Lochmanolenellus Lieberman, 1998

Webster, Mark & Bohach, Lisa L., 2014, Systematic revision of the trilobite genera Laudonia and Lochmanolenellus (Olenelloidea) from the lower Dyeran (Cambrian Series 2) of western Laurentia, Zootaxa 3824 (1), pp. 1-66 : 25-27

Genus Lochmanolenellus Lieberman, 1998

Type species. Wanneria mexicana prima Lochman in Cooper et al., 1952, by original designation.

Other species. Lochmanolenellus pentagonalis n. sp., Lo. subquadratus n. sp., and Lo. trapezoidalis n. sp. Also included are four poorly known morphotypes that might represent new species but are left in open nomenclature, all described herein: Lo. cf. primus 1, Lo. cf. primus 2, Lo. cf. subquadratus 1, and Lo. cf. subquadratus 2.

Diagnosis. Cephalon pentagonal, subquadrate, or trapezoidal in outline, anterior margin broadly curved; genal spine bases located opposite or anterior to lateral margins of L2; distal portions of posterior cephalic margin strongly divergent, parallel, or strongly convergent when traced toward genal spine bases. Broad-based intergenal spines retained at morphological maturity, length approximately one-quarter to one-half of sagittal cephalic length; base located slightly distal to adgenal angle; outer margin very weakly to very strongly laterally flared relative to orientation of cephalic margin between intergenal and genal spines. Anterior cephalic border broader well rounded dorsally anterior to LA, somewhat more broadly dorsally arched at and between bases of genal and intergenal spines (entire border of much lower dorsal convexity on compacted specimens). Ovoid lateral swellings on L2, and sometimes also on LO, L1, and L3. Prominent extraocular platform. Prominent intergenal ridge retained at morphological maturity; interrupts cephalic border furrow as it merges with border at base of intergenal spine. Network of raised ridges form walls of fine, polygonal, mesh-like prosopon on dorsal surface of much of cephalon; central bosses within some polygons on extraocular area and adjacent areas developed into large, pustule-like structures. Thorax (known from one specimen) of at least 20 segments; not clearly differentiated into prothorax and opisthothorax; T3 weakly macropleural, macrospinous; broad-based axial spine on T15 (length unknown).

Occurrence. MEXICO: Sonora State: Unit 4 of Puerto Blanco Formation. U.S.A.: Montezuma Range, Esmeralda County , Nevada: middle member of Poleta Formation. Slate Ridge, Esmeralda County, Nevada: middle member of Poleta Formation. Magruder Mountain, Esmeralda County, Nevada: middle member of Poleta Formation. CANADA: Mount Robson area, British Columbia and Alberta: middle member of Mural Formation. Cariboo Mountains, British Columbia: middle member of Mural Formation. Cassiar Mountains, British Columbia: Unit 4 of Rosella Formation. All stratigraphically constrained occurrences are in the lower portion of the Dyeran Stage, Waucoban Series.

Discussion. The three new species described herein considerably increase known disparity within the genus, particularly in terms of characters relating to cephalic outline, the width of the extraocular area, the orientation and length of genal/intergenal spines, and ocular lobe length. The generic diagnosis (previous) emends that provided by Lieberman (1998) in light of the new data. Several character states listed in the original generic diagnosis are true for mature cephala of virtually all olenelloid trilobites (e.g., the lateral margins of L3 being convex outwards; the anterodistal margins of L3 being formed by the ocular lobes), and are therefore now omitted. Novel additions to the generic diagnosis include mention of the lateral swellings on the glabella, the extraocular platform, and the cephalic prosopon. Details of the thorax are also included for the first time. However, the thorax is known from only a single specimen, and it is possible that future discoveries will reveal disparity among congeneric species in the thoracic traits listed here and/or a different basal condition for the clade-defining node. The diagnosis herein lists a range of conditions for traits relating to cephalic outline because the basal condition for the clade is ambiguous. The inflation of the central bosses within some polygons of the cephalic prosopon into pustule-like structures represents arguably the strongest putative synapomorphy of the genus (but see the caveat following).

