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New long-stemmed eocrinoid from the Furongian Point Peak Shale Member of the Wilberns Formation, central Texas

Published online by Cambridge University Press:  09 March 2015

Samuel Zamora,
Colin D. Sumrall and
James Sprinkle
Samuel Zamora
Affiliation:
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington DC, 20013-7012, USA,Instituto Geológico y Minero de España, C/Manuel Lasala, 44, 9ºB, 50006, Zaragoza, Spain 〈samuel@unizar.es〉
Colin D. Sumrall
Affiliation:
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA; 〈csumrall@utk.edu〉
James Sprinkle
Affiliation:
Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin TX, 78712-0254, USA; 〈echino@jsg.utexas.edu〉
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Abstract

Llanocystis wilbernsensis n. gen. n. sp. (Eocrinoidea, Echinodermata) is described based on three specimens from the Furongian Point Peak Shale Member of the Wilberns Formation in central Texas. It displays a unique morphology including a very long stem constructed with holomeric columnals, few feeding appendages, and a polyplated theca. The specimens occur in an intraformational conglomerate deposited in a proximal carbonate platform environment and represent an example of the poorly documented “pelmatozoan” radiation that occurred in proximal facies by the end of the Cambrian.

Information

Type
Articles
Information
Journal of Paleontology , Volume 89 , Issue 1 , January 2015, pp. 189 - 193
Copyright
Copyright © 2015, The Paleontological Society 

Introduction

Furongian echinoderms are rare world-wide (Zamora et al., 2013). The few known examples suggest that stemmed echinoderms in soft siliciclastic substrates were dominated by free-living forms that include likely precursors of glyptocystitoid rhombiferans (Jell et al., Reference Jell, Burrett and Banks1985; Ubaghs, 1998; Dzik and Orłowski, Reference Dzik and Orłowski1993; Zamora, Reference Zamora2012). However, in proximal carbonate environments, stemmed echinoderms were dominated by trachelocrinids and other columnal-bearing eocrinoids that lived permanently attached to stabilized substrates, skeletal debris, or intraformational conglomeratic clasts (i.e. Sprinkle, Reference Sprinkle1973; Sumrall et al., Reference Sumrall, Sprinkle and Guensburg1997; Sprinkle and Guensburg, Reference Sprinkle and Guensburg1997, fig.4).

The abundance of “pelmatozoan” remains in proximal and high-energy carbonate environments (see for example Sumrall et al., Reference Sumrall, Sprinkle and Guensburg1997; Kruse and Zhuravlev, Reference Kruse and Zhuravlev2008) suggests that during the Furongian (Late Cambrian) an important radiation of “pelmatozoans” occurred in such environments. This coincides with the appearance of extensive carbonate hardgrounds (Brett et al., Reference Brett, Liddell and Derstler1983) that were rapidly occupied by forms that migrated from offshore environments (Zamora et al., Reference Zamora, Clausen, Álvaro and Smith2010) as recorded by large numbers of blastozoan holdfasts and columnals (Brett et al., Reference Brett, Liddell and Derstler1983; Sumrall et al., Reference Sumrall, Sprinkle and Guensburg1997; Kruse and Zhuravlev, Reference Kruse and Zhuravlev2008; Zamora et al., Reference Zamora, Álvaro and Vizcaïno2009). Unfortunately, the geologic record fails to provide a good picture of this radiation because of 1) the limited preservation potential of Furongian outcrops from proximal environments, and 2) the difficulty in finding complete specimens preserved in these environments because of the unfavorable taphonomic conditions.

Llanocystis wilbernsensis n. gen. n. sp. is an example of this aforementioned radiation. It possesses a very long columnal-bearing stem that allowed high-level suspension feeding in the water column. Moreover, the polyplated theca has very small plates bearing minute epispires along their sutures. Llanocystis wilbernsensis occurs in the Point Peak Shale Member of the Wilberns Formation in central Texas that has been interpreted as part of a shallow carbonate ramp affected by storms.

