Field characteristics

At Badger Flat, the Al Rose Formation is approximately 100 meters thick but consists of poor outcrop (Fig. 1). The unit varies in thickness throughout the region and has been thinned structurally through the area (Stevens, Reference Stevens1986). The Al Rose is composed of splintery, siliceous shales with interbedded thin limestones, 2–7 cm thick. Limestones and shales, where cropping out, are laminated. Fossils were collected largely from float.

Ross (Reference Ross1967, plate 11, left-hand column) showed the lowest prolific fauna on his stratigraphic column—including “ampyxinid, aff. Parabolinella sp., Phyllograptus anna, Didymograptus protobifidus, and D. artus”—at the base of the Badger Flat Limestone rather than at the top of the Al Rose Formation. We believe that this is probably not a correct interpretation since the black limestones of the basal Badger Flat Limestone are apparently without fossils. This interval is underlain by a seam of splintery shales that do indeed yield pendent didymograptid graptolites along with Globampyx sexsegmentatus n. sp., which is likely what Ross listed as an ampyxinid and is the most abundant macrofossil of the Al Rose Formation below that level. It seems more likely to us that the collections referred to by Ross should actually belong to the topmost bed of the Al Rose Formation. The obvious lithological boundary between the Al Rose Formation and the overlying Badger Flat Limestone Formation is at the first robust black limestone bed that lies just above this shale interval.

Age and correlation of the Al Rose Formation

Graptolites from the Al Rose Formation are quite numerous. They represent a typical open-shelf biotope for the Early Ordovician, dominated by pendent didymograptids and phyllograptids. Oceanic elements such as sigmagraptines and isograptines are absent. Because much of the material is derived from local float, it is not possible to determine stratigraphic ranges of species. However, in situ material excavated from small pits in the fissile shales do not show any significant differences between the lower part of the formation and near the top, and it is likely that only one biozone is represented in the collections. The specimens are flattened in shales but are otherwise not greatly distorted.

The following species have been identified: Didymograptellus kremastus Vandenberg, Reference Vandenberg2017 (= Didymograptus (Didymograptellus) protobifidus Elles, Reference Elles1933, of North American usage, e.g., Berry, Reference Berry1960, see also comments in Cooper and Fortey, Reference Cooper and Fortey1982); Didymograptus (Expansograptus) similis Hall, Reference Hall1865; Phyllograptus typus Hall, Reference Hall1858; Tetragraptus amii Elles and Wood, Reference Elles and Wood1902; T. serra Brongniart, Reference Brongniart1828; T. quadribrachiatus Hall, Reference Hall1858. The specimens of Phyllograptus typus are well-enough preserved to observe the characteristic virgellar spine that distinguishes the species from superficially similar specimens of Pseudophyllograptus. Although several of the species have long ranges, comparison with other Lower Ordovician graptolite faunas from measured sections around the Laurentian platform (Berry, Reference Berry1960; Braithwaite, Reference Braithwaite1976; Cooper and Fortey, Reference Cooper and Fortey1982; Williams and Stevens, Reference Williams and Stevens1988) locates the overlap of these species within the equivalents of the Chewtonian and early Castlemainian of the Australian graptolitic standard. This equates with the Blackhillsian Stage of the Ibexian Series in North American terminology (Ross et al., Reference Ross, Hintze, Ethington, Miller, Taylor and Repetski1997), the later part of the earlier half of the Arenigian Stage in terms of the classical European usage (Fortey and Owens, Reference Fortey and Owens1987), which is Floian in terms of the new standard ratified by the International Union of Geological Sciences.

The shelly faunas are abundant, but the trilobites constitute mostly new species, and some of the material is not adequate to permit formal nomenclature. However, the pelagic trilobite Carolinites genacinaca Ross, Reference Ross1951 is typical of the Pseudocybele nasuta (J) Zone in Utah, Idaho, and Nevada, and the Al Rose Formation is therefore likely to be the equivalent of the appropriate part of the Wahwah Formation and Garden City Formation in those sections (Ross, Reference Ross1951; Hintze, Reference Hintze1953, Reference Hintze1973). The occurrence of the trilobite genera Lachnostoma and Protopresbynileus is consistent with this, even though they are represented by different species to the east.

The basal part of the overlying Badger Flat Limestone Formation comprises well-laminated black limestones that are singularly devoid of macrofossils. It seems likely that they represent anoxic conditions. Four meters above the base, we recovered the pelagic trilobite species Carolinites angustagena Ross, Reference Ross1967, which has been identified from early Whiterockian strata in Nevada (Fortey and Droser, Reference Fortey and Droser1999).