The closest relatives to Lochmanolenellus , and therefore the precise phylogenetic placement of the genus within the Olenelloidea, are herein deemed uncertain. Lochmanolenellus primus was included in cladistic analyses of olenelloid trilobites ( Lieberman, 1998, 1999). Perhaps surprisingly, those analyses found that Lo. primus shared more recent common ancestry with biceratopsids such as Bristolia , Fremontella , Nephrolenellus , Peachella , and Biceratops than with Laudonia, from which it had been taxonomically split ( Lieberman, 1998, 1999). Placement of the lower Dyeran Lochmanolenellus within the Biceratopsidae rather than the “ Laudoniidae ” also implied the existence of long ghost lineages within the Biceratopsidae , because all other unambiguous biceratopsids occur in (middle? to) upper Dyeran strata (Palmer & Repina in Whittington et al., 1997, fig. 254; Lieberman, 1999, fig. 23). However, the unexpected phylogenetic placement and resulting stratigraphic incongruence might be an artifact of the limited data available to Lieberman (1998, 1999): thoracic data were unknown for Lochmanolenellus , and ontogenetic characters were not considered in the analyses. The undescribed material previously identified as “ Laudonia ” (described herein) now provides new data—including thoracic and ontogenetic data for Lochmanolenellus —that suggest that Lochmanolenellus should be excluded from the Biceratopsidae and grouped instead with taxa such as Laudonia and “wanneriids” (e.g., Wanneria and Elliptocephala ) that are also known from lower Dyeran strata (Table 1). Determination of whether these similarities are symplesiomorphic or serve as synapomorphies of Lochmanolenellus and one or more of these taxa is beyond the scope of the present paper—a meaningful cladistic analysis must minimally also include representative holmiids, olenellids, Fritzolenellus , Mummaspis , and as-yet-undescribed “wannerid” species from the early Dyeran (Webster, in preparation).

Lochmanolenellus shares many traits in common with Laudonia, the most obvious being similarity in genal spine placement and cephalic outline (but see previous), and the retention of long, prominent intergenal spines at morphological maturity (see also Table 1). Based on comparison of the two Laudonia species with Lochmanolenellus primus, Lieberman (1999, pp. 117–118) provided a long list of differences between the genera. Unfortunately, a few of the supposed differences were erroneous, being based on mis-coded traits (e.g., the supposed presence of a plectrum in Lochmanolenellus ; and the supposed smooth merging of the ocular lobe into the extraocular area in Lochmanolenellus ). Other supposed generic differences listed by Lieberman (1999) relate to traits that are highly prone to compaction-related taphonomic overprint (e.g., the relative depth of glabellar furrows over the axis; the relative transverse convexity of the glabella; the relative dorsal convexity of the anterior cephalic border; and the degree of dorsal vaulting of the extraocular area). The pre-compactional state of these traits is conservatively better treated as unknown in Laudonia, all specimens of which are preserved in a compacted state. Other generic differences listed by Lieberman (1999) are no longer tenable given the increased disparity within both genera documented herein, resulting from examination of larger sample sizes of the Laudonia species and the description of three new species of Lochmanolenellus (e.g., the supposed differences in proportional length of LA; relative width of the anterior cephalic border; orientation of the preocular furrow; the relative width and degree of dorsal arching of the interocular area; the degree of anterior tapering of the glabella from its posterior margin to S1; the proportional width of the extraocular area; the orientation of the genal spine; and in the orientation of the proximal portion of the posterior cephalic margin). None of the previously listed traits consistently or unambiguously diagnose a Laudonia clade from a Lochmanolenellus clade. In fact, the present study reveals that Laudonia and Lochmanolenellus are best distinguished from each other by a set of four (or perhaps five) traits, discussed following. As will be shown, even these traits are not entirely free from ambiguity, and taphonomy in particular must be carefully considered.

Firstly, both species of Laudonia bear a long tropidium extending from adjacent to the lateral margins of LA to the cephalic border between the genal and intergenal spines. A tropidium appears not to be developed in any species of Lochmanolenellus (although a single specimen of Lochmanolenellus trapezoidalis bears a much shorter ridge-like structure in an analogous position [Fig. 21.4], and it is therefore conceivable that Lo. trapezoidalis exhibits intraspecific polymorphism for presence or absence of a short tropidium).

Secondly, all species of Lochmanolenellus bear ovoid lateral swellings on L2, and sometimes also on LO, L1, and L3. Such prominent swellings appear to be absent from both species of Laudonia (see also Lieberman, 1998 character 42). The lateral swellings are fainter but still evident on compacted material of Lochmanolenellus pentagonalis relative to non-compacted material of the same species (compare Fig. 11 to Fig. 12 ), so the absence of prominent lateral swellings on Laudonia appears to be a biological reality rather than a taphonomic artifact. Nevertheless, pending discovery of non-compacted material of Laudonia, support for using this trait to distinguish the genera must remain tentative.

