Paleobiology of <i>Climactichnites</i>, an Enigmatic Late Cambrian Fossil

Abstract

<i>Climactichnites wilsoni</i> Logan, 1860, is redescribed from field investigations and specimens in various museums. <i>Climactichnites youngi</i> Todd, 1882, and <i>C. fosteri</i> Todd, 1882, are placed in synonymy. The species is known only from its trail, consisting of raised bars and impressed furrows, bounded by two parallel, raised lateral ridges. This structure is interpreted as being formed from damp sand, redistributed and molded by the animal. At a few localities, an oval impression occurs at the origin of the trail; a new locality for this rare feature was found in Wisconsin. In Quebec a trail crossing over itself was found; this phenomenon was known from only one other locality. All occurrences of <i>Climactichnites</i> are in the Late Cambrian; specimens are known from New York, Quebec, Ontario, Wisconsin, and Missouri. This fossil is probably confined to the Dresbachian, the earliest stage of the tri-part Late Cambrian. All examples of trails are in sandstones; these are interpreted as sand flats that were just above water during low tide in the shallow epicontinental sea. A variety of animals have been proposed as the trail maker; they include several different kinds of arthropods, mollusks, and “worms.” Each proposal has weak points and none of the suggested animal groups has appropriate morphology to produce the marking. The animal is reconstructed as relatively low, broad, and about twice as long as wide. It is hypothesized that the tough body integument secreted mucus which facilitated movement and aided in preservation of the trail. A large flap covered most of the outside body and extended laterally over the muscular foot. Free edges of the flap on either side of the body compressed and molded damp sand into parallel bounding ridges. Respiratory organs may have been present below the flap edges, kept moist by being partially enclosed. By moving the edges of these lateral flaps, the organism may have been able to swim when in water. Most trails of <i>Climactichnites</i> are interpreted as a consequence of feeding activity. If so, food was taken in through a circular mouth located anteriorward on the ventral surface; this morphologic feature is inferred from circular markings seen on a few bars. As reconstructed, the sand under the animal was compressed anteriorly and laterally; the animal then brushed particles forward into a small dune-like bar, probably by the action of cilia on its ventral surface. Sand dwelling microorganisms displaced by brushing were concentrated centrally, obviating ingestion of large amounts of sediment. What sediment was taken in was released at irregular intervals through a posterior anus; medial marking on parts of some trails are interpreted as fecal strings. The complex clamping and brushing behavior, implies a well-developed nervous system. No body fossils are known in the Vendian or in the Paleozoic that could have constructed this form of trail. The proposed method of feeding, if correctly interpreted, is unique. Thus the trail of <i>Climactichnites</i> may constitute the work of an otherwise unknown phylum in the animal kingdom.