Morphology and Diagenesis of Dimorphosiphon Talbotorum N. Sp., An Ordovician Skeleton-Building Alga (Chlorophyta: Dimorphosiphonaceae)

By Boyd, Donald W

ABSTRACT-

Dimorphosiphon talbotorum n. sp. from the Upper Ordovician Bighorn Dolomite of Wyoming is an ancient representative of the erect skeleton-building green algae. Circumstantial evidence indicates that the original skeletal material was aragonite. If so, D. talbotorum and its Ordovician relatives in eastern North America, Europe, and Kazakhstan are reminders that even morphologically simple aragonite producers could thrive during one of Earth’s “calcite sea” intervals.

Unlike previously described Ordovician members of its family, the new species is represented by selectively silicified thalli showing three-dimensional details of internal tubes and a highly variable external form. It differs from a similar taxon, D. rectangulare, in having branched as well as unbranched thalli and in morphology of radial tubes. The new species is the first thoroughly documented record of Dimorphosiphon in western North America. Its stratigraphic position in the Richmondian part of the Bighorn Dolomite correlates with previously reported occurrences of the genus in the Red River Formation of North Dakota and Manitoba.

The Wyoming fossils typically occur as scattered components of wackestone and include both silicified and calcitic individuals. The change in skeletal composition from aragonite to calcite apparently took place before tubes decayed, thus preserving tube morphology. Subsequent silicification varied in degree among specimens, probably reflecting differences in permeability of the secondarily calcitic skeletons.

INTRODUCTION

THIS PAPER describes a new species of Dimorphosiphon Heg, 1927, an ancient skeleton-building alga. Like its modern relative Halimeda Lamouroux, 1812. Dimorphosiphon grew upright on equatorial seafloors and contributed significant quantities of skeletal calcium carbonate to the accumulating sediments. For example, Heg (1961) reported that it is the main component of upper Middle Ordovician limestone beds in the Oslo region of Norway. During the Middle and Late Ordovician, Dimorphosiphon dispersed among low-latitude paleocontinents of those times (Poncet and Roux, 1990; Fortey and Cocks, 2003). Previously named species of Dimorphosiphon are represented by calcitic fossils in limestones of Norway, Kazakhstan, Scotland, and eastern North America: these descriptions necessarily have been based on thin sections. By contrast, the silicified skeletons described here have been freed from their matrix by acid dissolution of the host rock. As a result, the new species is the first early Paleozoic dimorphosiphonacean alga to be well characterized in regard to intraspecific variability of thallus form. Furthermore, it is the first thoroughly documented record of Dimorphosiphon in western North America.

LOCALITY AND STRATIGRAPHY

The study collection is from nearly flat-lying (6E dip) beds of the Upper Ordovician Bighorn Dolomite on Hunt Mountain in the Bighorn Mountains of northern Wyoming (Fig. 1). Scattered exposures of fossiliferous rocks occur in meadows capping the ridges in the southern half of sec. 24, T55N, R91W. That section straddles the boundary between the Hidden Tepee Creek quadrangle and Leavitt Reservoir quadrangle as well as the boundary between Sheridan and Big Horn Counties. Specific collecting sites include exposures in NW, SW sec. 24, and the knob-bearing USC&GS azimuth marker “Hudson 1957″ in SE, NW, SE sec. 24 (Fig. 2). Dimorphosiphon-bearing beds occur in a stratigraphic interval estimated to be at least 4.6 m thick with the lowest bed approximately 12 m above the base of the Horseshoe Mountain Member (Fig. 3). The abundant algal fossils typically occur as scattered components of wackestone. Both silicified and nonsilicified individuals are commonly present in the same layer. In some beds, lenticular chert nodules represent nonselective silicification of packed skeletons (Fig. 4). The discontinuous nature of these concentrations and the pinching and swelling of adjacent laminae suggest that these lenses were produced by soft-sediment flowage that disrupted thin grainstone layers.

Small gastropods, brachiopods, and echinoderm columnals are minor components of the alga-bearing beds. Silicified favositid and halysitid corals are common float specimens in the area and occur in situ, together with solitary rugosans, in beds overlying the alga- bearing interval in Figure 2.