Biofacies of the Al Rose Formation

The benthic fauna of the Al Rose Formation is different from its contemporaries that have been studied in detail in the western United States. Shallow-shelf trilobite faunas in the Lower to Middle Ordovician formations across the Great Basin are dominated by faunas including prolific asaphids (in the Blackhillsian), bathyurids, and pliomerids with many additional dimeropgyids, cheirurids, and illaenids (Adrain et al., Reference Adrain, McAdams and Westrop2009); the last two families are often more significant in “mound” or bioherm faunas, such as that at Meiklejohn Peak, Nevada (Ross, Reference Ross1972). Fortey and Droser (Reference Fortey and Droser1996, fig. 4) indicated that a deeper-water Nileid Biofacies was present in collections from Ike's Canyon, Nevada, while the most off-shelf biofacies dominated by olenid trilobites has yet to be found in the western United States. The fauna described in the following is also typical of the Nileid Biofacies, and as in its typical development in the Valhallfonna Formation of northern Spitsbergen (Fortey, Reference Fortey1975a), it also has abundant raphiophorids and some agnostids as well as the eponymous family (note that we assign Protopresbynileus to Nileidae). In the systematic part that follows, close species comparisons are made with trilobites from the Al Rose and those of Lachnostoma, Globampyx, Cloacaspis, and Rhombampyx from Spitsbergen. Pliomeridae are rare although the family is extraordinarily diverse in the Great Basin as currently documented by J.M. Adrain and his colleagues (2009). Further, no example of a bathyurid was collected. Pelagic trilobites of the genus Carolinites provide the most conspicuous common element linking with the shallower-water faunas to the east. A total count of sclerites from the Al Rose Formation is only meaningful in the most general way as collections from individual beds have not been made. A sample of 100 sclerites taken at random from the collections breaks down as follows: Raphiophoridae 50%, Nileidae 30%, Agnostidae 8%, Asaphidae 5%, Telephinidae 2%, Olenidae 3%, Pliomeridae 1%, Indeterminate 1%. Lingulate brachiopods are numerous, as are plumulitid machaeridians. It is remarkable how agnostids and raphiophorids are apparently absent from contemporary strata in the Pogonip Group, despite the fact that many hundreds of specimens have been recovered by acid solution there. Olenid trilobites are rather rare in the Al Rose Formation, suggesting only that an olenid-dominated environment may have lain further offshore.

As far as the habitat of the Al Rose Formation is concerned, numerous flat bedding planes covered with graptolites such as Phyllograptus provide evidence that the seafloor may have been depleted in oxygen at times. However, the abundance of the blind benthic trilobite Globampyx on other bedding planes demonstrates that at other times there was enough bottom oxygenation to support large populations of what were probably particle feeders from suspension (Fortey and Owens, Reference Fortey and Owens1999). However, most of the other species of trilobites (agnostids excepted) were not blind and had well-developed eyes. The olenid has the longest eyes of any described species. It therefore does not seem likely that the habitat was below the photic zone, where atheloptic trilobites dominate. The fact that articulated trilobites are rather common compared with more inshore environments does suggest that the bottom sediments were not disturbed by strong currents. Many of the articulated specimens are likely to have been molts, and these remain unreworked, which makes the same point.

The Nileid Biofacies was probably widely distributed along the western paleocontinental margin of Laurentia in the earlier Ordovician, although it has not been preserved in many places. For example, Norford and Ross (Reference Norford and Ross1978) described a small fauna undoubtedly of this kind from British Columbia. Raphiophorid-bearing strata advance toward the cratonic interior later in the Ordovician. Possibly, some relatively marginal sites were overridden by allochthonous terranes (e.g., Vinini Formation in Nevada) that may serve to conceal the former extent of the deeper shelf biofacies. However, Ketner and Alpha (Reference Ketner and Alpha1992) dispute some of the geological evidence relating to the emplacement of the Vinini Formation. The Al Rose Formation is important in providing an accessible locality to examine this relatively marginal trilobite biofacies and its associated faunas, and it proves the presence of an onshore–offshore sequence of biofacies east of the Great Basin similar to those elsewhere on the flanks of Ordovician Laurentia.

Type species

Agnostus sidenbladhi Linnarsson, Reference Linnarsson1869, late Tremadocian of Sweden (see Ahlberg, Reference Ahlberg1989).

Type species

Arthrorhachis longicollis Raymond, Reference Raymond1925, middle Table Head Formation, western Newfoundland.