Locality and geological setting

Specimens of Llanocystis wilbernsensis n. gen. n. sp. were collected in Long Valley, 21 km (13 miles) SE of Brady, near the NW edge of the Llano Uplift, McCulloch County, central Texas. Stratigraphically, the collecting locality lies within the Point Peak Shale Member of the Wilberns Formation (see Bridge et al., Reference Bridge, Barnes and Cloud1947; Barnes and Bell, Reference Barnes and Bell1977 for details on the stratigraphy). The Wilberns Formation contains five members (Fig. 1): Welge Sandstone, Morgan Creek Limestone, Point Peak Shale, and San Saba Limestone/Pedernales Dolomite (sensu Bridge et al., Reference Bridge, Barnes and Cloud1947). The Point Peak Shale Member is composed of interbedded micaceous, calcareous siltstones, silty limestones, intraformational conglomerate, and stromatolitic limestone (Longacre, Reference Longacre1970). The average thickness of the member is about 49 meters (160 feet), and it thickens from the southeastern to the northeastern part of the uplift (Bridge et al., Reference Bridge, Barnes and Cloud1947). The lower contact with the Morgan Creek Limestone is gradational and the upper contact with the overlaying San Saba Member ranges from sharp to gradational (Longacre, Reference Longacre1970).

Figure 1

Stratigraphic section of the Upper Cambrian Wilberns Formation showing the occurrence of Llanocystis wilbernsensis n. gen. n. sp. (right) in the more shaly Point Peak Member.

The Point Peak Shale Member (Fig. 1) is moderately fossiliferous, and contains trilobites (Bridge et al., Reference Bridge, Barnes and Cloud1947; Longacre, Reference Longacre1970), calcareous and stromatolitic algae (Ahr, Reference Ahr1971), sponges (Rigby, Reference Rigby1975), brachiopods, graptolites, and the echinoderms described herein. The existence of these stemmed echinoderms was mentioned by Barnes and Bell (Reference Barnes and Bell1977, p. 4, 87) and Sumrall et al. (Reference Sumrall, Sprinkle and Guensburg1997, p. 1091), and attached one-piece holdfasts that probably belong to this eocrinoid were briefly described by Spincer (Reference Spincer1996). Lithologically, specimens of Llanocystis wilbernsensis are associated with intraformational conglomerate that is rich in glauconite and echinoderm ossicles. The presence of these conglomerates and associated microbial reefs and stromatolites (see Ahr, Reference Ahr1971; Spincer, Reference Spincer1996) suggest this member was deposited in a very proximal, energetic, shallow shelf environment.

The Point Peak Shale Member of the Wilberns Formation is Furongian (late Cambrian) in age. Longacre (Reference Longacre1970) concluded that the boundary between the Idahoia and Ellipsocephaloides Zones occurs near the base of the member. These levels correspond with the North American lower Sunwaptan Stage (Ludvigsen and Westrop, Reference Ludvigsen and Westrop1985) that correlates with the global Furongian Series, Jiangshanian Stage (Peng et al., Reference Peng, Babcock and Cooper2012).

Systematic paleontology

Subphylum Blastozoa Sprinkle, Reference Sprinkle1973

Class Eocrinoidea Jaekel, Reference Jaekel1918

Family uncertain

Genus Llanocystis new genus

Type species

Llanocystis wilbernsensis n. gen. n sp. from the Late Cambrian (Furongian) Point Peak Shale Member, Wilberns Formation, central Texas.

Diagnosis

Same as the species by monotypy.

Etymology

The name of the genus (Llanocystis) refers to the Llano Uplift in central Texas where the specimens were collected.

Llanocystis wilbernsensis new species

Figures 2, 3

Figure 2

Llanocystis wilbernsensis n. gen. n. sp. from the Point Peak Shale Member of the Wilberns Formation, Texas. 1–3, 7, 9–11, Holotype specimen USNM 178555. (1) general view of the polyplated theca composed of small plates of equal size; (2) same view of the specimen submerged in water; (3) detail of upper half of the theca and feeding appendages (submerged in water); (7) basal part of the theca showing the slightly larger plates that connect with the stem; (9) lateral view of the proximal stem; (10) axial view of the stem; note the wide lumen; (11) oblique view of the theca-stem connection; (4, 5, 8). Paratype specimen USNM 178556. (4) complete specimen immersed in water showing the theca and a very long stem composed of columnals, (5) proximal part of the stem; note the wide lumen that connects with the theca; (8) distal part of the stem showing the alternation of thin and thick columnals; (6). Paratype specimen USNM 178554 showing complete, but poorly preserved theca and the proximal part of the stem.

Figure 3

Camera lucida drawings of two best-preserved Llanocystis wilbernsensis n. gen. n. sp. specimens; (1) enlarged holotype USNM 178555 showing numerous, small plates covering most of theca (stippled areas damaged or covered), larger plates near stem attachment (bottom), and two (of four?) large feeding appendages attached to summit; (2) near-natural-sized paratype USNM 178556 showing less completely plated theca attached to long holomeric stem.