AGE AND CORRELATION

Several publications deal with invertebrate taxa collected from the upper Bighorn Dolomite at a locality approximately 1.6 km northeast of the alga-bearing outcrops. Macomber (1970) described brachiopods from a fossiliferous interval he termed the “Hunt Mountain beds.” His figure 3 shows the base of these beds to be 29 m above the Leigh Member, and laterally equivalent to part of the Clinton Member of the Stony Mountain Formation in the Williston Basin subsurface. He favored a Maysvillian age for his brachiopods. Kolata (1976) described several crinoid species from the same locality and concurred with Macomber on the Maysvillian age of the fauna. He did not specify the stratigraphic position of the “Hunt Mountain beds” within the upper Bighorn. Caramanica (1992) listed the corals he found in the “Hunt Mountain beds” and indicated that 13 m of strata separated that unit from the Leigh Member, less than half the thickness stated by Macomber. Caramanica also correlated the “Hunt Mountain beds” with the Stony Mountain Formation, although he considered that they had greater faunal and lithologic similarities to the Gunn Member rather than the overlying Gunton Member. His age interpretations imply that both these members are Richmondian. Sweet (1979) reached the same conclusion for the upper Bighorn Formation through graphic correlation of conodont data.

The Dimorphosiphon occurrence described here is the third report of the genus in western North America. Derby and Kilpatrick (1985) noted its importance as an index fossil for the B zone, one of four units comprising the Red River Formation in the subsurface Williston Basin of North Dakota. In surface exposures of southern Manitoba, Elias et al. (1988) found Dimorphosiphon to be common in Interval 1of the type section of the Fort Garry Member of the Red River Formation. They considered that interval to be correlative with the B zone in the subsurface. Interval 1yielded conodonts of the middle Richmondian Aphelognathus divergens Chronozone. The limitation of Dimorphosiphon in western North America to a small stratigraphicinterval of Richmondian rocks in Manitoba, North Dakota, and Wyoming suggests that the alga thrived only briefly in that region, and that its host strata there are time-equivalent.

MATERIAL STUDIED

Two hundred silicified specimens freed from matrix were examined. Most are club-shaped, 7-14 mm long, and preserve the bulbous distal end (Fig. 5.1, 5.9). The tapered form and very small proximal diameter suggest that little if any of the original length is missing. Two-branched specimens (Fig. 5.3) represent 10% of the collection, and one three-branched specimen is present (Fig. 5.4). Ten hand specimens containing partially silicified algae were lightly etched with diluted hydrochloric acid for study of the various stages of skeletal replacement. Eight thin sections were made from wackestone for study of unsilicified thalli (Fig. 6).

TERMINOLOGY

Morphologic terms employed here are those commonly used for fossil skeleton-building green algae (e.g., Bassoullet et al., 1983; Mu, 1990; Torres and Baars, 1992). The overall body, the thallus, is differentiated into two zones: inner (medulla) and outer (cortex). They differ not only in spatial relationship but in size and orientation of tubes that pass through them (Fig. 6). The medulla contains relatively large longitudinal tubes which give rise to lateral offshoots of smaller diameter. These typically branch one or more times and become perpendicular to the thallus surface at their distal ends. Thus the cortex is characterized by an abundance of lateral tubes oriented perpendicular to its outer surface. Bulbous expansions in these tubes are termed utricles.

Various terms have been applied to the longitudinal and lateral tubes that characterize Dimorphosiphon and similar fossil algae. Examples include “siphon” (Dragastan et al., 1997), “coenocyte” (Torres and Baars, 1992), “filament” (Mu, 1990), “thread” (Elliott, 1972), and “tube” (Heg, 1927; Roux, 1985). I have adopted Heg’s usage for reasons of the realistic image it conveys and its use in the original description of Dimorphosiphon.

PRESERVATIONAL MODES AND DIAGENETIC HISTORY

At the Hunt Mountain localities (Fig. 1), Dimorphosiphon thalli exhibit a variety of preservational states. In some hand specimens, silicified and unsilicified individuals occur within a few millimeters of one another (Fig. 7.2). Furthermore, those affected by silicification differ greatly in extent and selectivity of replacement. At one extreme, nonselective silicification of coquina- like concentrations produced nodules in which contiguous thalli are completely replaced and welded together (Fig. 4). At the other extreme, only tubes are silicified and the cortex is represented by crystalline calcite (Fig. 7.1, 7.2). Between these end members ar\e individuals in which both cortex and tubes are replaced, allowing essentially complete thalli to be freed by acid dissolution of the matrix (Fig. 5). Even these specimens differ in faithfulness to original microstructure, especially in regard to the cortex. Its originally perforate character is imperfectly preserved in many, and is lost entirely in some (Fig. 7.5). Perhaps some of the variation in selectivity of silicilication reflects differences in thoroughness of original calcification, both within a thallus and between thalli.

