Edrioasteroid ambulacral surface-area strategies

Most edrioasteroids allometrically increase ambulacral surface area by dramatically lengthening each of the five ambulacra with a concomitant slight increase in ambulacra width (McKinney and Sumrall, Reference McKinney and Sumrall2011). Because the ambulacra are structurally part of the thecal wall (Bell, Reference Bell1976; Sumrall, Reference Sumrall1993), increases in ambulacral length must be accommodated within the confines of the oral surface. Some taxa, including Bostryclavus and Euryeschatia, coil the distal tips of the ambulacra, thereby positioning the ambulacral tips in the centers of the interambulacral fields (Sumrall, Reference Sumrall1996; Sumrall and Zamora, Reference Sumrall and Zamora2011). Others, such as Pyrgopostibulla and Torquerisediscus, meander the mid- to distal portions of the ambulacra, effectively lengthening the ambulacra without extending the ambulacral tips into adjacent interambulacral areas (Sumrall, Reference Sumrall2001, Sumrall et al., Reference Sumrall, Brett and Cornell2006a). A third type of ambulacral lengthening is seen in Streptaster and Giganticlavus, where ambulacra spiral up to 180 degrees around the thecal ambitus (Bell, Reference Bell1976; Sumrall and Bowsher, Reference Sumrall and Bowsher1996).

Ambulacral surface area can also be increased by changing the number of ambulacra while keeping the relative length of each ambulacrum more or less constant, as in Thresherodiscus. This is not a teratological condition as seen in some specimens bearing supernumerary ambulacra (Bell, Reference Bell1976; Smith, Reference Smith1983; Sumrall et al., Reference Sumrall, Sprinkle and Bonem2006b; Sumrall and Zamora, Reference Sumrall and Zamora2011). Instead, this is a species where supernumerary ambulacral counts are the norm. Although five ambulacra are typical for Cystaster granulatus Hall, Reference Hall1871, numerous specimens have variable counts between four and nine (Bell, Reference Bell1976). In Thresherodiscus, branching ambulacra are normal, and no specimens are recorded that have unbranched ambulacra (Foerste, Reference Foerste1914; Sumrall and Gahn, Reference Sumrall and Gahn2006). Similarly, branching is seen in both known specimens of Bizarroglobus n. gen. and an undescribed taxon from the Ordovician of Iran (Guensburg, pers. comm., 2014). As with lengthening of ambulacra, the number of branches increases with maturity apparently in response to surface area-to-volume issues (Sumrall and Gahn, Reference Sumrall and Gahn2006).

Interestingly, the ultimate strategy for increasing surface area of ambulacra—decoupling them from the constraint of being attached to the theca and raising them into the water column—has not been achieved in edrioasteroids (see Guensburg and Sprinkle, Reference Guensburg and Sprinkle2009, for an alternate interpretation). This strategy is common in many echinoderm groups, including blastozoans such as trachelocrinids, hemicosmitoids, coronoids, lyracystids, and eumorphocystids, as well as in crinoids (Sprinkle, Reference Sprinkle1973, Reference Sprinkle1975; Ubaghs, Reference Ubaghs1978; Parsley, Reference Parsley1982; Brett et al., Reference Brett, Frest, Sprinkle and Clement1983; Sumrall, Reference Sumrall1997; Sumrall et al., Reference Sumrall, Sprinkle and Guensburg1997; Sprinkle and Collins, Reference Sprinkle and Collins2006). Although several blastozoan clades are noted for bearing branched recumbent ambulacra (i.e., the hemicosmitoid Caryocrinites and glyptocystitoid Strobilocystites), they generally do not have branching in the erect ambulacra once free from the theca except in lyracystids, where they branch once just after becoming erect. The presence of numerous erect brachioles results in an extensive food collecting apparatus even though the main ambulacral trunks remain unbranched. Crinoids typically have erect, branching ambulacra above the calyx incorporated into the arms, but also can bear branching ambulacra in the underlying cup, as seen in many camerates.