Diagnosis

Polyplated oval theca composed of small, tessellate, polygonal plates; plates uniform in size except those connecting with stem that are slightly larger; apparently four stout, biserial appendages arising from oral surface; epispires scattered, small; long heteromorphic stem formed from thick and thin columnals that may end in a distal holdfast.

Description

Theca oval in side view, probably circular in transverse cross section, composed of numerous small plates with at least three hundred per exposed side (Fig. 2.1, 2.2, 3.1); apparently four large appendages attach to edges of slightly depressed oral surface (Fig. 2.3); thecal length 35 mm and width 22 mm in large holotype; thecal plates polygonal, usually pentagonal or hexagonal, lack ornament (Fig. 2.2, 2.7); plate diameters very constant (1–2 mm) except those forming base of theca that are nearly twice as large; some plates bear sparse, small sutural pores (epispires), with scattered distribution across theca, circular in outline (Fig. 2.3, 2.7, 3.1), lacking rims and raised edges; pores 200µm in diameter in holotype; base of theca protruding, with several larger polygonal plates that connect with the stem (Fig. 2.7); plating of oral surface only known from side view (Fig. 2.3), hydropore, gonopore, and periproct not preserved in present material.

At least 2–4 feeding appendages (either brachioles or exothecal ambulacra) attached to oral surface (Fig. 2.3); only proximal portion of appendages preserved, exact number and distal morphology not evident; proximally, appendages comprised of large, biserial, flooring plates bearing shallow food groove and very small cover plates (Fig. 2.3); unclear how appendages relate to the features of oral surface; no evidence for smaller brachioles or other feeding structures arising from appendages (Fig. 2.3).

Stem-to-theca transition abrupt consisting of a change from thecal plating to a columnal facet. Stem very long, heteromorphic, holomeric (Fig. 2.9, 2.10), 65 mm long in most complete specimen with diameter of 4 mm proximally and 3 mm in distalmost preserved section, bearing alternating thick and thin columnals (Fig. 2.5, 2.8, 2.9); lumen wide, circular, 1.5 mm in diameter proximally in holotype (Fig. 2.5, 2.10); at least eight columnals per centimeter along length of stem; columnals are six times wider than long; articulations between adjacent columnals smooth without apparent crenulations.

Remarks

Llanocystis differs from the middle Cambrian Eustypocystis Sprinkle, Reference Sprinkle1973 by having a larger theca composed of more plates and a longer stem that is heteromorphic in the former and homeomorphic in the later. There are two species of trachelocrinid eocrinoids known in the Furongian of North America that differ in important ways from Llanocystis. Although trachelocrinids are similar in gross morphology bearing polyplated thecae and holomeric stems, neither species shows any evidence for epispire development, and both have the oral area raised into a spout-like projection (Sumrall et al., Reference Sumrall, Sprinkle and Guensburg1997). Trachelocrinus resseri Ulrich Reference Ulrich1929 (see Sprinkle, Reference Sprinkle1973) has a very long stem comprised of holomeric columnals, but they have spiny projections that are absent in Llanocystis. Moreover, the theca in T. resseri has only a few large plates. Tatonkacystis codyensis Sumrall et al. Reference Sumrall, Sprinkle and Guensburg1997 also has a very long stem comprised of columnals. However, it has more numerous, ridged, thecal plates mostly arranged in columns, and the stem has characteristic widely spaced nodals. Trachelocrinids are further characterized by erect feeding appendages with secondary brachioles spaced between every third erect floor plate. Unfortunately, this feature cannot be observed in Llanocystis because the medial-distal appendages are not preserved. However, no evidence for brachioles arising from the proximal appendages in Llanocystis is present.

Another epispire-bearing eocrinoid similar to Llanocystis has been figured by Stinchcomb (Reference Stinchcomb2007, p. 148) from the Furongian Davis Formation of southeastern Missouri. The single known specimen (termed a “Cambrian crinoid”) has an oval theca with fewer preserved plates on the exposed side, a less protruding base, and no large erect appendages near the summit. Instead, it has recumbent ambulacra extending laterally from the summit with a few thin elongate brachioles scattered alongside. The epispires on the basals and lower thecal plates are more elliptical than those in Llanocystis. The preserved stem is slightly shorter than the theca and slowly tapering distally; the proximal columnals are very thin and slightly irregular, and the lumen is not visible. Many one-piece conical holdfasts that may belong to this eocrinoid are cemented to hardgrounds and domal stromatolites in the Davis Formation.