Although all the nonsilicified Wyoming thalli are calcitic, there are two reasons to assume that they were originally composed of aragonite. First, modern marine chlorophytes (e.g., Halimeda) produce that mineral and there is no evidence that their ancient relatives did otherwise. Second, in the Bighorn wackestone, originally calcitic grains (e.g., brachiopod fragments and echinoderm columnals) exhibit original microstructure whereas gastropods, probably originally aragonite, consist of a mosaic of calcite crystals similar to that of the nonsilicified algae. It is worth noting that Dimorphosiphon originated and became widely dispersed during one of Earth’s “calcite sea” intervals. If the skeletons were built of aragonite, the taxon represents an exception to the generalization that seawater chemistry during such times favored precipitation of calcitic skeletons, especially for morphologically simple taxa (Stanley and Hardie, 1998).

If mineralization in a living Dimorphoxiphon was analogous to that in extant Halimeda, it was initiated by precipitation of aragonite on surfaces of the close-spaced radial tubes in the cortical area. As described by Macintyre and Reid (1995), intertube spaces in the Halimeda cortex become filled with a mesh work of small (1-3 m) aragonite needles. Transformation of the meshwork into equant micrite begins at tube surfaces and expands into the meshwork as needles degrade into rounded crystals of aragonite. In the next stage, still during the life of the plant, gradual rilling of inner- cortical and medullary spaces takes place as larger needles form in those areas. If calcification in Dimorphoxiphon proceeded in a similar way to that of Halimedii, variation in the rate of any or all of these processes would produce skeletal material unequal in both porosity and in the quality of molds of internal features such as medullary tubes. Such inequalities, perpetuated during syndepositional diagenesis, would determine the parts of a buried thallus vulnerable to replacement by silica-bearing pore water.

Scattered angular fragments of unsilicified Dimorphosiphon thalli in the wackestone (Figs. 6.2, 7.6) exhibit smooth margins truncating a mosaic of interlocking crystals. Preburial breakage is indicated by the absence of adjacent counterparts. The shape of the fragments suggests that the thalli were already dense and brittle objects at the time of breakage. If the damage had occurred to friable loosely constructed grains, the broken edges could be expected to be ragged rather than smooth. Very early recrystallization could be expected by analogy with the condition of Halimeda grains in modern sediments of the Belize lagoon where Reid et al. (1992, p. 151) reported alteration of the aragonite to microcrystalline Mg-calcite primarily by recrystallization.

Nonsilicified Dimorphosiphon specimens consist of a mosaic of calcite crystals interrupted by micrite-filled tubes. Widths of crystals in the mosaic arc typically in the 7-25 m range, with the larger individuals more common in the cortex. The silicified cortex in specimens such as the one in Figure 7.4 exhibits an irregular network of thin silica walls enclosing pores in the same size range as are crystal widths in nonsilicitied thalli. Apparently the silica- bearing fluid encountered a mosaic of interlocking crystals and replacement occurred only at crystal surfaces. A similar phenomenon was described by Mu and Riding (1988) in their study of silicified Permian algae from China. In that material, as with the Wyoming fossils, silicification postdated cementation and neomorphism. Calcite cement crystals in the outer medulla of some Chinese specimens are only peripherally replaced, leaving a delicate network of silica plates after dissolution of the calcite (Mu and Riding, 1988, figs. 6, 8).

Tube morphology was preserved in several ways. The thin tube wall, replaced by silica, is clearly visible in some specimens (Fig. 7.3, 7.4). but for many silicified thalli it is difficult to determine if silica replaced the wall or simply coated it. Calcitic thalli exhibit well-defined tube morphology (Fig. 6) due to the contrast between tube casts and the crystalline calcite. The casts consist of very fine-grained carbonate similar to that of the surrounding wackestone. Furthermore, the fillings appear continuous with the matrix where tubes are truncated at broken margins. These aspects suggest that the tubes were filled with sediment before final burial, although this seems unlikely in many cases given the small diameter of the tubes and the length of individual casts. The only apparent alternative is precipitation of micrite within the tubes. In any case, it seems likely that the tubes were filled before loss of the original skeletal microarchitecture and that the cast material was not susceptible to the diagenetic processes that produced the present mosaic of calcite crystals. Tube casts in silicified specimens vary in composition from carbonate (Fig. 7.1) to silica. Some of the latter probably represent secondary precipitation of silica in an empty tube whereas the pelletal aspect of others (Fig. 7.4) is evidence for replacement of carbonate sediment. In many thoroughly silicified specimens, longitudinal tubes are represented by molds rather than casts (Fig. 7.5). These voids apparently formed by dissolution of carbonate casts which had been present throughout the transformation of carbonate fabrics and subsequent silicification.

