Class Malacostraca Latreille, 1806
Superorder Peracarida, Calman, 1904
Order Hemicaridea Schram, 1981
Suborder Cumacea Kröyer, 1846
Infraorder Neocumacea, nov.next section
Diagnosis. – Eyes, when present at all, developed as an unpaired dorso-median organ fused to cephalothorax, generally without separate facets; pseudorostra and siphons well developed; ventral margin of carapace without medially directed shelf.
Remarks. – The distinctly apomorphic condition of the eyes and the development of the distinctive pseudorostrum and siphons, as opposed to what is seen in the Paleozoic forms, we believe necessitate the creation of a distinct infraorder to accommodate the living cumaceans. This infraorder should contain all the living cumaceans and should also include the Jurassic species Palaeocuma hessi Bachmeyer, 1960. Although the preservation of this Mesozoic form leaves a lot to be desired, we can see nothing that resembles the ventral branchiostegal shelf characteristic of the Paleozoic taxa. Currently, 8 families of living cumaceans are recognized: Bodotriidae, Ceratocumatidae, Diastylidae, Gynodiastylidae, Lampropidae, Leuconidae, Nannastacidae, and Pseudocumatidae.
Infraorder Ophthalmcumacea Malzahn, 1972
Diagnosis. – Distinctly lobed eyes, free from cephalothorax, bearing well-developed facets; carapace lacking pseudorostra, but with anterolateral lappets extending forward; large ventral branchiostegal shelf providing a partial floor to branchial chamber.
Remarks. – Malzahn (1972: 448) used this taxon to accommodate cumaceans with lobed, well-facetted eyes, but he did not place it in opposition to any other taxon. The diagnostic characters used here, viz., the lobed eyes and lappets (which we believe are precursors of the pseudorostrum), are perhaps intermediate between generally plesiomorphic conditions (stalked eyes, carapace without lappets or pseudorostrum) and the states seen within the neocumaceans. In this respect, as Malzahn (1972) first pointed out, the evolution of cumaceans would seem to parallel that of the tanaidaceans (Schram et al., 1986) wherein the Paleozoic infraorder Anthracocaridomorpha is also predominantly characterized by the presence of intermediate characters such as lobed eyes. The ventral branchiostegal shelf of the ophthalcumaceans, however, is a unique apomorphic feature and apparently shared by all the Paleozoic forms.
Family Ophthalmdiastylidae Malzahn, 1972
Diagnosis. – As only one family is currently recognized, the diagnosis of the family is the same as that of the infraorder.
Type genus. – Ophthalmdiastylis Malzahn, 1972.
Remarks. – Malzahn (1972) did not formally designate or diagnose a family, but its status is implicit in his description of the species and use of the higher taxon name Ophthalmcumacea.
Ophthalmdiastylis Malzahn, 1972
Diagnosis. – Rostrum wide and relatively long; pseudorostra very weakly, if at all, developed.
Type species. – Ophthalmdiastylis inflata Malzahn, 1972.
Remarks. – Malzahn (1972) did not clearly diagnose the genus. From the descriptions and illustrations it would appear that for both species of this genus a short, but true, rostrum characterizes O. inflata and O. costata. O. inflata appears distinguished by the smooth yet slightly papillose carapace, while O. costata is characterized by distinct longitudinal carinae on the carapace.
Ophthalmdiastylis parvulorostrum new species
Diagnosis. – Slender, somewhat laterally compressed carapace. Anteriorly serrate dorsal ridge extending longitudinally, terminating anteriorly in a slight frontal trough. Anterior lappets marked, extending slightly anteriad from the anterolateral edge of carapace lateral of the lobed eyes.
Description. – The carapace is about 1.3 mm in length. In dorsal view, the carapace is elongateand is somewhat pointed posteriorly. In lateral view, the carapace is ovoid, more constricted anteriorly. The height of the carapace is over 1/2 the length and the transverse width is under 1/2 that of the length. A reticulate sculpturing (Fig. 2.1) covers the carapace surface, which is characterized by irregular raised areas separated by narrow grooves. A mid-dorsal ridge is present, and this bears a set of 3 serrate teeth directed anteriorly (Figs. 2.1, 2.2). The tip of the short rostrum was equipped with 3 small spines (Figs. 2.3, 2.4); one can see their bases though the tips have broken off. One spine is terminal while the other two are located just posterior to it, one slightly in front of the other on the edges of the optic notches. A broad, ventral, branchiostegal shelf is present that extends the length of the carapace (29305, Fig. 2.5), There are no anterolateral spines, but the ventral shelf appears to merge smoothly into the anterior lappets (Figs. 2.1, 2.5). Ventrally, the carapace displays a narrow ventral ridge marking the bend that forms the branchiostegal shelf. The anterior lappets display a slight, spatulate form and extend anteriorly ventral and lateral to the compound eyes (Figs. 2.1, 2.2).
