Peronopsis
interstricta
Traditionally, trilobites are subdivided into two
major groups: the polymeroids and the agnostoids. Polymeroids are
garden-variety trilobites. They have a multisegmented thorax; they
usually possess eyes; and they can be macropygous, isopygous, or micropygous
(the head is smaller than the tail, the head & tail are equal-sized, or the
head is larger than the tail). Agnostoids have a two-segmented
thorax only; they are all blind; and they are all isopygous (the heads &
tails are equal-sized). Agnostoids are usually quite small.
Many modern arthropod phylogenetic studies conclude
that agnostoids should be excluded from the trilobites.
Below are two specimens of the common agnostoid Peronopsis
interstricta (White, 1874). They come from the upper Middle Cambrian Wheeler
Formation of the House Range in western Utah.
Classification: Arthropoda, Trilobita, Agnostida, Peronopsidae
Peronopsis interstricta (8.5 mm long each) (commercially prepared) from the
Wheeler Formation (Middle Cambrian) of western Utah’s House Range.