Diacanthaspis (s.l.) maquoketensis ( Walter, 1924 )

Adrain, Jonathan M. & Pérez-Peris, Francesc, 2023, Funeralaspis n. gen.: a new odontopleurine trilobite from the early Middle Ordovician (Dapingian) of Death Valley, eastern California, USA, and the classification of Ordovician odontopleurines, Zootaxa 5336 (4), pp. 509-529 : 523-526

Diacanthaspis (s.l.) maquoketensis ( Walter, 1924)

Figure 7

1924 Ceratocephala maquoketensis Walter , p. 229, pl. 18, figs 2, 3.

Material. Unique holotype, dorsal exoskeleton lacking pygidium, SUI 9233 ( Fig. 7 ), Maquoketa Formation ( lower Katian ), Florenceville, Howard County, Iowa, USA .

Diagnosis. Dorsal exoskeletal sculpture of moderately dense medium sized tubercles, lacking any dense background of finer tubercles; fixigenae relatively wide; LO forming a broad tuberculate shelf; SO very shallow; pair of slender occipital spines, length unknown, bases widely spaced and connected by transversely straight posterior margin; librigena with evenly tuberculate field, prominent tubercles aligned along an inflated lateral border, with probably lateral border spines, posterior three developed on base of genal spine; genal spine with several thorn-like tubercles at base of dorsal aspect; posterior pleural bands of thoracic segments with transverse rows of elevated, thorn-like tubercles or very short spines.

Description. Cranidium subtrapezoidal in outline, maximum width (tr.) across posterior border, strongly narrowing (tr.) anteriorly toward the posterior tip of the palpebral lobe, gently narrowing from forward of the palpebral lobe to the anterior border; anterior border short (sag., exsag.), slightly longer exsagittally, gently bowed anteriorly, sculpted with medium sized tubercles; anterior border furrow well defined, bowed forward, short sagittally, slightly longer exsagittally; glabella subrectangular in shape, with maximum width (tr.) across mid-part of L1, slightly narrowing anteriorly, anterior margin slightly bowed forward, medially almost transverse, anterolateral corners rounded, posterior margin transverse medially, laterally gently convex posteriorly, posterolateral corners rounded and slightly pointed, surface densely sculpted by small granules and larger tubercles, tubercles more densely arranged in the anterior half of the median glabellar lobe and in the lateral glabellar lobes; S1 relatively long (exsag.), wide (tr.) reaching one quarter of the total width of the glabella, distally deeply incised, directed posteromedially, proximally curving backwards with the most distal part slightly outwardly directed, effacing becoming a faint depression; S2 shorter (exsag.) than S1 and with similar width (tr.), subparallel to S1, running posteromedially from the axial furrow, bending backwards proximally, deeply incised distally, dramatically effaced proximally; SO deep laterally across L1, medially as a shallow depression that separate the median glabellar lobe and LO, shorter (exsag.) distally and longer (sag.) proximally, slightly expanded (exsag.) across the most proximal part of L1; LO relatively long (sag.), sagittally carrying a median occipital node ( Fig. 7.1 ), sculpted by small granules and medium size tubercles, posterior margin transverse, carrying two pairs of occipital spines (broken on specimen) directed posterolaterally, distally gently sloping downwards from horizontal ( Fig. 7.4 ); L1 suboval in shape, long (exsag.), narrower (tr.) anteriorly, slightly widening (tr.) posteriorly, anterior margin anteriorly and gently outwardly bowed, posterior margin gently bowed posteriorly, posterior proximal part weakly isolated from the medial glabellar lobe by a faint S1; L2 shorter (exsag.) and narrower (tr.) than L1, suboval in shape, proximally anterior margin greatly bowed anteriorly and gently outwardly, posterior margin bowed posteriorly and slightly medially; L3 obscured, expressed as an independent inflation in the lateral aspect of the anterior glabellar lobe ( Fig. 7.2 ), almost indistinguishable; axial furrow wide (tr.) across the glabella, deeply incised across L1, shallowing anteriorly and almost effaced across L2, posteriorly to the glabella bending obliquely posterolaterally, very shallow, almost indiscernible, expressed as a faint depression; interocular fixigenal field sub-triangular in shape, long (exsag.), narrow (tr.) anteriorly, slightly widening (tr.) posteriorly, with maximum width (tr.) across the posterior tip of the palpebral lobe, densely sculpted by tubercles of similar size to the glabellar tubercles; posterior part of the fixigenal field short (exsag.) and considerably wide (tr.), anterior and posterior margin running outwards and slightly backwards, most distal part strongly bending backwards, sculpted with small granules and sparsely arranged small tubercles; posterior border furrow deeply incised, relatively long (exsag.), most proximal part transverse, distally gently bending backwards and directed obliquely posterolaterally; posterior border narrow (exsag.) adaxially, lengthening gradually abaxially, most proximal part transverse, narrow (tr.), distal part wider (tr.), running obliquely laterally slightly posteriorly, sparsely sculpted by medium size tubercles and tiny granules; palpebral lobe set opposite to L1, anteriorly continuous with the eye ridge; eye ridge long (exsag.), narrow (tr.), with maximum width (tr.) posteriorly, gently narrowing (tr.) anteriorly, forwardly directed, bending slightly adaxially, sparsely sculpted by medium tubercles and small granules; palpebral furrow shallow, describing an arc directed inwards, anteriorly continuous with the eye ridge furrow; eye ridge furrow deeply incised, slightly shallower posteriorly, anteriorly effaced at S2; anterior branch of facial suture runs posteriorly from the anterior margin of the cranidium, slightly outwardly directed, opposite the posterior half of L1 describing an outwardly directed arc, ending at the base of the palpebral lobe opposite to the most posterior part of L1; posterior branch of the facial suture laterally directed, deflected slightly posteriorly, strongly bending posteriorly at the most distal part ( Figs 7.1, 7.2 ).