Sprinkle and Gahn (Reference Sprinkle and Gahn2007) briefly described and illustrated several epispire-bearing eocrinoids from the Lower Ordovician (Ibexian) Garden City Formation of northeastern Utah and southeastern Idaho that are also similar to Llanocystis; these eocrinoids are currently being studied.

The summit of Llanocystis is somewhat difficult to interpret based on the poor overall preservation. It likely has oral plates (sensu Sumrall, Reference Sumrall2010; Sumrall and Waters, Reference Sumrall and Waters2012) based on the presence of larger plates in the summit (Figs. 2.3, 3.1), but their identity and distribution cannot be determined. The summit of the holotype has two well-preserved, short proximal stumps of appendages that arise from the back of the theca to the left and right sides as preserved (Fig. 2.3). Lying directly in front of these appendages are what appear to be poorly preserved facets for two additional appendages that arise from the upper front surface of the theca. However, the front facets are poorly preserved and may be a taphonomic artifact. A fifth possible appendage placed centrally between the right and left brachioles is likely a disarticulated ossicle (Fig. 2.3). This suggests a four-ray ambulacral system as described by Sumrall and Wray (Reference Sumrall and Wray2007), implying that ambulacra B-E are present and A has been paedomorphically lost; this has also been termed a 2-0-2 ambulacral arrangement (Zhao et al., 2008). However, other interpretations may be possible.

The appearance of a weakly developed marginal series (Fig. 2.4) appears to be a taphonomic artifact of post mortem thecal collapse and subsequent weathering.

Etymology

The species is named for the Upper Cambrian Wilberns Formation from which the specimens were collected.

Types

Specimens are housed at the National Museum of Natural History (Smithsonian Institution) under repository numbers USNM 178555 (holotype) and USNM 178556, 178554 (paratypes).

Occurrence

Intraformational conglomerate beds of the Point Peak Shale Member, Wilberns Formation, NW Llano Uplift, central Texas, southern USA.

Acknowledgments

S. Z. was funded by a Post Doctoral grant at the Smithsonian Institution (Springer Fund) and by a Ramón y Cajal Grant (RYC-2012-10576) and project (CGL2013-48877) from the Spanish Ministry of Economy and Competitiveness. Additional funding was provided by NSF for the Assembling the Echinoderm Tree of Life Project DEB-1036260. J. S. was supported by the Geology Foundation, University of Texas at Austin. We appreciate the support of I. Pérez (Zaragoza University, Spain) for her assistance with photographs and to N. Hughes (University of California, Riverside) for his help on the geology of the area. Two reviewers, W. I. Ausich and R. L. Parsley, provided valuable input to improve the manuscript.

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Figure 0

Figure 1Stratigraphic section of the Upper Cambrian Wilberns Formation showing the occurrence of Llanocystis wilbernsensis n. gen. n. sp. (right) in the more shaly Point Peak Member.

Figure 1

Figure 2Llanocystis wilbernsensis n. gen. n. sp. from the Point Peak Shale Member of the Wilberns Formation, Texas. 1–3, 7, 9–11, Holotype specimen USNM 178555. (1) general view of the polyplated theca composed of small plates of equal size; (2) same view of the specimen submerged in water; (3) detail of upper half of the theca and feeding appendages (submerged in water); (7) basal part of the theca showing the slightly larger plates that connect with the stem; (9) lateral view of the proximal stem; (10) axial view of the stem; note the wide lumen; (11) oblique view of the theca-stem connection; (4, 5, 8). Paratype specimen USNM 178556. (4) complete specimen immersed in water showing the theca and a very long stem composed of columnals, (5) proximal part of the stem; note the wide lumen that connects with the theca; (8) distal part of the stem showing the alternation of thin and thick columnals; (6). Paratype specimen USNM 178554 showing complete, but poorly preserved theca and the proximal part of the stem.

Figure 2

Figure 3Camera lucida drawings of two best-preserved Llanocystis wilbernsensis n. gen. n. sp. specimens; (1) enlarged holotype USNM 178555 showing numerous, small plates covering most of theca (stippled areas damaged or covered), larger plates near stem attachment (bottom), and two (of four?) large feeding appendages attached to summit; (2) near-natural-sized paratype USNM 178556 showing less completely plated theca attached to long holomeric stem.