Thirdly, all species of Lochmanolenellus bear a prominent extraocular platform upon which the ocular lobes are located, and such a platform seems to be absent from both species of Laudonia (with the possible exception of one specimen; see following). The presence of an extraocular platform is evident even on compacted material of Lochmanolenellus pentagonalis ( Fig. 12 ), so the apparent absence of the structure from Laudonia is likely to be genuine. However, one specimen of Laudonia amputata (USNM 443753; Fritz, 1992, pl. 11, fig. 2) shows a fold in the extraocular area that might be consistent with compaction-related deformation of an extraocular platform, and it is therefore conceivable that La. amputata exhibits intraspecific polymorphism for presence or absence of an extraocular platform.

Fourthly, the genera also seem to differ in one aspect of the polygonal, mesh-like network of raised ridges that dominates the cephalic prosopon. In Lochmanolenellus pentagonalis , Lo. trapezoidalis , and Lo. subquadratus , the central bosses within some polygons on the extraocular area and adjacent areas are developed into large, pustulelike structures (Figs 3.1, 11, 16, 20, 21). In the two Laudonia species, the polygons are clearly visible but the central bosses are never enlarged in such a way. Unfortunately, the presence or absence of the large, pustule-like central bosses cannot be determined in Lo. primus because the only unambiguously assigned specimen is completely exfoliated and all traces of original prosopon have been lost.

The presence or absence of an axial spine on T15 might serve as another trait that differentiates the two genera. Such an axial spine is present on the thorax of Lochmanolenellus pentagonalis , but appears to be absent from T15 of Laudonia bispinata (see also Lieberman, 1998, character 73). However, caution is warranted because, as noted previously, the axis of the thorax of La. bispinata is not well preserved—it is therefore conceivable that the apparent absence of the axial spine is a taphonomic artifact. Furthermore, T15 is currently only known from one species within each of the two genera, and it is risky to assume that a trait observed in one species is diagnostic of the basal node for the genus.

There is an interesting parallel in terms of mature cephalic morphology between Lochmanolenellus and Bristolia : the increasingly exaggerated cephalic outlines of Lochmanolenellus pentagonalis , Lo. primus , Lo. subquadratus , and Lo. trapezoidalis form a phenetic gradient that closely matches that seen between Bristolia mohavensis (Crickmay in Hazzard, 1933), Br. harringtoni Lieberman, 1999 , Br. bristolensis ( Resser, 1928) , and Br. insolens (see Webster, 2011 for illustrations of these species). As argued previously, Lochmanolenellus is probably not very closely related to Bristolia (contra Lieberman, 1998, 1999). A comparative phylogenetic study of the ontogeny and variation within species of these two genera could offer useful insight into whether and how ontogenetic trajectories of shape change facilitate or constrain such convergence. Data presented herein offer a first step towards such a study.

Figures

FIGURE 11. Morphologically mature specimens of Lochmanolenellus pentagonalis n. sp. preserved in limestone. 1, 2, Holotype cephalon; LACMIP 12550 a / b (part and latex peel of counterpart, respectively), x 4. 3, Cephalon; LACMIP 12549.1, x 6. 4, Cephalon, latex peel of counterpart; LACMIP 12548.5, x 5. 5, Cephalon; GSC 137492, x 7. 6, Cephalon with deformed right genal region, latex peel of counterpart; LACMIP 12548.1, x 5. 7, Cephalon; GSC 137493, x 10. 8, Cephalon; LACMIP 12551.2, x 7. 9, Hypostome, ventral view; LACMIP 12551.1, x 6. All in dorsal view unless specified. 1 – 4, 6, 8, 9 from middle member of Poleta Formation at LACMIP locality 17052, Slate Ridge, Esmeralda County, Nevada; 5 and 7 from Unit 4 of Rosella Formation, GSC locality 95190, Cassiar Mountains, northern British Columbia.

FIGURE 12. Morphologically mature specimens of Lochmanolenellus pentagonalis n. sp. preserved in a compacted state in shale and siltstone. 1, Cephalon, latex peel of external mold; FMNH PE 58483, x 3. 2, Cephalon, latex peel of external mold; FMNH PE 58484, x 6. 3, Fragmentary cephalon representing largest known specimen; FMNH PE 58485, x 2. 4, 5, Cephalon and thorax, with enlargement of posterior portion of thorax; MCZ 110679, x 4 and x 8, respectively. All in dorsal view. 1 – 3 from collection ICS- 10241, middle member of Poleta Formation, IS-S section, Montezuma Range, Esmeralda County, Nevada; 4, 5 from middle member of Mural Formation at Mumm Peak, Mount Robson area, Alberta.