SDSNH 26273 shows a pair of large, well-preserved eyes protruding from beneath the frontal lobe and separated by a small rostrum (Figs. 2.2, 2.3, 2.4). The lobed eyes are equipped distally with a well-developed set of facets rounded to hexagonal in outline (Figs. 2.3, 2.4). The lobes themselves are decorated with a faint scale-like sculpturing (Figs. 2.3, 2.4).
Portions of endopods of thoracic limbs are preserved on SDSNH 26279 (Fig. 3.1). The details are obscure since the distal portions of the limbs are not preserved. The individual limbs are arranged parallel and appear to be remains of pereiopods, and if tentative comparison can be made to modern forms the appendages bore rather elongate bases (Fig. 3.2).
Etymology. – A reference to the tiny rostrum.
Holotype. – SDSNH 26273 (Fig. 2.1 – 2.4). San Diego Natural History Museum.
Other material. – One fairly complete carapace paratype (SDSNH 29305); another partial specimen (SDSNH 26279); and a fragmentary and poorly preserved specimen (SDSNH 29309) tentatively assigned to this species as well.
Occurrence. – The holotype specimen and the questionable fragmentary specimen were collected from locality SDNHM 3191 in Arkansas. The paratypes (SDSNH 26279, 29305) was collected from locality SDNHM 3267 in Kansas.
Remarks. – The carapace of O. parvulorostrum is much more slender, elongated and laterally flattened than that of C. imoensis n. sp., another cumacean from these same localities. O. parvulorostrum differs from O. inflata Malzahn, 1972 in its smooth carapace surface and smaller rostrum, and from O. costata Malzahn, 1972 by its lack of longitudinal carinae on the carapace. A reconstruction of O. parvulorostrum is offered in Figs. 5.1 and 5.2
Carbocuma new genus
Diagnosis. – Carapace highly vaulted and barrel-like, almost bulbous in lateral view. A deep mid-dorsal trough runs longitudinally, and anteriorly expands to form a well-developed frontal lobe. A small rostrum is present. Small, sinuous lappets developed laterally. Well-developed lateral branchiostegal shelf on ventral aspect of carapace. Large, faceted, paired eyes.
Type species. – Carbocuma imoensis n. sp. by monotypy.
Etymology. – Named after the Carboniferous period in which the fossils are found, and the genus Cuma, which was erected by H. Milne Edwards and from which the name of the group Cumacea arises (Schram, 1986).
Remarks. – The relatively large size of the species along with the globose form and distinctive mid-dorsal ridges and grooves justify a separate genus for this taxon.
Carbocuma imoensis new species
Diagnosis. – As this taxon is monotypic, the diagnosis of the species is that same as that for the genus.
Description. – The approximate length of the carapace is 1.75 mm. In dorsal view, the carapace is ovoid in outline, though complete posterior and anterior margins are not preserved together on the same specimen on any of the material at hand. In lateral view, the dorsal carapace is more rounded, although the ventral aspect is slightly flatter and has a bulbous mid-region that dips anteriorly into the frontal lobe (Fig. 3.3). The surface of the carapace is rather lightly textured with faint papillae, almost scale-like in places (Fig. 3.4, 3.5). The height of the carapace is roughly 4/5 of the length, as is the transverse width. A deep median trough marks the dorsal surface of the carapace (Fig. 3.3) and is flanked by large, parallel ridges that bear less pronounced troughs on their crests. Wide mid-dorsal indentations extend from below the smaller troughs, anteriorly to spread around the frontal lobe, and ventrally to the mid-lower region of the carapace. The frontal lobe itself is well developed and displays what appears to have been a small rostrum (Figs. 3.3, 3.4). A prominent ventral branchiostegal shelf extends the length of the carapace and is developed as a spine anteriorly, only the stump of which is typically preserved (Fig. 3.5). A distinct keel marks the point at which the branchiostegal shelf meets the side of the carapace, and similar narrow longitudinal ridges mark the interior edges of the shelf (Fig. 3.5). The ridges and lateral keels converge posteriorly, and anteriorly as well but to a lesser extent. The ventral margins of the carapace floor a large branchiostegal chamber and extend toward a mid ventral gap, with two smaller ridges along the margins adjacent to the gap (Fig. 3.5).
