Taxonomy
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.
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 be only distantly related to crustaceans, being at least a member of 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 difficulty is further compounded by evidence that the fossils of P. spinodorsalis may in fact be moults (exuviae), and not of the actual animal.
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