SYSTEMATIC PALEONTOLOGY

Repository.-Types, figured specimens, and other specimens studied are housed in the collection of fossil invertebrates. Department of Geology and Geophysics, University of Wyoming.

Suprageneric taxa.-Following Heg (1927), workers on Dimorphosiphon have noted the striking skeletal similarities between that genus and extant Halimeda (e.g., Elliott, 1972), and all have concurred in assigning Dimorphosiphon to the green algae (Chlorophyta). Usage has been less consistent at intermediate taxonomic levels, where different authors have used different names for the same rank in referring Dimorphosiphon to class, order, or family (e.g., Elliott, 1972; Shuyskiy, 1987; Mu, 1990; Nitecki et al., 2004). The present study contributes nothing to the unresolved questions concerning affinities and nomenclature at those taxonomic levels. Several proposed classifications involving these ranks are listed by Dragastan et al. (1997, table 1). In the present paper, I utilize Shuyskiy’s (1987) Dimorphosiphonaceae for the family assignment. It has the merit of being based on fossils, including Dimorphoxiphon. thus avoiding implications of biological attributes associated with family names based on extant genera.

Family DIMORPHOSIPHONACEAE Shuyskiy, 1987

Genus DIMORPHOSIPHON Heg, 1927

Type species.-Dimorphosiphon rectangular? Heg, 1927.

Diagnosis.-Subcylindrical thallus approximately 10 mm long. 2.5 mm wide, with rounded ends. Interior dominated by 7-25 longitudinal nonseptate tubes approximately 0.2-0.3 mm wide, more or less concentrated in central area; smaller (0.08-0.12-mm-wide) radial tubes branch from longitudinal ones at nearly right angles and divide one or more times before reaching outer surface of thallus.

Occurrence.-The type species is abundantly represented in upper Middle Ordovician strata of Norway. It was first described from the Mjs Limestone of the Oslo area (Heg, 1927) and later from the Kalstad Limestone of the Trondheim area (Heg, 1932). In the latter paper, Heg changed the specific name to rectangularis, a revision not adopted by all subsequent authors dealing with the species. Other Middle Ordovician occurrences are in the Ouareau Formation of the St. Lawrence Lowlands of Quebec (Guilbault and Mamet, 1976) and, according to Poncet and Roux (1990), in Kazakhstan. Dimorphosiphon rectangulare in eastern Kazakhstan was described as lower Upper Ordovician by Gnilovskaya (1966), and it is present in the lower Upper Ordovician Craighead Limestone of southern Scotland (Elliott, 1972).

Other described species include D. diadromum Gnilovskaya, 1966 and D. magnum Gnilovskaya, 1972. Both are present in Kazakhstan, the former in Upper Ordovician strata (Gnilovskaya, 1966) and the latter in Lower Silurian rocks (Gnilovskaya, 1972). Upper Silurian representatives of D. magnum have been described from Quebec (Bourque et al., 1982).

In western North America, Dimorphosiphon has been identified in thin sections of the Upper Ordovician Red River Formation, both from subsurface cores (Derby and Kilpatrick, 1985) and from outcrop samples (Elias et al., 1988). The cores are from the B zone limestone in the Williston Basin of western North Dakota whereas the surface samples are from the type section of the Fort Garry Member in southeastern Manitoba. In both cases, the material was identified only to genus.

