Rare arthropods from the Burgess Shale, Middle Cambrian, British Columbia

Abstract

Six species of arthropods from the Walcott Collection, U.S. National Museum, are described. Molaria spinifera Walcott is known from over 100 specimens, a sample that reveals the morphology fairly fully. Between one and 12 specimens of the other species are known, and yield limited information. M. spinifera had a smooth, convex exoskeleton, not trilobed, the cephalic shield being a quarter-sphere in shape, eight trunk tergites diminishing in size posteriorly and the cylindrical telson having a short ventral spine and a long, jointed posterior spine. The cephalon bore a pair of short, slim antennae and three pairs of biramous appendages. There were eight pairs of similar biramous appendages on the trunk. The biramous appendage had a large basal podomere, a segmented inner walking branch, and a lobate outer branch arising from the basal podomere and bearing marginal lamellae. The sagittal length of cephalon, trunk and telson ranged from 8 to 26 mm, the posterior spine slightly exceeding this length; the smallest specimens are similar to the largest. The animal lacked eyes, and was probably benthic and may have been a scavenger and deposit feeder. Habelia optata Walcott was superficially similar to M . spinfera , the trunk being of 12 tergites; there was no cylindrical telson, but a ridged and barbed spine inserted into the 12th tergite, the spine having a joint at about two-thirds its length. The external surface of the exoskeleton was tuberculate; the pleurae of the tergites curved back increasingly strongly posteriorly, the tips being spinose. The cephalon appears to have borne a slim, short pair of antennae and two pairs of biramous appendages; the proximal portions of the jointed inner branches may have been adapted for grinding food. The first six trunk somites bore biramous limbs, the inner branch being a relatively long walking leg, the outer a lobe having marginal lamellae; on the posterior trunk somites there is no trace of the inner branch, but the outer was present. H. optata lacked eyes and was probably a benthic animal. Only the smooth exoskeleton of a possible second species, H? brevicauda Simonetta, is known, of which the posterior spine is short and bluntly rounded. The new genus and species Sarotrocercus oblita is erected for a few specimens, in which the body is about 1 cm in length, and behind which is a slim spine having a group of spines at the tip. From beneath the anterolateral margin of the cephalic shield a large eye projected, and the cephalon bore also one pair of large, jointed appendages. Behind these were pairs of lobed appendages bearing marginal lamellae, one on the cephalon and one on each of the nine trunk somites. This small species may have drifted and swum in the higher water layers, the occasional carcass lying on the sea bottom having been preserved. The single specimen of Actaeus armatus Simonetta is over 6 cm in length. The exoskeleton of this specimen is divided into cephalic shield with marginal eye lobe, 11 trunk tergites and a triangular terminal plate. The anterior cephalic appendage was Leanchoil -like, the stout proximal portion being curved and ending in a group of claws, the next two podomeres bearing long, slim extensions. The head shield also bore three pairs of biramous appendages, consisting of a small jointed inner branch and a large lobed outer branch with marginal lamellae; appendages like these outer branches are preserved beneath the trunk tergites. Only two specimens are identified as Alalcomenaeus cambricus Simonetta (length 3-4 cm). The exoskeleton is divisible into cephalic shield, trunk of probably 12 tergites, and an ovate terminal plate which has lateral bands. The cephalon has a marginal eye lobe and an anterior appendage which is broad proximally, the long distal portion being slim. The holotype shows a series of lobed appendages, the first three cephalic. Between them project the curved, pointed terminations of inner branches. The second specimen suggests that these lobed appendages bore marginal filaments, and reveals the inner branches as blade-shaped, and spinose on the inward-facing margin. These biramous appendages were present on all the trunk somites, being largest anteriorly. These remarkable appendages suggest a benthic scavenger, able to hold on to, and tear up, a carcass. ‘ Leanchoilia protogonia ’ Simonetta is most probably a composite, a poorly preserved Leanchoilia superlata lying on an unidentified, branching organism. The five species showing appendages extend greatly the known range of variation in morphology of the Burgess Shale arthropods. Affinities are discussed, but familial and higher classification is postponed, pending completion of work on all the arthropods from the shale.