The holotype specimen, SDSNH 26275, preserves one large eye (Fig. 3.4). Its apparently short lobe bears a densely facetted field of ommatidia. Just ventral to the eyes, between them and the anteroventral spine, the margin of the carapace is developed as a somewhat sinuous, weakly developed set of lappets (Fig. 3.3, 3.4).
Specimen SDSNH 29308 illustrates preservation of pereiopods (Fig. 3.6). These are incomplete, but the elongate basis is virtually in tact and is marked by a scale-like cuticular decoration, not uncommon in living cumaceans too. Specimen SDSNH 29310 shows preservation of pereiopods and a faceted eye fragment.
Etymology. – The name imoensis is from the type locality of the specimen.
Holotype. – SDSNH 26275 (Figs. 3.2, 3.4). San Diego Natural History Museum.
Additional specimens. – SDSNH 26274, 26277, 26278, 29307, and 29308. Another partial specimen, SDSNH 29310 is also tentative assigned to this species.
Occurrence. – SDSNH 26275 (the holotype) and SDSNH 26274, 26277, and 29308 come from locality SDNHM 3191 in Arkansas. SDSNH 26278, 29307, and 29310 come from locality SDNHM 3267 in Kansas.
Remarks. – This constitutes the most numerous and distinctive of cumacean species in these two localities. Reconstructions of the various views of the cephalothorax are provided in Figs. 5.3 to 5.5.
Securicaris new genus
Diagnosis. – Slender, laterally compressed carapace. Sharp dorsal ridge, anteriorly giving way to a relatively elongate and narrow frontal lobe almost 1/3 the length of the carapace. Ventro-lateral ridge extends anteriorly to form large spine. Carapace decorated with a scale-like terracing.
Type species. – Securicaris spinosus n. sp. by monotypy.
Etymology. – From the Latin, securis (f.), meaning axe-like.
Remarks. – The rarest of the cumaceans reported herein, known from a single specimen. Though the specimen is incomplete, the sum of what is preserved seems to indicate that it is probably a cumacean. The general habitus accords with the other species described here, although it is unfortunate that only one specimen is at hand for study.
Securicaris spinosus new species
Diagnosis. – Since there is but a single specimen, the diagnosis of the species is the same as that of the genus.
Description. – The fairly complete carapace is 1.5 mm in length. It is slender and elongate (Fig. 4.1). In lateral view the dorsal ridge is rather concave, more so than the ventral edge which is flatter. The height of the carapace is roughly 1/2 that of the length, and the transverse width is just 1/3 of the length. There is a sharp, narrow mid-dorsal ridge or crest, which flattens out anteriorly to form a triangular frontal lobe, widest at its most anterior end. A well-defined lateral ridge is present on the right-hand side and marks the transition to a ventral branchiostegal shelf. This lateral ridge extends anteriorly to form a rather delicate, well-developed, anterolateral spine, though the tip is absent. Ridge and spine are not preserved on the left-hand side. In ventral view, the ventral aspect of the left valve of the carapace is folded under the branchiostegal shelf of the right side, a vagary of the preservation of this specimen. The surface of the carapace between the mid-dorsal ridge and ventral keels is curved and decorated with a distinct scale-like sculpture creating the effect of overlapping terraces.
Etymology. - A reference to the delicate anterolateral spine of the carapace.
Holotype. – The single known specimen, SDSNH 26276 (Fig. 4.1).
Occurrence. – The specimen was collected from locality SDNHM 3191 in Arkansas.
Remarks. – Though of similar size to Ophthalmdiastylus parvulorostrum, the carapace of S. spinosus appears to be more laterally flattened. The frontal lobe is narrower and longer than any of the other fossil cumaceans described herein, and there is a clear set of anterolateral spines that are lacking in Ophthalmdiastylus. The posterior dorsal area of the carapace on SDSNH 26276 does not seem to be complete, so the lateral outline as reconstructed (Fig. 5.6) is a surmise of the original form. Nevertheless, despite the singular and fragmentary nature of the specimen, all the features noted above justify the erection of a separate taxon.
Remarks. – In addition to the cephalothoracic material described and named above, the specimens at hand include three pleonal fragments (SDSNH 29310) that seem referable to the Cumacea (Fig. 4.3). These specimens are narrow and elongate consisting of 3 to 4 articulated segments and are virtually identical to the long, narrow pleons characteristic for Cumacea. From their size and preservation, these pleons might be those of Carbocuma imoensis, but without their being attached to a cephalothorax there is no way to be certain.
The specimens are highly sculptured (Fig. 4.3) with paired mid-dorsal ridges. The cuticle is marked with distinct areas bearing different textured decorations. These range from papillose (Fig. 4.4) to scale-like (Fig. 4.5) in form. These cuticular decorations are again very similar to what occurs on living cumaceans.