Remarks

Nielsen (Reference Nielsen1997) noted that nearly 100 taxa of Ordovician age had been assigned to Geragnostus. He opted to partition this large number of taxa into a number of subgenera, although the basis for placing a particular form into one of these rather than another was acknowledged to be somewhat arbitrary. Since the species from the Al Rose Formation is very similar to the type species of the subgenus Novoagnostus, we are obliged to use this name if we use any at all. However, it is clear that even within the type species, N. longicollis Raymond, Reference Raymond1925, there is variation in the length of the pygidial axis. For example, Whittington's (Reference Whittington1965, pl. 1, fig. 7) type series includes a specimen with an axis that occupies almost 70% of the pygidial length. This is more than the proportion taken up on the type species of Geragnostus (Geragnostus), G. sidenbladhi (Linnarsson, Reference Linnarsson1869); for example, the pygidium illustrated by Ahlberg (Reference Ahlberg1989, fig. 3F) has an axis only 62% pygidial length. Therefore, it cannot be true that species of Geragnostus (Novoagnostus) are characterized by “a shorter pygidial axis, although the axis still accounts for 60% of the pygidial length or more” as Nielsen (Reference Nielsen1997, p. 486) states. Nielsen also assigns Geragnostus clusus Whittington, Reference Whittington1963 to Novoagnostus even though one of Whittington's (Reference Whittington1963, pl. 1, fig. 10) specimens has an axis nearly 80% pygidial length, in what is presumably a highly variable species. Comparative pygidial axial length is clearly not a satisfactory character to delineate a subgenus. Possibly the pygidial terminal piece being well defined posteriorly and with a tendency to be wider (tr.) than the axial ring in front of it may be of use in distinguishing a species group within Geragnostus, but it is clear that no current revision is yet satisfactory. It also is likely that revision of the whole group will result in synonymizing of species since agnostid species are generally widespread, and this is not reflected in the current taxonomy of Geragnostus.

Geragnostus cf. G. (Novoagnostus) longicollis Raymond, Reference Raymond1925
 Figure 2.1–2.3

cf.

Reference Raymond1925 Agnostus longicollis; Raymond p. 12, pl. 1, fig. 5

cf.

Reference Whittington1965 Geragnostus longicollis (Raymond, Reference Raymond1925); Whittington p. 301, pl. 1, figs. 1–12,14,16,17.

Figure 2. (1–3) Geragnostus cf. G. (Novoagnostus) longicollis Raymond, Reference Raymond1925: (1) dorsal exoskeleton, LACMIP 42370.1, LACMIP Type 14950; (2) latex cast from dorsal exoskeleton, LACMIP 42370.2, LACMIP Type 14951; (3) pygidium, LACMIP 42370.3, LACMIP Type 14952, referred with caution to this species. (4) Hintzeia sp. pygidium, LACMIP 42370.6, LACMIP Type 14955. (5, 6) Rhombampyx sp.: (5) cranidium, LACMIP 42370.4, LACMIP Type 14953; (6) axial shield, LACMIP 42370.5, LACMIP Type 14954. Scale bars = 3 mm.

Remarks

Geragnostus longicollis was fully described from good material from western Newfoundland by Whittington (Reference Whittington1965), and rather poor material from the Al Rose Formation adds nothing to our understanding of the species. One obvious difference from the holotype (Whittington, Reference Whittington1965, pl. 1, fig. 1) is the relatively shallow furrows on the pygidial axis of our material, but different preservation may account for this. The most important shared character between our best specimen illustrated on Figure 2.2 herein and the type material is that the pygidial axis tapers backward before slightly expanding at the terminal piece, which is as long (sag.) as the axis in front of it. This is not so clear on a second specimen (Fig. 2.3) with a more effaced pygidial axis, and it is possible that this represents a second agnostid species; the preservation is imperfect for systematic treatment, and it is referred with reservation to Geragnostus cf. G. longicollis.

Type species

Globampyx trinucleoides Fortey, 1975 (Fortey, Reference Fortey1975b), early Whiterockian, Valhallfonna Formation, Spitsbergen.

Remarks

The inclusion of the new species described in Globampyx requires that the original diagnosis of the genus be emended to include species with six, as well as five, thoracic segments. Raphiophorids are unusual among post-Cambrian trilobites in being variable in the number of free segments in the thorax, which can be as few as two in Taklamakania but five, six, or seven in other taxa. If it is accepted that the larger number is more likely to be the primitive condition, then the presence of six segments in G. sexsegmentatus n. sp. is not an apomorphy and thus not a possible basis for separating the taxon from Globampyx.

Holotype

Axial shield, LACMIP 42370.8, LACMIP Type 14957 (Fig. 3.2).

Figure 3. Globampyx sexsegmentatus n. sp.: (1) axial shield, LACMIP 42370.7, LACMIP Type 14956; (2) holotype, axial shield, LACMIP 42370.8, LACMIP Type 14957; (3) dorsal exoskeleton, LACMIP 42370.9, LACMIP Type 14958; (4) latex cast from imperfect dorsal exoskeleton, LACMIP 42370.10, LACMIP Type 14959; (5) well-preserved cranidium, LACMIP 42370.11, LACMIP Type 14960; (6) latex cast from complete exoskeleton, LACMIP 42370.12, LACMIP Type 14961; (7) limestone block showing abundance of this species in the Al Rose Formation, LACMIP 42370.13, LACMIP Type 14962. Scale bars = 3 mm.

