worms). However, this type of asexual reproduction (budding) in order to form an aggregation is alien to mollusks and thus supports the non-molluscan affinities of microconchids. The occurrence of two types of asexual reproduction is a peculiar feature of Helicoconchus, and its adaptational meaning is unclear.

Fig. 5.

Eroded side view of microconchid tubeworm Helicoconchus elongatus gen. et sp. nov. aggregation, NHM PI AN 1186.

Fig. 6.

Coiled lateral bud on the side of a tube of microconchid tubeworm Helicoconchus elongatus gen. et sp. nov., NHM PI AN 1187.

Central pits.—Common central pits in the diaphragms of Helicoconchus (Fig. 7) are atypical for microconchids. Usually microconchids have simple slightly concave diaphragms without pits. However, Weedon (1990) has reported domelike central projections in some diaphragms of Devonian to Lower Triassic microconchids. He interpreted the folding of posterior micro-lamellar sheets of the diaphragms as perforations. We believe these unusual structures were not related to real perforations, but are the result of a folding process comparable to that in Helicoconchus. In spite of the different orientation of the central deflections of diaphragms described by Weedon (1990) from those in Helicoconchus, it could indicate that microconchid diaphragms had a certain degree of plasticity during the early phase of their formation. Alternatively, the central pits of Helicoconchus could reflect soft body characteristics, but in this case one would expect to find central pits in all the diaphragms. This probable early plasticity of diaphragms in microconchids is different from the other encrusting members of tentaculitoid tubeworms such as cornulitids and trypanoporids. Similar deflections could be present in some tentaculitids, interpreted as the perforations by Blind (1969). Most likely the diaphragms were not strongly calcified during secretion and were deformed by body movements (causing the pits and the mix of concave, planar and convex cross-sections) before final calcification.

Fig. 7.

Binary fission budding and diaphragms in microconchid tubeworm Helicoconchus elongatus gen. et sp. nov., NHM PI AN 1188.

Fig. 8.

Scanning electron image of two microlamellar walls of microconchid tubeworm Helicoconchus elongatus gen. et sp. nov. abutting each other in a polished and etched cross-section, NHM PI AN 1189.

Shell structure.—Helicoconchus has no pseudopunctae or pores (Fig. 8), which have been interpreted as shell strengthening structures in microconchids (Vinn and Taylor 2007). However, there is probably not a direct correlation between the presence of pseudopunctae and the erect growth of the tube (a mechanically more demanding growth form than a planispiral shell) in microconchids. The helically coiled microconchid M. aberrans from the Middle Triassic had fewer pseudopunctae (if any at all) than its contemporary the planispiral microconchid species M. valvatus (Vinn 2010b).