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

Fortey and Peel (1989, p. 9-10, figure 7) treated 'Paraplethopeltis' as a subgenus of Hystricurus, noting that Hystricurus (Paraplethopeltis) sp. nov. A shows the features which link 'Paraplethopeltis' with Hystricurus. Similarly, Berg and Ross (1959, p. 112-113) regarded Hintze's species as belonging to Hystricurus. Other holaspid features than surface sculptures strongly support the close relationship between 'Paraplethopeltis' and Hystricurus: in effect, there are several morphologic intermediates such as Hystricurus? sp. I (Ross, 1951b, plate 17, figure 1-3) and Pachycranium? sp. (Ross, 1951b, plate 17, figures 4, 5). However, larval morphologies of 'Paraplethopeltis' described (Figure 8) are distinct enough from those of Hystricurus (Figure 2) for each to be treated as a separate genus. The fusiform glabella of 'Paraplethopeltis' larvae strongly indicates that the genus is a member of the Proetida; the holaspid resemblances to Hystricurus allow us to place 'Paraplethopeltis' in the Hystricuridae.

Of phylogenetic significance is that 'Paraplethopeltis' have an intermediate condition of larval and adult features between Hystricurus and the Ptychopariida including the Plethopeltidae. In particular, such larval features of 'Paraplethopeltis' as transglabellar furrows, no preglabellar field, and no regular tuberculation pattern (Figure 8) are the retained primitive features found in the ptychopariide type larvae (Fortey, 1990, text-figure 16 g).

'PARAPLETHOPELTIS GENACURVUS Hintze, 1953

Figure 8.7, 8.12, 8.13, 8.15

Paraplethopletis? genacurvus INTE,1953, p. 202-204, pl. 7, figs. 1-5.

Hystricurus? sp. aff. H.? genacurvus BERG AND Ross, 1959, p. 112113, pl. 21, figs. 21, 23.

Additional diagnosis.-Larvae with fusiform glabella, smooth exoskeleton, transglabellar furrows, and re-entrant posterior margin. See Hintze (1953, p. 202-203) for the diagnosis of adults.

Description.-Two protaspid instars, designated "early" and "late," are recognized by morphologic details. Numerical measurements are given in average.

Early protaspides about 0.7 mm long and 0.59 mm wide, suboval in outline; anterior margin rounded and posterior margin indented. Axis spindle-shaped, of independent convexity, with four glabellar lobes and occipital ring; LA longer than subequal L1/L2/L3; LI/L2/L3 are rectangular, and L2 broadest; occipital ring smaller (tr. and sag.) than L1; glabellar furrows continuous across glabella; behind occipital ring, indistinct protopygidial ring developed. Palpebral lobe only recognized by weak outward-curvature of facial suture.

Late protaspides about 1.04 mm long and 0.84 mm wide. Posterior cranidial marginal furrow distinct and runs almost straight halfway across pleural region and then strongly curves backwards. Anterior branch of facial suture parallel before converging anteriorly; posterior branch strongly divergent backwards before it curves backwards and inwards. Protopygidium accounts for 33 percent of shield length; axis with two rings and terminal piece; two pleural furrows shallower.