Diagnosis

A Globampyx species distinguished from others by having six, rather than five, thoracic segments and an anteriorly comparatively less-inflated glabella.

Description

A common small species, entire exoskeletons 1 cm long or less, of which much is accounted for by the genal spines, so that the main body of the animal is not much more than 5 mm long. The length of the latter (sag.) is close to its width at the midpoint of the thorax. Much of the convexity is on the cephalon, the thorax being nearly horizontal away from the axis. The cephalon lacks an anterior median spine and is just under half total axial length. The cranidium is twice as wide as long, and at its widest, close to its anterior margin, the glabella is slightly more than one quarter cranidial width. Our material is mostly slightly crushed, so it is not possible to be certain of the original convexity of glabella; however, a well-preserved external mold shows a relatively gradual increase in width and height anteriorly before an almost semicircular frontal lobe. There is a suggestion of a median tubercle at the high point of the glabella, as in G. trinucleoides, but no trace of the muscle impressions on the glabella are preserved. Occipital ring narrow and curved slightly backward. Posterior border furrow fades out toward glabella, the border being widest at about two-thirds its length. Facial sutures run around the edge of the cranidium, and the yoked free cheeks probably comprised a strip of doublure separating the very long needle like genal spines, which double the length of the whole animal and seem to curve inward slightly posteriorly.

Thorax widens backward to third segment and thereafter narrows; thoracic axis follows a similar pattern, but even at its widest is not wider than the adjacent thoracic pleura. Thoracic segments are all similar to one another, in touch along their whole length, with sharply truncate pleural tips, which are downturned. Pygidial margin continues this near-vertical peripheral wall, which gradually widens before a posterior notch. Length of pygidium less than half that of thorax, and pygidium more than three times as wide as long. Axis tapers noticeably posteriorly and reaches border. Most specimens show little in the way of pygidial pleural furrows, but this feature is variable, as it is in G. trinucleoides, and up to three pygidial axial rings and an equivalent number of more-or-less transverse pleural furrows may be visible. There is no indication of surface sculpture on the exoskeleton.

Etymology

Referring to six thoracic segments.

Remarks

Two species of Globampyx are known previously, both from early Middle Ordovician (Whiterockian) strata. The type species, G. trinucleoides from the Valhallfonna Formation, Spitsbergen, is similar to G. sexsegmentatus in size and, like it, can be abundant on certain bedding planes. It differs in having a distinctly pyriform glabella and in having only five thoracic segments (Fortey, Reference Fortey1975b, pl. 29, fig. 4). The other species from the Glenogle Shales of British Columbia, G. sinalae Norford and Ross, Reference Norford and Ross1978, is more like G. sexsegmentatus in general proportions but like G. trinucleoides has only five thoracic segments. G. sinalae carries a surface sculpture of fine rugae on the fixed cheeks, unlike the other two species. G. trinucleoides has a sculpture of fine punctae, and G. sexsegmentatus appears to be smooth. Since G. sexsegmentatus is older than either of these species, it is not unreasonable to suppose that the shorter thoraces of the younger species were achieved by failure of release of one thoracic segment from the front of the pygidium. Some support for this is given by the fact that the pygidium of G. trinucleoides accounts for 25% of the exoskeletal length, and this proportion is the same as the length of the pygidium plus sixth thoracic segment on G. sexsegmentatus. The preservation of most specimens of the latter species leaves something to be desired, and so it would be difficult to put this observation on a more quantified footing.

Type species

Rhombampyx rhombos Fortey, 1975 (Fortey, Reference Fortey1975b), Valhallfonna Formation, Spitsbergen.

Remarks

One raphiophorid shield (exoskeleton lacking free cheeks) and cranidium among dozens of Globampyx differ from that genus in possessing a stout anterior spine carried on a relatively wide glabella and a small pygidium with pleural furrows. The overall small size and short, wide, furrowed pygidium, together with details of the occipital structure, are different from Ampyx, in which the pygidia are typically longer (sag.) than the thorax. The Al Rose material is closely similar to the type species of Rhombampyx, R. rhombos Fortey, Reference Fortey1975, but the material available is not sufficient to characterize a new species. Nonetheless, it differs from R. rhombos in a less anteriorly protruding glabella, narrower (tr.) cheeks, with narrow posterior border furrows, and much longer (tr.) pleurae on the anterior thoracic segment. Ampyx and its relatives were already widespread in the Floian across Baltica and marginal Gondwana, but raphiophorids in general appear widely in Laurentia only at the Middle Ordovician (Whiterockian) (Ross, Reference Ross1967; Fortey and Droser, Reference Fortey and Droser1999). The Al Rose species of Rhombampyx is older than the earliest Whiterockian species from Spitsbergen and clearly different from it.

Type species

Carolinites bulbosus Kobayashi, Reference Kobayashi1940 from Middle Ordovician of Caroline Creek, Tasmania.