Librigenal field subtriangular in outline, bordered by anterior and posterior sutural ridges ( Fig. 7.2 ), narrow (tr.) anteriorly, widening (tr.) posteriorly, slightly vaulted dorsally at the eye lobe ( Figs 7.3, 7.4 ), distally gently sloping downwards from horizontal, sculpted by medium tubercles, more densely arranged proximally at the base of the eye lobe; anterior sutural ridge very narrow (tr.); posterior sutural ridge short (exsag.), lengthening (exsag.) gently distally, smoothly merging with the lateral border at the base of the genal spine; lateral border furrow greatly outwards and forwards bowed, relatively shallow, narrow (tr.), posteriorly slightly wider (tr.), with maximum width (tr.) at the intersection with the posterior border furrow; posterior border furrow really shallow and short (exsag.), slightly deeper and longer (exsag.) distally, maximum length at the intersection with the lateral border furrow, running outwards and slightly backwards; lateral border relatively wide (tr.), outwardly and forwardly arched, sculpted by medium size tubercles, posteriorly extended into a long (exsag.) genal spine; lateral border carrying 14 (?) border spines ( Fig. 7.1 ), subtriangular in shape, distal tip pointed, anterior spines small, anteriorly and slightly outwardly directed, increasing in size posteriorly, posterior spines relatively big, outwardly and slightly anteriorly directed.

Rostral plate and hypostome unknown.

Most posterior part of the thorax not preserved. Nine segments present, 8 axial rings and 9 thoracic pleurae preserved ( Fig. 7.1 ); axial ring subrectangular in shape, relatively short (sag.) and wide (tr.), dorsally vaulted ( Fig. 7.3 ), surface bearing coarse tubercles almost transversely aligned; thoracic pleurae divided in anterior and posterior bands by a pleural furrow; pleural furrow short (exsag.), gently lengthening (exsag.) distally, running almost transversely adaxially, gently bending backwards abaxially; anterior pleural band short (exsag.), anterior margin almost transverse, gently bending backwards laterally, extended into a short pleural spine distally, spine directed outwardly and posteriorly; posterior band longer (exsag.) than anterior pleural band, anterior and posterior margin transverse proximally, distal half gently bending posteriorly, laterally extended into a prominent spine, spine approximately of the same width (tr.) as the posterior pleural band, as long (exsag.) as the posterior band proximally, gradually shortening (exsag.) distally, directed outwards and slightly backwards; posterior spines more strongly posteriorly directed; anterior and posterior pleural band carrying coarse tubercles, tubercles aligned, forming a transverse row, tubercles of anterior pleural band slightly smaller and more densely arranged than tubercles of the posterior band ( Figs 7.1, 7.2 ).

Pygidium unknown.

Discussion. Diacanthaspis (s.l.) maquoketensis does not seem to have been been mentioned in the literature since it was proposed, most likely because the only photographs available were Walter’s (1924, pl. 18, figs 2, 3) tiny and harshly lit images of a small external mold, which indicated little more than that an odontopleurid of some kind was represented. A latex cast reveals a fairly well preserved outstretched specimen lacking the right librigena and pygidium. With its tuberculate anterior cranidial border, paired occipital spines, slender genal spine base, and tall librigenal field, the species is clearly an odontopleurine, which based on the arguments above would for the present be treated as Diacanthaspis (s.l.).

Of the other Katian Laurentian species not part of or potentially part of the core group, there is little to compare. Diacanthaspis (s.l.) playfairi is all but uninterpretable, as are the Sandbian D. (s.l.) hystrix and D. (s.l.) lalage . The sole Upper Ordovician non-s.s. species that can be meaningfully compared is the Sandbian D. (s.l.) trippi, which is known from good photographs of three outstretched specimens. This species is quite similar to D. (s.l.) maquoketensis , especially in the size and density of the tuberculate sculpture, particularly the prominent transverse rows of tubercles on the posterior pleural bands of the thoracic segments and the cranidial posterior border. None of the specimens have the posterior part of the occipital ring well preserved and it is unclear whether paired occipital spines were present, but they are not ruled out. The size and position of the glabellar lobes is also comparable between the species. Obvious differences include the fact that D. (s.l.) trippi was evidently blind, whereas D. (s.l.) maquoketensis has a definite eye (albeit quite small). The librigena of D. (s.l.) trippi was described as having around 20 lateral border spines, whereas there seem to be only 14 present in D. (s.l.) maquoketensis . Finally, the thoracic pleural spines of D. (s.l.) trippi are more strongly turned posteriorly in all three specimens, whereas those of D. (s.l.) maquoketensis run in a much more lateral direction.

Figures

FIGURE 7. Diacanthaspis (s.l.) maquoketensis (Walter, 1924), from the Maquoketa Formation (lower Katian), Florenceville, Howard County, Iowa, USA. 1–4. Dorsal exoskeleton lacking pygidium, holotype, latex cast from external mold, SUI 9233, dorsal, oblique, anterior, and left lateral views, x7.5.