Waptia - Description

Description

Waptia fieldensis had a maximum body length of 8 centimetres (3.1 in). The exoskeleton was very thin and easily distorted from fossilisation. It possessed a large bivalved carapace that was narrow at the front with wide posterior margins that covered the cephalon and most of the thorax. The cephalon had five short somites (body segments) with three to five pairs of small and poorly preserved feeding appendages.

A single pair of long and slender antennae is present, the segments of which are relatively few and elongated. They were most probably used as sensory organs, with most segments possessing small setae (bristles). A pair of short lobed structures (possible antennules) are also present, their position corresponding to the location of the second pair of antennae in modern-day crustaceans. In between them is a small triangular rostral plate with a narrow and sharp central ridge.

The well-developed compound eyes were stalked. The striking morphological similarities of the eyes of Waptia fieldensis to that of extant mysid shrimps make it very likely that they were capable of producing true visual images or were at least sensitive to motion. A median eye may have also been present. Nervous tissue, including a putative brain, has been identified.

The thorax is divided into two groups of somites. The anterior group is composed of four somites, each with well-developed walking limbs with lengths that extend past the margins of the carapace. Their morphological details are not well preserved but their distal segments appear to possess a multitude of small spines. It is unknown if these limbs are biramous, but they are assumed to be the endopodites, the inner branches (ramus) of the leg, with the exopodites, or outer branches, absent or not preserved.

The posterior thoracic group is made up of six somites, each possessing a pair of long multi-jointed appendages. The segments of these appendages are longer near the body and taper towards the flexible distal segments, extending past the carapace. They bear a fringe of long, slender filaments, all of which are directed towards the middle of the body, a characteristic shared by extant crustaceans. Though usually squashed into blade-like shapes from the fossilisation process, the filaments were slender cylindrical tubes when the animal was alive. The filaments at the tipmost segment are often bunched together. These appendages are believed to be the exopodites and may have functioned as gills and as swimming limbs. They are possibly biramous, with some specimens showing traces of what may be small endopodites at their bases.

The abdomen is composed of five somites, all of which lack appendages except the last. The back-facing margins of these somites bear small spines and four or more larger spines. The last abdominal somite forks into a pair of flattened spatulate appendages (the uropods) that function as a tail fan (caudal rami). In addition to stabilising the body while swimming, a quick flick of the tail fan can rapidly propel the animal backwards, which may have functioned as a means of escaping predators like in modern shrimp. Traces of four fused segments are evident in the three faint lines dividing each lobe of the tail fan. A long telson is present at the end of the body. Traces of a rounded stomach, small digestive glands, and an intestine that terminates into a tiny anal opening on the telson can also be observed in some specimens.