Synonymy

See discussion in McCormick and Fortey, Reference McCormick and Fortey1999.

Figure 4. Carolinites genacinaca subsp. indet. Ross, Reference Ross1951: (1) cast from imperfect cranidium, LACMIP 42370.14, LACMIP Type 14963; (2) small cranidium, LACMIP 42370.15, LACMIP Type 14964; (3) latex cast from partial thorax and pygidium, LACMIP 42370.16, LACMIP Type 14965; (4) pygidium, LACMIP 42370.17, LACMIP Type 14966. Scale bars = 3 mm.

Remarks

This pelagic species has been claimed by McCormick and Fortey (Reference McCormick and Fortey1999) as the most widely distributed Ordovician trilobite. It has been described several times from paleoequatorial localities, mostly from better-preserved material than that from the Al Rose Formation, for example, by Ross (Reference Ross1951), Hintze (Reference Hintze1953), Fortey (Reference Fortey1975b), and McCormick and Fortey (Reference McCormick and Fortey1999). Further description would be superfluous here. Critical features in determining Carolinites species include the relative size of the bacculae on the cranidium. A large cranidium from the Al Rose Formation has been slightly flattened and is identical to a similarly preserved specimen from Spitsbergen figured by Fortey (Reference Fortey1975b, pl. 38, fig. 1). Pygidia of Carolinites genacinaca genacinaca Ross, Reference Ross1951 taper more posteriorly than do those of other species of the genus, and the posterior axial ring tends toward effacement—both features seen on the specimen with the posterior part of the thorax figured here in Figure 4.3. C. genacinaca genacinaca has a range through the J Zone (Pseudocybele nasuta) of the platform sequences in Utah and Nevada and has been distinguished from C. genacinaca utahensis Hintze, Reference Hintze1953, the status of which is not finalized. Perfectly preserved silicified material from the Great Basin should clarify these taxa, but the Al Rose material is closer to C. genacinaca genacinaca in the proportions of its bacculae.

Type species

Pseudonileus willdeni Hintze, Reference Hintze1953.

Remarks

Classification of effaced asaphoids is notoriously difficult, and the species considered in the following is no exception. The type species of Protopresbynileus was described from the zone G fauna of Ibex, Utah, by Hintze (Reference Hintze1953). It was originally regarded as close to Presbynileus (Asaphidae), and to reflect this assumed relationship, Fortey (Reference Fortey1975b) included Protopresbynileus within it as a subgenus. This is now regarded as implausible. Hypostomes are very important in asaphoid classification (Fortey et al., Reference Fortey, Wernette and Hughes2022, p. 318), and the hypostomes of the type species of Presbynileus, P. ibexensis Hintze, Reference Hintze1953, and that of Protopresbynileus are fundamentally different. This is a far more important feature than general dorsal effacement, which is manifestly polyphyletic in Asaphida. The hypostome of Presbynileus is deeply forked, as it is in the subfamily Isotelinae of Asaphidae, whereas that of Protopresbynileus has wide (tr.) rounded borders, a tapering middle body interrupted by prominent smooth maculae, and a markedly shallow fork with a median tooth. It is, in fact, identical to the hypostome regarded typical of the family Nileidae and that of Nileus itself (see, e.g., Schrank, Reference Schrank1972). The hypostome associated with a new species of Protopresbynileus from the Al Rose Formation is nearly identical to that of the type species of the genus illustrated by Hintze (Reference Hintze1953, pl. 15, fig. 16). Hence, we believe that the true affinities of Protopresbynileus lie within Nileidae rather than within Asaphidae. Presybnileus is certainly a member of Asaphidae, and the three species named by Hintze (Reference Hintze1953) all share common hypostomal characters. It might be objected that P. willdeni and P. divergens n. sp. show a ventral median cephalic suture, whereas Nileidae have yoked cheeks. However, Owens et al. (Reference Owens, Fortey, Cope, Rushton and Bassett1982) noted that the early nileid Platypeltoides had a median suture, and the same feature has now been recognized in early cyclopygids, so it is a plesiomorphic character for Cyclopygoidea (sensu Fortey and Chatterton, Reference Fortey and Chatterton1988) and hence not relevant to classification at the family level.

If this is correct, a species from Spitsbergen attributed with question to Protopresbynileus, P.? glaber Fortey, 1975 (Fortey, Reference Fortey1975b), should rather be attributed to Presbynileus sensu stricto because, although its hypostome is not known, it does show good panderian openings on the thoracic doublure, which is a feature of Asaphidae and unknown in Nileidae. Note that Whittington (Reference Whittington2000) regarded Nileus as related to another effaced group, Illaenidae, rather than part of Asaphida, a view based on similarities of thoracic structure. This is a single character. Nileidae share the following with other Asaphida: a distinctive globular protaspis, a median suture that is present plesiomorphically in nileids (rather than a wide rostral plate as on illaenids), petaloid facets on the thoracic segments, transverse hypostome with wide, ear-like lateral borders, and hypostomal maculae that are smooth and frequently inclined forward. This larger number of important synapomorphies suggests that the similarity between Illaenus and Nileus is rather a matter of convergence. Nileidae should be retained in Asaphida.