Differences between Bizarroglobus and Thresherodiscus

Bizarroglobus and Thresherodiscus both bear branching ambulacra that increase the surface area of the ambulacral system. However, the nature of branching and details of the thecal organization are substantially different, suggesting that these two genera are not closely related. Bizarroglobus has the ambulacra as a well-developed 2-1-2 ambulacral symmetry. Ambulacra leave the oral area and proceed down the theca, forming five main ambulacral trunks. From these main ambulacral trunks, fully developed side ambulacra branch left and right primarily alternately down the theca throughout its entire length. Early branched ambulacra often do not reach the thecal periphery, terminating in the center of interambulacral areas, whereas later-formed ambulacra generally extend down the theca terminating within a few mm of the thecal base at the attachment surface (Fig. 1.2).

Figure 1

Bizarroglobus medusae n. gen. n. sp. Two known specimens from Middle Ordovician Kanosh Shale, western Utah. (1–2), Paratype 1806TX8, highly excavated oral surface and exposed aboral surface showing (1) 2-1-2 ambulacral arrangement on peristome, slightly raised branching ambulacra, pustules on thecal plates, and anal pyramid in CD interray (lower center); and (2) tips of ambulacral branches and wrinkled, partly shrunken, tiny-plated, peripheral rim, X2.5. (3), Holotype 2063TX1, oral view of large domed specimen attached to hardground; note eroded (B ray) and partly crushed oral surface with damaged peristome near center, better preserved C, D, E and partial A-ray branched ambulacra, numerous pustules on thecal plates, hidden pores especially along D ambulacrum, and periproct in CD interray, ×1.5. (4), damaged A-ray ambulacrum showing at least six branches, fairly thick, tessellate, thecal plating between ambulacra, and imbricate, pustular plates along vertical, upper edge, ×3. (5), Enlarged B-ray edge of theca; note tiny-plated peripheral rim attached to hardground surface, ×6. (6), Enlarged C and D rays with partial branched ambulacra, well-preserved periproct in CD interray, pores along slightly eroded D ambulacrum vs. numerous pustules along C ambulacrum, and beginning of imbricate plates along thecal edge, ×3.

The ambulacra of Thresherodiscus follow a more even, dichotomous branching pattern. Three main ambulacral trunks leave the peristome, recalling the triradiate stage of ambulacral development (Sumrall and Wray, Reference Sumrall and Wray2007). At approximately equal distances from the centrally located peristome, each of these trunks bifurcate, forming six ambulacra. These ambulacra generally dichotomously branch, but single extra branches on one side or the other occur commonly (Sumrall and Gahn, Reference Sumrall and Gahn2006).

Respiratory structures in edrioasteroids are relatively uncommon, occurring primarily in many Cambrian taxa bearing epispires along sutures between the tessellate interambulacral plates (Smith, Reference Smith1985; Smith and Jell, Reference Smith and Jell1990; Guensburg and Sprinkle, Reference Guensburg and Sprinkle1994; Zamora et al., Reference Zamora, Sumrall and Vizcaïno2013). These structures are also generally confined to the oral surface. Only two Ordovician edrioasteroids, Thresherodiscus and Bizarroglobus, have respiratory structures, but they differ substantially in construction. In Thresherodiscus, a series of modified plates that border the ambulacra possess diplopore-like structures that are covered by bulbous protuberances of thin stereom (Sumrall and Gahn, Reference Sumrall and Gahn2006). Presumably, coelomic fluid was circulated through these structures for gas exchange. In Bizarroglobus, many of the interambulacral plates adjacent to the ambulacra have a large pore. These pores may be internal because they are visible only in parts of the theca that have been eroded. Thin plates with internal pores would place coelomic fluids in proximity to the ambient seawater.

The peripheral rim of Bizarroglobus is also unusual for edrioasteroids. Typically, the rim is differentiated from the distal portions of the theca or pedunculate zone by a circlet of enlarged plates. These plates are extremely thick and are attached to the substrate (Sumrall and Parsley, Reference Sumrall and Parsley2003). Additional peripheral rim plates occur in irregular circlets that decrease in size distal to the proximal circlet of peripheral rim plates. Furthermore, all of these plates are in contact with the substrate. This general format is seen in nearly all isorophid edrioasteroids, including Thresherodiscus (Sumrall and Gahn, Reference Sumrall and Gahn2006).

