|Period||Cambrian 517 million years ago (Emu Bay-Burgess Shale)|
Waptia fieldensis was a species of arthropod from the Middle Cambrian Burgess Shale Lagerstätte of Canada. It grew to a length of about 8 cm (3 in) and resembled modern shrimp in both morphology and habit. It had a large bivalved carapace and a segmented body terminating into a pair of tail flaps. It was an active swimmer, feeding on organic particles it gathers from the seafloor substrate. Based on the number of individuals, Waptia fieldensis is the third most abundant arthropod from the Burgess Shale Formation, with thousands of specimens collected. It was among the first fossils found by the American paleontologist Charles D. Walcott in 1909. He described it in 1912 and named it after two mountains near the discovery site – Mount Wapta and Mount Field. Waptia fieldensis is the only species classified under the genus Waptia. Although it bears a remarkable resemblance to modern crustaceans, its taxonomic affinities remain unclear. It is currently classified as a stem group crustacean and tentatively included in the clade Crustaceomorpha.
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.
Fossils of Waptia fieldensis 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.
Waptia fieldensis was one of the first fossils discovered by Charles D. Walcott from the Burgess Shale in August 1909. A rough sketch of Waptia is present in his diary for August 31, 1909, alongside sketches of Marrella and Naraoia. A formal description for the species was published by Walcott in 1912. The species was named after the two mountains connected by the Fossil Ridge containing the Burgess Shale locality, Mount Wapta and Mount Field of Yoho National Park, British Columbia, Canada. The name of Mount Wapta itself comes from the First Nation Nakoda word wapta, meaning "running water"; while Mount Field was named after the American telecommunications pioneer Cyrus West Field.
Specimens of Waptia fieldensis were recovered from the Burgess Shale Lagerstätte of Canada, which dates from the Middle Cambrian period (510 to 505 million years ago). The locality was once about 200 m (660 ft) underwater; it was located at the bottom of a warm and shallow tropical sea adjacent to a submarine limestone cliff (now the Cathedral Limestone Formation). Undersea landslides caused by the collapse of parts of the limestone cliff would periodically bury the organisms in the area (as well as organisms carried by the landslides) in fine-grained mud that later became shale. Based on the number of individuals, Waptia fieldensis constitutes about 2.55% of the total number of organisms recovered from the Burgess Shale, and 0.86% of the Greater Phyllopod bed. This makes them the third most abundant arthropods of the Burgess Shale (after Marrella and Canadaspis). The National Museum of Natural History alone houses more than a thousand specimens of the species from the Burgess Shale. Waptia fieldensis are often found disarticulated, with parts remaining in close proximity to each other. Several possible specimens of W. fieldensis were also recovered from the Middle Cambrian Spence Shale member of the Langston Formation in Utah in 2008.
Waptia fieldensis is the only species accepted under the genus Waptia. It is classified under the family Waptiidae (established by Walcott in 1912), the order Waptiida (established by Leif Størmer in 1944), and (tentatively) the clade Crustaceomorpha. Early restorations of Waptia fieldensis by Charles R. Knight in 1940 erroneously depicted it as a primitive shrimp, but modern paleontologists now consider it among the Burgess Shale arthropods that are of unknown and uncertain taxonomic placement.
Side view with carapace pushed over forward so as nearly to reverse the position of the dorsal line.
Dorsal view of a specimen flattened on the shale. Both photographs are from Walcott's 1912 paper first describing W. fieldensis. Some authors have suggested that it may be allied to crustaceans, but like many Cambrian crustaceomorphs, the mouthparts were not preserved, making it impossible to reliably classify them within the crustacean crown group. Others propose that it may only be distantly related to crustaceans, being at least a stem group of crustaceans or even of all arthropods. Despite being one of the first species recovered and being the third most abundant fossils of the Burgess Shale, there is still no in-depth work on the morphology of Waptia fieldensis. In 1975, an apparently very similar species was described from the Lower Cambrian (515 to 520 million years ago) Maotianshan Shale Lagerstätte of Chengjiang, China. It was originally placed within the "ostracod"-like genus Mononotella, as Mononotella ovata. In 1991, Xian-Guang Hou and Jan Bergström reclassified it under the new genus Chuandianella when additional discoveries of more complete specimens made its resemblance to W. fieldensis more apparent. Like W. fieldensis, Chuandianella ovata had a bivalved carapace with a median ridge, a pair of caudal rami, a single pair of antennae, and stalked eyes. In 2004, Jun-Yuan Chen tentatively transferred it to the genus Waptia. However, C. ovata had eight abdominal somites in contrast to five in W. fieldensis. Its limbs were biramous and were undifferentiated, unlike those of W. fieldensis. Other authors deemed these differences to be enough to separate it from Waptia to its own genus. In 2002, a second similar species, Pauloterminus spinodorsalis, was recovered from the Lower Cambrian Sirius Passet Lagerstätte of the Buen Formation of northern Greenland. It was also identified as a possible waptiid. Like C. ovata it had biramous undifferentiated appendages, but it also had only five abdominal somites like W. fieldensis. However, the poor preservation of the P. spinodorsalis specimens, particularly of the appendages on the head, make it difficult to ascertain its taxonomic placement. This is compounded by the evidence that the fossils of P. spinodorsalis may in fact be moults (exuviae), and not of the actual animal.
Aside from sharing a striking superficial similarity, Waptia fieldensis were also very shrimp-like in their habits. They were bottom-dwelling (nektobenthic) deposit feeders. The lack of strong feeding appendages indicates that they were more likely to feed by sifting through the sea bottom for edible organic particles. Their tails and the feather-like filaments on their posterior thoracic somites also imply that they were active swimmers.