Hystricurid trilobite larvae from the Garden City Formation (Lower Ordovician) of Idaho and their phylogenetic implications

Journal of Paleontology, Sep 1997 by Lee, Dong-Chang, Chatterton, Brian D E

Late stage with 0.64 mm in width and 0.80 mm in length, and differs from early stage in having more divergent posterior facial suture, longer (sag.) preglabellar field, increment of protopygidial axial ring, and more strongly curved posterior cranidial marginal furrow.

Discussion.-This species, represented by only larval materials, is questionably assigned to Hystricurus. Although no development of inner series tubercles on the protopygidium and no rachial series on the cranidium suggest that this species belongs to Hystricurus, development of outer series tubercles on protopygidium distinguishes this species from Hystricurus paragenalatus and Hystricurus n. sp. A.

Material.-UA 11008, UA 11013 (protaspides from R550.3).

HYSTRICURUS? sp. B

Figures 3.3, 5.3, 5.4

Description.-Early protaspid stage about 0.52 mm long and 0.42 mm wide, suboval in outline. Two pairs of tubercles on anterior border; three pairs of inner series tubercles developed in regular spacing; palpebral and posterior cranidial border tubercle present. Occipital ring rectangular, with tubercle. Protopygidium semicircular and bears two axial rings and terminal piece; rachial series tubercles on anterior two rings; two pairs of outer series developed along margin.

Late stage with 0.79 mm in length and 0.63 mm in width differs from early one in having one more pair of inner series tubercles located opposite glabellar front, more divergent posterior facial suture, longer (sag.) preglabellar field, increment of protopygidial axial ring, and more distinct and strongly curved posterior cranidial marginal furrow.

Discussion.-Absence of inner series tubercles on the protopygidium and a rachial series on the cranidium suggests that this species belongs to Hystricurus. However, the larvae of this species differ from Hystricurus in having two pairs of tubercles on the anterior border and an outer series of tubercles on the protopygidium. These two features occur in Parahystricurus and Amblycranium so that this species may link Hystricurus to the latter two genera.

Material.-UA 11010 (protaspis from R5-50.3); UA 11056 (protaspis from RS-76.4).

Genus PARAHYSTRICURUS ROSS, 1951b

Type species.- Parahystricurus fraudator Ross, 1951 b, from lower part of Garden City Formation of southern Idaho.

Taxonomic remarks.-Parahystricurus was distinguished from Hystricurus in having a triangular posterior fixigenal area, a more elliptical glabella and smaller palpebral lobes (Ross, 195 lb, p. 57). Just as in Hystricurus, the Parahystricurus species show a considerable morphologic variation of the cranidium; compare Parahystricurus fraudator (type species; Ross, 195 lb, plate 12, figure 1-16) and Parahystricurus carinatus (Ross, 1951b, plate 13, figure 23-27). In particular, the latter species has a more similar cranidial morphology to Hystricurus than other Parahystricurus species. However, the transitory pygidium of the complete carapace of Parahystricurus carinatus described in this work (Figures 5.2, 6) is greatly dissimilar from pygidial morphologies of Hystricurus (Whitfield, 1889, plate 13, figure 15, 20; Ross, 1951b, plate 9, figure 1-13), in having a narrow, steeply inclined marginal border and a row of spines along the margin of the pleural region. These pygidial features seem significant in distinguishing Parahystricurus from Hystricurus; and they are reminiscent of those of the Dimeropygidae (Chatterton, 1994). Further, Parahystricurus larvae are different from Hystricurus in having an inner series of tubercles on the protopygidium.