The peripheral rim of Bizarroglobus differs from typical edrioasteroids by lacking a differentiated proximal plate circlet and, consequently, lacking a clear demarcation between the interambulacral fields and peripheral rim. The more distal circlets decrease in size regularly as in a typical peripheral rim, but it is also unclear how many circlets are in contact with the substrate. This is similar to the situation seen in mature specimens of Kailidiscus (Zhao et al., Reference Zhao, Sumrall, Parsley and Peng2010). In Kailidiscus, small specimens bear a typical looking peripheral rim, except for a demarcation between the interambulacral fields and the rim. The rim plates are thickened and in contact with the substrate. Larger specimens that have come free of the substrate have a similar wrinkling around the margin as seen in the smaller detached paratype of Bizarroglobus.

Stratigraphy and locality

Bizarroglobus medusae n. gen n. sp. is known from two specimens collected from the lower Middle Ordovician Kanosh Shale in trilobite Zones L or M. The large holotype was collected in the upper Kanosh Shale at the K-North Section at Fossil Mountain, and the smaller paratype was collected from the lower Kanosh Shale several kilometers to the south at the J Measured Section of Hintze (Reference Hintze1973), both in the western Ibex area, southwest of Delta, Millard Co., western Utah. The Kanosh Shale at these Ibex area localities has a sparse echinoderm fauna (see Sprinkle and Guensburg, Reference Sprinkle and Guensburg1997) that has gradually increased over the past 25 years, but has not been extensively studied. The edrioasteroid described here is the first new echinoderm to be described from the Kanosh Shale since the 1980s.

The larger domed holotype was identified in a collection of sponges at Brigham Young University, but had not been recognized as an echinoderm. Parts of this specimen were covered by a thin shale layer and required relatively little preparation; however, other portions of the theca, including the peristome and parts of several ambulacra, had already been removed by weathering. The specimen is attached to a hardground surface that is associated with a few one-piece blastozoan holdfasts.

The paratype was found on a broken nodule, with the aboral surface free of matrix and requiring little preparation. Because the bottom surface showed features consistent with an unknown edrioasteroid that had numerous branched ambulacra, but none of the oral surface was visible except around the periphery, it was necessary to prepare the heavily entombed oral side that was covered by about 6 mm of hard concretionary micrite. This material was removed through a combination of micro-drilling, hand picking, and air abrasion. This accounts for the differences in the quality of preservation seen on opposite sides of the specimen. Although not ideal, it was the only way that any information about the oral surface could be obtained.

The two known specimens of this new edrioasteroid are preserved differently. The large, domed holotype is attached to a small, brown, grainstone slab that apparently was a hardground. The peripheral rim is partly exposed around the lower margin of this domed specimen on the flat bedding surface, which is littered with marginals of the flattened eocrinoid Mandalacystis and small, attached holdfasts of this or other eocrinoids (Lewis et al., Reference Lewis, Sprinkle, Bailey, Moffitt and Parsley1987; Wilson et al., Reference Wilson, Palmer, Guensburg, Finton and Kaufman1992). This specimen is still attached to the surface where it was living when it was killed and buried, probably by a severe storm. The bed bearing this fossil was likely exposed on the Kanosh Shale slope for many years before it was found and collected, explaining the extensive weathering of most exposed parts of this specimen.

In contrast, the smaller paratype specimen was apparently ripped off the surface where it was living by storm currents before death and burial. The carcass was carried some distance by the waning currents, then buried horizontally (probably summit-side up) in the fine-grained micritic sediment. The peripheral rim shrank and became wrinkled soon after death, the interior had been filled with sediment, and concretionary material developed around the specimen. During present-day weathering, freezing and thawing caused the nodule to break open along the flat base of the specimen, which slowly weathered on the Kanosh Shale slope prior to discovery. Because the flat surface of the specimen in its nodule was not exposed for very long, this side of the edrioasteroid is relatively well preserved.

Systematic paleontology

Class Edrioasteroidea Billings, Reference Billings1858

Order Isorophida? Bell, Reference Bell1976

Description

Based on flattened, weathered-out paratype, and larger, more domed, but partly eroded holotype attached to a limonite-encrusted, grainstone hardground.

Theca medium-to-large, paratype nearly circular, between 32.5 and 35.6 mm in diameter; originally in nodule approximately 13 mm high, now after excavation of oral surface between 6.8 and 7.4 mm high. Holotype slightly less circular, 59.6 mm maximum diameter (including one peripheral rim 3.4 mm in width) by 50.9 mm minimum diameter (without peripheral rim), profile domed to low globular-shaped with oral surface approximately 15.5 mm high, and low nearly vertical sides above flat peripheral rim.