Discussion.-When describing his specimens from the Mjos Limestone, Heg (1927) simultaneously characterized his new genus and the type species. He noted that dichotomous division of the radial tubes takes place irregularly, and that a third order of branching takes place near the thallus surface where a clublike dilation of a radial lube gives rise to several smaller branches which increase in width distally. More commonly, according to his statement, the radial tubes are straight and constant in width between their origin and the thallus surface where \they open in a pit. He attributed the rarity of the third-order branching to erosion of the thallus surface. Multiple branching (one to three times) of the lateral tubes is shown by Mu (1990, tig. 2a) as characteristic of Dimorphosiphon, but it is not clear whether his diagram is based on Heg’s description or on independent evidence. Gnilovskaya (1972, p. 79) expanded the diagnosis for Dimorphosiphon to include Russian species with branched as well as unbrunched thalli and a greater range in the number of central tubes (10-45).

Three other Ordovician genera resemble Dimorphosiphon in general skeletal architecture but differ in various details. In Dimorphosiphonoides Guilbault and Mamet, 1976 medullary tubes are concentrated centrally and are of equal or smaller diameter than the radial tubes. Lowvillia Guilbault and Mamet, 1946 differs from Dimorphosiphonoides in that radial tube origins are clustered at intervals along the length of the medullary tubes. Palaeoporella Stolley, 1893 is characterized by a large axial canal from which smaller longitudinal tubes arise.

The question of segmentation: Several workers dealing with specimens of Dimorphosiphon have followed Heg (1927) in considering the pieces to he detached components of a segmented skeleton (e.g., Elliott, 1972; Wray, 1977; Derby and Kilpatrick, 1985). The evidence for original linkage is indirect, since no connected individuals have been described or illustrated. Heg emphasized the resemblance of internal morphology of Dimorphosiphon to that of individual segments of Halimeda, a modern articulated green alga, and doubted that the short thallus of the former would have such robust longitudinal tubes if it were not part of a larger entity. He noted that the longitudinal tubes in Halimeda change at or near the node from discrete individuals to an intergrown or welded mass from which new tubes arise. No evidence for this phenomenon has been documented for Dimorphosiphon, either by Heg or later workers, and I have found no evidence of it in the Wyoming collection. This fact does not preclude linkage because crowding, thickening, and dividing of longitudinal tubes might be confined to the noncalcified zone where adjacent segments meet. That is the case in modern Halimeda specimens I have examined. Tubes of these heavily calcified fan- shaped plates pass from one segment to another through a narrow circular collar which rims a shallow noncalcified area. Thus, although collars of adjacent plates are in contact, no molds are formed in the zone of unusual tube morphology. It should also be noted that both the collar and the concave inner surface it surrounds resemble the constricted form of the proximal ends of some specimens in the Wyoming Ordovician collection (Fig. 5.5).

In spite of the internal similarities to Halimeda, the most common external morphology of well-preserved Wyoming specimens is more compatible with the growth form postulated by Bourque et al. (1982). In their figure 5, they portray Dimorphosiphon individuals as nonsegmented, club-shaped objects growing upright on a sand door. The alternative possibility, in which each Dimorphosiphon fossil represents one segment of an originally upright articulated skeleton, is represented by Mu and Riding’s (1983, fig. 12) reconstruction of a Permian gymnocodiacean alga. In their drawing, the terminal segments include club-shaped and bifid forms strikingly similar to typical Wyoming specimens. However, their description of the Permian species notes that circular openings occur at both ends of the segments, an expected condition for all but terminal pieces. By contrast, unbroken Wyoming specimens have a domical large end lacking a central opening (Fig. 5.1, 5.2, 5.9). If these thalli were parts of articulated skeletons, they would all be terminal segments.

DIMORPHOSIPHON TALBOTORUM new species

Figures 5-7

Diagnosis.-Thallus commonly subcylindrical, variably tapered, up to 2 cm long with domical larger end up to 3 mm in diameter; forked thalli less common. Medulla contains numerous longitudinal, sinuous tubes giving rise irregularly throughout length to narrower radial lubes. Radials vary in angle of divergence but become perpendicular to exterior in dense cortex and end in surface pores. Longitudinal tubes highly variable in number of radial offshoots; branching of radial tubes uncommon and nonhierarchical.

Description.-Club-shaped thalli vary from nearly cylindrical to expanded and flattened at distal end (maximum width = 3 mm). Bifid thalli common; trifid rare. Longitudinal tubes consistent in diameter (range = 0.2-0.3 mm; av. = 0.22 mm: n = 18) and wall thickness (~5 m) but variable in number in transverse sections (range = 7-25; av. = 15; n = 20). Radial tubes half or less the diameter of longitudinal ones (range = 0.05-0.15 mm; av. = 0.11 mm; n = 19). typically maintaining same diameter from origin to thallus exterior and not branching in cortex. Cortex ~0.3 mm thick in thalli of 2.0 mm diameter (n = 4).

