HYSTRICURID TRILOBITE LARVAE FROM THE GARDEN-CITY FORMATION (LOWER ORDOVICIAN) OF IDAHO AND THEIR PHYLOGENETIC IMPLICATIONS

Hystricurids are considered to constitute the earliest (early Ibexian or Tremadocian) family of Proetida, based on comparisons of hystricuri d larvae with younger proetide and Cambrian ptychopariide larvae. The hystricurid larvae share a fusiform glabella falling short of the ante rior margin with other proetide larvae, which were derived from Cambri an ptychopariide larvae with a forward-expanding glabella. Two discret e 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 (discont inuous or transglabellar), an indented posterior margin, and a lack of tuberculation. Each of these hystricurid lineages represented by a di stinct 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 pty chopariide larvae. Several larval features of these primitive proetide s are considered to have originated by early onset of post-larval feat ures of the ptychopariides (peramorphic pattern). Newly described hyst ricurid species are Hystricurus n. sp. A, Hystricurus? sp. A, Hystricu rus? sp. B, 'Paraplethopeltis' n. sp. A, 'Paraplethopeltis' sp. B, and Hyperbolochilus cf. marginauctum.