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

Development of three rows of tubercles on the thoracic pleural region of the meraspid carapaces (Figure 5.2, 5.11) contrasts with that of two main (inner and outer) series on the protaspid pleural region (Tripp and Evitt, 1983, text-figure 2). The most proximal row of meraspid thoracic pleurae (more evident in ventral view; Figure 5.11) corresponds to the protaspid inner series. Topographically, the row at mid-width of the thoracic pleurae is continuous with cephalic tubercles behind the palpebral lobe and on the anterior fixigenal field, which are equivalent to the protaspid outer series. Interestingly, it is apparent that this middle row is serially homologous with the tubercle on the midwidth of the posterior cranidial border, which is equivalent to the "posterior cranidial border tubercle" (bt) of the protaspides (Tripp and Evitt, 1983, text-figure 2). Such correspondence of "bt" with the outer series seems more evident in larvae of other hystricurid genera (Figures 5.3, 7.1-7.3) and it also applies to meraspid dimeropygids (compare Tripp and Evitt, 1983, plate 32, figure 6 [cranidium] and figure 12 [thoracopygidium]). Thus, it is suspected that at least in hystricurids, the "bt" belongs to the outer series, along with the middle row of the thoracic pleurae. A similar observation was made by Chatterton (1994, p. 547), who argued that the swelling close to the fulcrum of Dimeropyge speyeri, which was believed to belong to the outer series, may incorporate the "bt" of the protaspides. If this is the case, the tubercle at the distal extremity of the posterior cranidial border of the larvae (Figure 3.4) is serially homologous with the meraspid tubercles at the distal extremity of each thoracic pleura and along the pygidial margin (Figure 6.1). These tubercles seem to comprise an additional series of tubercles to three main series of the protaspis (Tripp and Evitt, 1983, text-figure 2). However, it is not clear which of two rows on librigenae (one on the field and the other on the border) is homologous with this tubercle series on thoracic pleurae (Figure 6.1).

Material.-UA 11007 (protaspides from R5-86); UA 11022, UA 11017, UA 11058 (meraspides from R5-86); UA 11015 (meraspis from R5-76.4); UA 11018 (holaspid cranidium from R5-76.4); UA 11021 (holaspid pygidium from R5-86).

Genus AMBLYCRANIUM Ross, 1951b

Type species.-Amblycranium variabile Ross, 1951b, from the lower part of Garden City Formation of southern Idaho.

AMBLYCRANIUM VARIABILE Ross, 1951b

Figures 3.5, 7.1-7.7, 7.9

Amblycranium variabile Ross, 1951b, p. 64-66, pl. 13, figs. 10-18.

Additional diagnosis.-Larvae with rachial, inner, and outer series of short spines (instead of tubercles) on both cranidium and pygidium, and two tubercles on posterior cranidial border. See Ross (1951b, p. 64-66) for detailed adult diagnosis.

Description.-All protaspid specimens are considered to belong to one instar, because the plots of length and width fall within one morphometric grouping (Figure 4). Shields 0.5-0.58 mm long and 0.36-0.43 mm wide, suboval in outline excluding librigenae. Protocranidium trapezoidal in outline and occupies 66-68 percent of whole shield length. Glabella barrel-shaped, moderately convex upwards, and tapers forwards, occupying 61-77 percent of cranidial length. Preglabellar field less than 10 percent of cranidial length; preglabellar median furrow shallow. Anterior cranidial border furrow as distinct as preglabellar furrow. Palpebral lobe small, slightly raised and located immediately behind glabellar front. Posterior cranidial marginal furrow weakly impressed and gently curves posteriorly distally. Anterior facial suture convergent and posterior suture gently divergent. Protopygidium semicircular in outline; axis strongly tapers backwards, with two axial rings and terminal piece; anteriormost interpleural furrow shallow; marginal border gently slopes downwards.