Cambrian Ocean World by John Foster

Author:John Foster
Language: eng
Format: epub
Publisher: Indiana University Press
Published: 2014-01-10T16:00:00+00:00

Isoxys acutangulus and I. longissimus

Isoxys is a soft-bodied, bivalved arthropod that is widely known throughout Cambrian deposits in North America and elsewhere. The first species of this genus was named by Walcott from a specimen found in the Cambrian of Tennessee, and the first specimen of I. acutangulus was named from the Mount Stephen Trilobite Beds. Isoxys was later found at the Walcott Quarry and several other sites in the area. The distinctive carapace valves (or head shield) of Isoxys have anterior and posterior extensions along the dorsal articulation, none longer than those of Isoxys longissimus. For many years we knew only the carapace of this animal, but recent specimens have finally shown us what the rest of the body looked like. Isoxys had a head with large, bulbous eyes and – anteriorly – two long, grasping appendages; most of the rest of the head was covered by the head shield. The thorax consisted of 13 segments with long, paddle-like appendages, and there was a short, tail fluke–shaped telson. Interestingly, the head shield covered most of the body segments but lifted clear of them posteriorly so that there was a significant space between the last thoracic segment and the posterior “spike” of the head shield.

Isoxys acutangulus is known from about 300 specimens from the Burgess Shale, but there are only a handful of the long-spined I. longissimus known. Isoxys was probably a free-swimming genus, as indicated by its paddle-like limbs; the large eyes and long grasping appendages of the head suggest that it was a predator of small prey items living in the water column.67

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