Holotype

Cranidium, LACMIP 42370.24, LACMIP Type 14973 (Fig. 5.7).

Figure 5. Protopresbynileus divergens n. sp.: (1) small complete exoskeleton, LACMIP 42370.18, LACMIP Type 14967; (2) immature complete exoskeleton, LACMIP 42370.19, LACMIP Type 14968; (3) free cheek, LACMIP 42370.20, LACMIP Type 14969; (4) latex cast from hypostome, LACMIP 42370.21, LACMIP Type 14970; (5) small cranidium, LACMIP 42370.22, LACMIP Type 14971; (6) free cheek showing median suture clearly, LACMIP 42370.23, LACMIP Type 14972; (7) holotype cranidium, LACMIP 42370.24, LACMIP Type 14973; (8) pygidium, LACMIP 42370.25, LACMIP Type 14974; (9) pygidium, LACMIP 42370.26, LACMIP Type 14975; (10) thorax and pygidium, LACMIP 42370.27, LACMIP Type 14976. Scale bars = 4 mm.

Diagnosis

Protopresbynileus species with highly divergent anterior branches of the facial sutures producing wide (tr.) anterior lobe on cranidium; small eye; eight thoracic segments; pygidium with defined axis and four pleural ribs.

Description

If the exoskeletal parts are correctly associated, this is the commonest asaphoid in the Al Rose Formation. Specimens are all flattened so that the original convexity has to be inferred; the holotype shows distinct cracks in the exoskeleton, which indicates compression of a convex cranidium. However, the critical specific characters are not seriously affected by compression. Articulated material indicates exoskeletal width (tr.) about three-quarters length (sag.), with the thorax just longer than the pygidium. There is a size series of cranidia, the largest being the holotype. On all except the smallest, the maximum width is longer than its length, and on the holotype this width is equal across the anterior lobe and the posterior margin. On smaller specimens, the maximum width is slightly more across the posterior fixigenal limbs. The glabella is not defined except at its posterior end, where it is 0.5–0.6 cranidial width. A pair of basal muscle impressions on the holotype might mark the occipital furrow. A prominent median glabellar tubercle lies in front of these, just behind the transverse line connecting the posterior limits of the palpebral lobes. The palpebral lobes themselves are at one-third glabellar length (but this is on flattened material, and if the anterior glabellar lobe originally was downturned, the lobes would appear further forward) and close in, toward the glabellar region; highly curved. As is invariably the case with asaphoids, the smooth palpebral lobes decrease in size during growth, from less than one-fifth cranidial length in the largest cranidium to one-third in the late meraspis. Very narrow anterior border, perhaps better described as a rim. Facial sutures unusually divergent in front of eyes, at least 60° to sagittal line on flattened profile; this angle is lower on small cranidia; decreases anteriorly to curve around anterior margin. Posterior branch initially recurves at a right angle to sagittal line and distally curves backward to cut the posterior margin at an acute angle.

Free cheeks are assigned confidently because of the unusually acute angle between the anterior and posterior branches of the facial sutures. Exsagittal length (not including doublure) approximately the same as transverse width; genal angles rounded from a small size. Doublure wide, and still wider toward genal angle, where it curves toward midline. Apparently unequivocal median suture. Eye lobe strongly curved and apparently no eye socle. Terrace ridges on doublure dense medially, this being the most prominent sculpture on the trilobite.

Thorax has eight segments (see remarks that follow) with axis quite well defined with zetoidal furrows and narrow (sag.) half rings. Segments are similar to each other except that the anterior segment has a much wider facet than the posterior six extending almost to the axis. A degree 6 meraspis is similar except for a relatively longer pygidium, presumably indicating two unreleased segments.

Pygidium twice as wide as long, with well-defined, funnel-shaped axis up to a third pygidial width at anterior margin, but rapidly tapering initially, and then less so posteriorly to tip (not sharply defined) at three-quarters pygidial length. Up to six axial rings are defined, the first three distinctly, of only slightly decreasing length (sag.). Pleural fields with four ribs and weaker interpleural ridges stopping at paradoublural line (anterior half rib passes on to border). Doublure has concave inner margin and even width along its length except where presumably excavated around the tip of the axis.

Hypostome much wider transversely than long, with wide, ear-shaped posterolateral borders separated by a wide shallow notch. Anterior wings prominent. Triangular forward-facing maculae prominent; some evidence of transverse terrace ridges on middle body.