Oral area wide, showing a ‘2-1-2’ pattern of ambulacral branching on paratype (Fig. 1.1), shared ambulacra 6.4 mm wide, cover plates biserial, oral cover plates appear large, domed; oral frame difficult to see on damaged holotype (Fig. 1.3).

Hydropore poorly known from paratype, linear raised area below mouth and adjacent to C ambulacrum, covered with small elongate plates, just adoral to elongate depression in thecal plating.

Ambulacra long, of narrow width and relatively low, alternately branching five to eight times to produce six to nine branches per ambulacrum and approximately 36 branches total on paratype (Figs. 1.1, 2.2), at least 26–27 branches reaching the aboral surface (Figs. 1.2, 2.3); at least 36–37 ambulacral branches on holotype where B ambulacrum mostly eroded (Fig. 1.3, 2.1); ambulacra imbedded in theca on weathered thecal surfaces, appear to form thecal wall on broken thecal edges (Fig. 1.4), branch off main trunk at high angle, between 50 and 90 degrees, most early branches are long but terminate before peripheral rim; on paratype, rays B and E branch first to the right, rays A, C, and D branch first to the left, terminations relatively abrupt, most distal ones just onto peripheral rim in both paratype (Fig. 1.2) and holotype (Fig. 1.5); floor plates not commonly exposed, food groove at least one-third of ambulacral branch width; cover plates biserial, relatively low-domed, rarely with pustules.

Figure 2

Line drawings of ambulacral branching patterns in holotype and paratype of Bizarroglobus medusae n. gen. and sp. (1), Domed adoral side of large holotype 2063TX1 attached to hardground slab showing branching pattern for four of the five ambulacra, and small marginal areas where peripheral rim exposed (compare with Fig. 1.3), ×1.5. (2), Slightly domed adoral side of smaller paratype 1806TX8 after extensive excavation showing oral surface features and ambulacral branching for all five ambulacra (compare with Fig. 1.1), ×2. (3), Nearly flat aboral surface originally exposed in limestone nodule showing extensions of branched ambulacra from adoral side and wrinkled peripheral rim around sediment-filled central cavity (compare with Fig. 1.2), ×2. A, B, C, D, E=ambulacral rays, ap=anal pyramid, hp=hydropore, p=peristome, pr=peripheral rim, ?=unknown morphology, stippled areas=damaged areas in theca showing internal sediment, or in (3), sediment in thecal interior surrounded by wrinkled peripheral rim.

Interambulacra wide but partly covered by multiple ambulacral branches, oral surface plates tessellate, polygonal, but relatively small and moderately thick (0.75–0.9 mm), plates becoming wider, thinner, and more imbricate on transition to sloping and vertical ambitus, and apparently smaller and imbricate with almost invisible sutures on peripheral rim; pustules widespread on oral surface plates around ambulacral branches (Fig. 1), up to 0.3 mm high, one or more per plate, sometimes grouped into rows perpendicular to the distal ambulacra (Fig. 1.2), becoming smaller and more closely spaced on peripheral rim plates (1 per plate?; Fig. 1.2, 1.5).

Peripheral rim attached to hardground surface around parts of domed holotype, between 3.4 and 3.7 mm wide in different places, folded in and wrinkled on paratype flat bottom, approximately 3.2 mm wide; plates becoming smaller from proximal to distal edge, pustules becoming smaller and more numerous (Fig. 1.5), bottom surface unplated based on matrix-filled central cavity on paratype (Fig. 1.2).

Anal pyramid best preserved in large holotype (Fig. 1.3), located approximately 60% of distance from mouth to inner edge of peripheral rim, slightly closer to D ambulacrum than to C ambulacrum, sitting in slight depression surrounded by small plates, pyramid approximately 5.2–6.0 mm in diameter, plated with approximately 18 long, triangular plates (Fig. 1.6); periproct in smaller paratype more disrupted, approximately 70% of distance from mouth to folded edge of theca, pyramid partly open, approximately 2.5–3.0 mm in diameter, plated with approximately 14 smaller, triangular plates.

Respiratory structures present as exposed pores between 0.2 and 0.3 mm in diameter, commonly alongside ambulacra, most on sutures between ambulacral and interambulacral plates, no pores where plate surfaces and pustules well preserved (Fig. 1.6); almost no pores visible on smaller paratype, especially on well-preserved aboral surface (Fig. 1.2).