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

ABSTRACT-Hystricurids are considered to constitute the earliest (early Ibexian or Tremadocian) family of Proetida, based on comparisons of hystricurid larvae with younger proetide and Cambrian ptychopariide larvae. The hystricurid larvae share a fusiform glabella falling short of the anterior margin with other proetide larvae, which were derived from Cambrian ptychopariide larvae with a forwardexpanding glabella. Two discrete morphotypes of hystricurid larvae are recognized. The first type is characterized by the development of a pattern of regularly distributed tubercles on the shield and the presence of a preglabellar field. The second is characterized by the presence of glabellar furrows (discontinuous or transglabellar), an indented posterior margin, and a lack of tuberculation. Each of these hystricurid lineages represented by a distinct larval morphotype is considered ancestral (or sistergroup) to a different group of younger proetides. The second larval morphotype is considered a phylogenetic intermediate between the first type and ptychopariide larvae. Several larval features of these primitive proetides are considered to have originated by early onset of post-larval features of the ptychopariides (peramorphic pattern).

Newly described hystricurid species are Hystricurus n. sp. A, Hystricurus? sp. A, Hystricurus? sp. B, 'Paraplethopeltis' n. sp. A, 'Paraplethopeltis' sp. B, and Hyperbolochilus cf. marginauctum.

INTRODUCTION

HYSTRICURID TRILOBITES have been regarded either as the earliest member of the Proetida (Fortey and Owens, 1975; Fortey, 1990), or as Ordovician relatives of Cambrian ptychopariides such as the Solenopleuracea (Ludvigsen and Westrop in Ludvigsen et al., 1989) and Catillicephalacea (Shergold, 1991). Both opinions were based on post-larval similarities of related taxa. There is little doubt that the hystricurids represent a phylogenetically bridging taxon between Cambrian ptychopariide and long-lived post-Cambrian proetide trilobites (Fortey, 1990, p. 560). Larval features of the hystricurids described in this work permit us to evaluate previous hypotheses of the phylogenetic relationships of the group (Chatterton, 1971, p. 71; Owens, 1973, text-figure 11; Fortey and Owens, 1975; Fortey, 1990) and to reappraise hystricurid taxonomy.

The specimens figured were collected from the lower part of the Ibexian Garden City Formation at "Locality 5" of Ross (1951b, figure 1), exposed on the east side of Hillyard Canyon, southern Idaho. The age of all the trilobites described herein is early Ibexian or Tremadocian (Figure 1.1, 1.2). Zonal assignment of each sampling horizon was made by comparison of the fauna collected in this work with Ross (1951a, b, 1953) and Hintze (1953); the sampling horizons range from Zone B to E (Ross, 1951b) or the Symphysurina Zone to the Tesselacauda Zone (Hintze, 1953). All specimens are housed in the Paleontological Collection of the University of Alberta (labelled UA numbers). Most specimens (unless noted) were photographed using backscattered electrons in a scanning electron microscope. The newly described hystricurids lack sufficient holaspid sclerites for us to name new species, so we have left them in open nomenclature.

The distinct tuberculation pattern of some hystricurid larvae is described using the terminologiy of Tripp and Evitt (1983, text-figures 2-4), where three main series of tubercles, "rachial," "inner," and "outer" are recognized; this designation system is also used for the pygidial tuberculation. A similar system was proposed by Adrain and Chatterton (1994, figure 1) to describe patterns of juvenile cranidial spine distribution in otarionine meraspides. Their G1-3, Fx1-4, and P1-2 are equivalent to the "rachial" and "inner" series and "palpebral" tubercles of Tripp and Evitt (1983), respectively.

SYSTEMATIC PALEONTOLOGY

Order PROETIDA Fortey and Owens, 1975

Family HYSTRICURIDAE Hupe, 1953

Genus HYSTRICURUS Raymond, 1913

Type species.-Bathyurus conicus (Billings, 1859) from Canadian "Calciferous Sandrock" of the Mingan Islands, Quebec.

Taxonomic remarks.-Raymond (1913, p. 60) distinguished Hystricurus from Bathyurus Billings, 1859 (a bathyurid) by its forward-tapering glabella and dorsally raised palpebral area of the fixigenae (=his "the elevated ridge on the fixed cheeks outside the dorsal furrow"). However, several species lacking both features have been reported, since Ross (1951b, p. 39-40), in an extensive work on Hystricurus, added several features to the generic diagnosis; for example, Hystricurus globosus (Stitt, 1983, plate 5, figure 2) has a barrel-shaped glabella. Morphologic deviations of the other species from the type species, Hystricurus conicus (see Whitfield, 1889, plate 13, figure 15, 20) are summarized in Table 1. Such a wide variation confounds the taxonomic concept of the genus Hystricurus. The pygidial features are not helpful in confidently classifying these trilobites, because the disarticulated pygidium has seldom been confidently associated with the cranidium of a particular species (e.g., Ross, 195 lb). Furthermore, complete carapaces of hystricurid trilobites have rarely been discovered; Hystricurus ravni (Poulsen, 1927, plate 18, figure 5) is the only species previously known from a complete carapace. Thus, in order to elaborate the taxonomy of Hystricurus and allied genera, it seems informative to compare larvae that represent a complete individual. Hystricurus larvae described below are characterized by the development of an inner series of tubercles only on the protocranidium, compared to other hystricurid genera. This may serve to distinguish Hystricurus from other hystricurid genera.