Etymology

Referring to the highly divergent anterior branches of the facial sutures.

Remarks

This species is easily distinguished from the type species, P. willdeni, by its strongly divergent anterior branches of the facial sutures and by the inner margin of the pygidial doublure, which is concave forward rather than convex forward. The eyes are smaller, and the better-developed pygidial furrows may also be a distinguishing feature (but the type material of P. willdeni is not well preserved, and flattening may exaggerate the furrows). Another species requiring consideration is Presbynileus (P.) latifrons Dean, Reference Dean1989 from the Outram Formation, Alberta, which is the same age as P. divergens. Dean (Reference Dean1989) did not firmly assign a hypostome to this species, so whether it might be better assigned to Protopresbynileus is uncertain. However, this species does have strongly divergent anterior branches of the facial sutures like P. divergens and similarly short postocular fixed cheeks (tr.). Interestingly, the vincular furrow on the doublure of the free cheek (Dean, Reference Dean1989, pl. 37, fig. 11) is much like that of the nileid Poronileus (Fortey, Reference Fortey1975b, pl. 41, fig. 6), and the comparatively well-defined glabella apparently expands forward in front of the eyes like that in other primitive cyclopygoids. It seems possible that Presbynileus latifrons might prove to be another Protopresbynileus species; its strongly ridged surface sculpture and wide pygidial and cephalic doublure clearly distinguish it from P. divergens. Dean (Reference Dean1989, pl. 37) illustrated a selection of unassigned asaphoid hypostomes, none of which is typically nileid.

As noted, the family assignment of P. divergens is particularly difficult. One additional feature requires mention—it has eight thoracic segments, like Asaphidae. Nileids typically have seven. However, the sister group of Nileidae and Cyclopyidae is Taihungshaniidae (following Fortey and Chatterton, Reference Fortey and Chatterton1988), with eight segments in the thorax, and hence this is likely to be the primitive number for the group as a whole. If this is the case, the presence of eight segments is a plesiomorphic character and hence not relevant to the classification of the genus. A degree 6 meraspis is very similar to a meraspis of the same degree of the nileid Symphysurus arcticus Fortey, 1975 (Fortey, Reference Fortey1975b, pl. 21, fig. 16). The peculiar combination of characters shown by P. divergens provides justification for naming the species formally, while recognizing that the material from the Al Rose Formation has certainly been flattened during compaction, and full-relief material would be desirable.

Type species

Lachnostoma latucelsum Ross, Reference Ross1951 from the late Ibexian of the Garden City Formation.

Remarks

Fragmentary remains of this species do not permit formal taxonomic recognition. They can be compared with the commonest trilobite fossils of the Pseudocybele nasuta (J) Biozone of Utah and Nevada, Lachnostoma latucelsum Ross, Reference Ross1951 (also see Hintze, Reference Hintze1953; Fortey and Droser, Reference Fortey and Droser1999). The same species was described from western Canada by Dean (Reference Dean1989), so its morphology is well known. The same comments apply to the flattening of this species as were previously noted for Protopresbynileus divergens. A small and imperfect cephalic shield shows the presence of genal spines and proves that its transverse width was at least twice its length. The cranidium itself is comparatively long and narrow, with the greatest width at the posterior margin along the strap-like postocular cheeks, this somewhat exceeding the sagittal length. The glabella is not defined as material is preserved, except by axial pits at its posterior end. Palpebral lobes probably about 25% cephalic length; this is, longer than in mature L. latucelsum, where they are proportionately about half as long. However, the principal difference between Lachnostoma cf. L. disputa Fortey, 1975 (Fortey, Reference Fortey1975b) and L. latucelsum is in the much lower divergence of the anterior branches of the facial sutures of the Al Rose material, which means that the width across the anterior part of the cranidium is less; in addition, the cranidial border is not clearly defined in the latter. The likely associated larger pygidium has a long, narrow axis like the type species and unfurrowed pleural fields, but the material is too incomplete to say much about the border. A smaller pygidium is much like those attributed to L. latucelsum by Ross (Reference Ross1951).

