Newfoundlandops, Adrain & Pérez-Peris, 2021

Adrain, Jonathan M. & Pérez-Peris, Francesc, 2021, Middle Ordovician (Darriwilian) cheirurid trilobites from the Table Cove Formation, western Newfoundland, Canada, Zootaxa 5041 (1), pp. 1-73 : 30-31

Newfoundlandops n. gen.

Type species. Newfoundlandops karimae n. sp., from the Table Cove Formation (Darriwilian), western Newfoundland, Canada (Laurentia) .

Other species. Monotypic.

Etymology. From Newfoundland and the Greek noun ops, eye. Gender is masculine.

Diagnosis. Glabella inflated and subspherical with sculpture of dense granules; fixigena with small number of large, low tubercles in smaller specimens; all specimens with genal spine, length reduced through ontogeny but still a robust, thorn-like spine in largest specimen; S1 does not fully isolate L1; librigena with broad lateral border, very shallow or obsolete lateral border furrow, and dense granular sculpture on all surfaces, including position of border furrow; pygidium of three segments; pygidial axis with three axial rings but no terminal piece; each pygidial axial ring with transverse row of small tubercles; adaxial pleural region of all three pygidial spines with independently swollen anterior and posterior pleural bands separated by oblique, posterolaterally running, pleural furrow; distal parts of spines nearly flat, with gentle dorsal concavity; distal spines with central row of small tubercles, curved to match posterior curvature of spine, more prominent in smaller specimens; spine tips positioned at posterior margin of spines, acutely angled but blunt; apart from tubercle rows, dorsal pygidial sculpture of dense granules.

Discussion. It would be best to compare the morphology of Newfoundlandops karimae to that of the oldest known, contemporaneous, species of Sphaerexochus . This not possible in detail, as those Darriwilian species are based on internal molds of relatively poorly preserved specimens, illustrated with tiny photographs. Instead, the most useful comparisons are with slightly younger, Sandbian, species known from silicified material. In particular, we will focus on S. pulcher Whittington and Evitt, 1954 , from the Edinburg Formation, Virginia, USA, S. hapsidotus Whittington and Evit, 1954 , from the Lincolnshire Formation, Virginia, USA, and S. arenosus Chatterton and Ludvigsen, 1976 , from the Esbataottine Formation, Northwest Territories, Canada.

In cranidial morphology, N. karimae seems very close to these three species. All of the species share inflated, nearly spherical glabellae, small fixigenae, and genal spines of the same relative lengths. They share nearly identical dimensions in dorsal view. They share a pervasive sculpture of dense granulation on all dorsal surfaces. There are only two substantive differences. Species of Sphaerexochus have S 1 in posterior contact (if sometimes weakly) with the occipital furrow to completely isolate L1. In N. karimae , a faint furrow seems present posteriorly in small specimens (Pl. 11, fig. 16) and certainly L1 is independently inflated from the median glabellar lobe, but in larger specimens (Pl. 11, figs 1, 4) S1 clearly terminates about a third of the distance of L1 posteriorly. Secondly, the fixigena of N. karimae retains a small number of low tubercles, and there are tiny tubercles also on the posterior border and genal spine (Pl. 11, figs 11, 12, 16, 25). However, the fixigenal tubercles are actually themselves covered in granules. They are more evident in smaller specimens. Apart from these two features, the cranidium of N. karimae is more or less identical with those of the Sphaerexochus species.

The librigena share with those of Sphaerexochus pervasive granular sculpture on the lateral border and field, similar to that covering the cranidium in both genera. The presence of dense granular sculpture on the librigenal field seems to be uniquely shared within all of Cheiruridae . The eye of N. karimae is very small, with prominent lenses (Pl. 11, fig. 17). It is nearly identical in size, lens number, and prominence with that of S. arenosus ( Chatterton and Ludvigsen, 1976, pl. 13, fig. 46). All of the Sphaerexochus species have librigenae with a broad, deep lateral border furrow about the same width as the remaining field, bounded posteriorly and anteriorly by sutural ridges, and lacking sculpture. While the librigena of N. karimae lack such a well defined furrow, a depression of similar unusual width is developed in the same position (see especially Pl. 11, fig. 19), across which the granular sculpture runs uninterrupted from the lateral border onto the field. Librigenae of the Sphaerexochus species differ from that of N. karimae in two prominent ways. Some of the derived morphology of Sphaerexochus seems to be associated with enrollment. All species have a deep coaptative furrow on the underside of the posterior part of lateral border such that when seen in internal view ( Chatterton and Ludvigsen, 1976, pl. 13, figs 35, 36), the region has a prominent flange jutting backward above the doublure, on the far side of the coaptative furrow. This furrow can be seen in all species of Sphaerexochus for which information is available and functioned to receive the pleurae of the first thoracic segment during enrollment (e.g., Ramsköld, 1983, pl. 28, fig. 12a). There is no similar furrow in N. karimae , in which the ventrolateral part of the lateral border remains normally inflated, with no break in morphology from more anterior regions (Pl. 11, fig. 20). Second, the librigenal fields in S. arenosus ( Chatterton and Ludvigsen, 1976, pl. 13, fig. 46) and S. hapsidotus ( Whittington and Evitt, 1954, pl. 32, figs 29–31) have prominent caecal pits (that of S. pulcher may have, too, but the photographs are too tiny to tell). These are absent from the librigena of N. karimae .