Etymology.-Named for the Curtis Talbot family, discoverers of the new species.

Types.-Holotype, UW A4052, Figure 5.1; paratype A, UW A4061, Figure 6.1; paratype B, UW A4062-a, Figure 6.2.

Occurrence.-Upper Ordovician (Richmondian) Bighorn Dolomite (Horseshoe Mountain Member); Hunt Mountain, northwestern Bighorn Mountains, Wyoming.

Discussion.-Typical specimens are tapered, and the larger end is considered to be distal in relation to growth direction. Where unbroken, this end is domical and bears surface pores from 0.05 mm to 0.1 mm in diameter. Although the smaller (proximal) end is typically a broken surface, several silicified specimens in the study collection exhibit at this end an abrupt inward sloping of the cortex suggestive of an original feature (Fig. 5.5). In none of these specimens does the cortex completely cover the base. In some cases this may reflect erosion, but a central depression rimmed by cortex in two specimens suggests that this area was never covered. In some specimens, the basal area exposes a thicket of diversely oriented radial tubes but relatively few longitudinal ones (range = 4-8; av. = 6; n = 13).

No structures indicative of the sites of reproductive organs have been recognized, a situation typical of Dimorphosiphon and similar Paleozoic algae (Gnilovskaya, 1972, p. 79; Bassoullet et al., 1983, p. 499; Mu, 1990, p. 149). Heg (1927, p. 7), noting the absence of such features in D. rectangulare, speculated that sporangia may have been situated externally.

Of the three previously described species of Dimorphosiphon, D. rectangulare, D. diadromum, and D. magnum, the Wyoming material most closely resembles D. rectangulare. The new species differs in its diversity of thallus shape and in characteristics of the radial tubes. The cylindrical form of D. rectangulare is only one of several shapes, including forked specimens, represented in the D. talbotorum study collection. Radial tubes typically continue through the cortex to the exterior without branching (Fig. 6) whereas in D. rectangulare they divide dichotomously toward the surface (Heg, 1961). Furthermore, the radial tubes arise from the longitudinal ones at diverse angles in D. talbotorum (Fig. 5.8) and vary greatly in abundance among the longitudinal tubes in any one medullary bundle.

Dimorphosiphon diadromum includes branched thalli but differs from D. talbotorum in several respects. It is distinguished by its large number (35-45) of central tubes, their concentration in the central part of the thallus, and the consistent convex-upward form of the first-order radial tubes. Straight second-order radials branch from widened ends of the first-order radials.

Dimorphosiphon magnum differs from the Wyoming species in having a slender (2.75 mm diameter) unbranched thallus and a small number (e.g., 10) of close-spaced central tubes. They are uniformly thick (0.27-0.30 mm diameter) and give rise to first-order radial tubes with distal diameters nearly equal to those of the central tubes. Second-order radial tubes are small, egg-shaped projections from the widened ends of first-order radials.

As noted above, fossils identified simply as Dimorphosiphon have been utilized for correlation in Upper Ordovician (Richmondian) strata in the eastern Williston Basin (Derby and Kilpatrick, 1985; Elias et al., 1988). Their descriptions and illustrations lack the detail necessary for species assignment. Given the geographic and stratigraphic locations of the Williston Basin Dimorphosiphon beds relative to the Wyoming occurrence, it seems likely that the northern algae and those described here will prove to be conspecific.

ACKNOWLEDGMENTS

C. Talbot and family, enthusiastic prospectors for fossils in northern Wyoming, discovered the alga-bearing beds that provided the specimens described here. Curtis recognized the uniqueness of the fossils and sent some to me for appraisal, thereby initiating the research that led to the present paper. I am indebted to J. Amory and E. Morozova for their translations of French and Russian papers. The figures herein reflect the varied skills of S. Horodyski, P. Ranz, and K. Trujillo. S. Swapp and N. Swoboda-Colberg provided SEM images of several specimens. The final manuscript is much improved as the result of valuable advice from reviewers M. Nitecki and B. Pratt and from associate editor S. Hagadorn.

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ACCEPTED 19 AUGUST 2005

DONALD W. BOYD

Department of Geology and Geophysics, University of Wyoming, Laramie 82071,

Copyright Paleontological Society Jan 2007

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