Note that the Al Rose species is clearly distinct from L. latucelsum, a species we have found to be a ubiquitous trilobite across the Great Basin in shallower facies and whose geographic range extended the length of the western Laurentian platform (Dean, Reference Dean1989). A somewhat similar fragmentary cranidium from the basal bed of the Whiterockian at Meiklejohn Peak, Nevada, was figured by Fortey and Droser (Reference Fortey and Owens1999, figs. 7–16). At the eastern margin of the Laurentian paleocontinent, two species of Lachnostoma are known from the top of the Kirtonryggen Formation and base of the overlying Valhallfonna Formation in Spitsbergen (Floian) (Fortey and Bruton, Reference Fortey and Bruton2013). The stratigraphically earlier L. platypyga Fortey and Bruton, Reference Fortey and Bruton2013 has a broader cranidial border than the species from California and a relatively wide pygidium. L. disputa Fortey, Reference Fortey1975 was originally considered a ptychopygine (Asaphidae) by Fortey (Reference Fortey1975b) but reassigned by Fortey and Bruton (Reference Fortey and Bruton2013); it offers the closest match to the Al Rose species, sharing with it a thin cranidial anterior border, narrow glabella, and weakly divergent anterior branches of the facial sutures. However, the large free cheeks of L. disputa figured by Fortey (Reference Fortey1975b) lack genal spines—although they do frequently become reduced in later ontogeny in Asaphidae. The pygidium attributed to L. disputa is very like that of L. platypyga and probably wider (tr.) than that of either L. latucelsum or the Al Rose species, although the preservation on the latter is unfortunately incomplete. L. disputa is slightly older than the Al Rose fauna (i.e., Bendigonian). In view of the several uncertainties, a comparative determination is given to our material.

Type species

Cloacaspis senilis Fortey, Reference Fortey1974, Lower Ordovician, Valhallfonna Formation, Spitsbergen.

Cloacaspis cf. C. ceryx anataphra Fortey, Reference Fortey1974
Figure 7

1967

aff. Parabolinella sp.; Ross, pl. 11.

Figure 7. Cloacaspis cf. C. ceryx anataphra Fortey, Reference Fortey1974 (1) cranidium, LACMIP 42370.32, LACMIP Type 14981; (2) fused free cheeks showing narrow doublure, LACMIP 42370.33, LACMIP Type 14982; (3) cranidium, LACMIP 42370.34, LACMIP Type 14983; (4) latex cast from small cranidium, LACMIP 42370.35, LACMIP Type 14984. Scale bars = 4 mm.

Remarks

This species is of interest as the only olenid trilobite in the Al Rose fauna. Indeed, apart from the “Olenid bed” at Little Rawhide Mountain, Nevada (Fortey and Droser, Reference Fortey and Droser1999), and specimens from Ike's Canyon, Nevada (Hopkins, Reference Hopkins2019), it is the only olenid from the Floian–Whiterockian of the Great Basin. Balnibarbiines differ from olenines in their large size, square to forwardly expanding glabellas, highly divergent anterior branches of the facial sutures, and pleural nodes adjacent to the axis on the thorax (where known). Many species have wide preglabellar fields, and all have large eyes close to the glabella. The cuticle of these olenids is very thin and readily distorted, which applies to most of our Al Rose material, and this places limits on the formal taxonomy. However, the cranidium of the Californian species is very like that of Cloacaspis ceryx anataphra Fortey, Reference Fortey1974 from the lower part of the Olenidsletta Member of the Valhallfonna Formation in northern Spitsbergen. This species was originally included in Balnibarbi by Fortey (Reference Fortey1974) on the basis of the anterior divergence of the facial sutures; however, a phylogenetic analysis of the Balnibarbiinae by Hopkins (Reference Hopkins2019) indicated that more characters supported attribution to Cloacaspis, which is followed here. The extremely long palpebral lobes are noted, which constrict narrow, strap-like postocular fixed cheeks; this species has the longest eye of any olenid. The anterior branches of the facial sutures, although short, are more divergent than they are in Bienvillia species with long eyes, but they are like Cloacaspis ceryx anataphra in this regard. That the species attained a large size with a square glabella typical of balnibarbiines is proved by a fragment of a very large cranidium, LACMIP 42370.59, LACMIP Type 15008. One significant difference is noted between the Spitsbergen species and that from the Al Rose Formation: a pair of fused free cheeks of the latter prove that the genal spines were not far advanced from the level of the posterior border whereas on the type material of C. ceryx anataphra, the origin of the genal spines was far enough forward to be opposite the eyes. This difference is probably of taxonomic significance, but the material available from California is not sufficient to recognize this formally.

Type species

Protopliomerops aemula Hintze, Reference Hintze1953, Pseudocybele nasuta Biozone, Wahwah limestone Formation, Utah.

Remarks

This is of interest as the only pliomerid in the Al Rose Formation, whereas pliomerids are ubiquitous in correlative faunas further to the east. It is evidently not the narrow kind of pygidium belonging to the zone fossil Pseudocybele in those sections. It may be compared with the pygidium of the species originally described by Hintze (Reference Hintze1953) as Pseudomera aff. P. insolita (Poulsen, Reference Poulsen1927) from the nasuta Zone and subsequently transferred to Hintzeia by Demeter (Reference Demeter1973). A number of species of Hintzeia from the Great Basin have now been described from superbly silicified material by McAdams and Adrain (Reference McAdams and Adrain2011). However, it is clear that pliomerid pygidia are often conservative, and it is not possible to confidently assign the relatively poorly preserved Al Rose material to any one of these species; hence, we are obliged to use open nomenclature here.