The hypostome of N. karimae (Pl. 12, fig. 3) is very like those of species of Sphaerexochus . Compared with that of S. arenosus ( Chatterton and Ludvigsen, 1976, pl. 13, figs 32, 42), there are no substantial differences. The general dimensions are identical, the lateral borders are of the same width, the middle body has the middle furrow in exactly the same position, and the posterior border is bilobate, with a median indentation. The entire ventral surface in both species is covered in dense granular sculpture. The only minor differences are that the hypostome of N. karimae is slightly longer relative to its width, and the posterior border lobes are somewhat larger.

While the cephala of these taxa share many features, differences in the pygidia are admittedly profound. The cephala are so similar and the pygidia so dissimilar, in fact, that the question of sclerite association in the Table Cove Formation species should be addressed before comparing pygidial morphology. In brief, the cephalic sclerites and pygidia are both reasonably common. There is literally no other cheirurid pygidial type that the cephalic sclerites could possibly be associated with, and no other cephalic material that the pygidia could possibly be associated with. Other cheirurids occurring at horizon TCM 18 ( Sphaerocoryphe becki , Laneites polydorus , etc.) have pygidia and other sclerites with well known and expected general morphology. Hence, the association of cephalic material on Plate 11 with the pygidia on Plate 12 is correct beyond any reasonable doubt.

All species of Sphaerexochus share a pygidium which at least in large adult specimens is highly tagmatized, with robust, club-like spines and often with the third ring and terminal piece fully merged. This morphology is unique to the genus, and is the main reason its affinities are difficult to determine. There are some apparent points of similarity with N. karimae , but not many. All of the taxa have three pygidial segments. The ventral doublure of N. karimae species forms a ventral rim, and has a subquadrate median embayment (Pl. 12, figs11, 14). This is a close match for the condition in Sphaerexochus ( Whittington and Evitt, 1954, pl. 20, fig. 14; Chatterton and Ludvigsen, 1976, pl. 13, figs 23, 28). The differences are many. Early species of Sphaerexochus do sometimes retain a few tubercles on their axial rings (e.g., Whittington and Evitt, 1954, pl. 21, fig. 1), but none have transverse tubercle rows like N. karimae . All species of Sphaerexochus have a terminal piece, although it is often partially or wholly fused with the third axial ring. The pygidium of Newfoundlandops karimae definitively lacks a terminal piece. Most obviously, the pygidial spines of N. karimae are dorsoventrally flattened, versus inflated, and the distal parts of the pleurae have distinct anterior and posterior pleural bands separated by an obliquely set pleural furrow, similar to that seen in cheirurines. There is no sign of pleural bands or furrows on Sphaerexochus segments. Some Ordovician species do have a shallow furrow more or less bisecting the robust pleural spines (e.g., Chatterton and Ludvigsen, 1976, pl. 13, fig. 22), but this develops late in ontogeny and does not seem to be a true pleural furrow. Early ontogenetic pygidia (e.g., Whittington and Evitt, 1954, pl. 17, figs 39, 43, 49, 50) are subcylindrical in their proximal pleural regions, with no hint of bands or furrows.

In sum, we are equivocal about the possibility of relationship, but consider on the basis of what is known that the positively shared cephalic features outweigh the pygidial differences. Sphaerexochus has to have a cheirurid sister taxon, and whatever it is, that sister taxon lacks the derived thoracopygidial morphology. Newfoundlandops is apparently the best candidate known, and so we tentatively assign it to Sphaerexochinae . More information and discoveries will likely be required to evaluate the hypothesis.

Figures

PLATE 11

PLATE 12