Megacheira (Order: Leanchoiliida, stem group arthropods)
The phylogenetic position of Leanchoilia is controversial. Some studies align it with the arachnomorphs, a group including trilobites and chelicerates (Wills et al., 1998; Cotton and Braddy, 2004), while others group Leanchoilia with Alalcomenaeus, Yohoia and Isoxys together in the Megacheira, the “great appendage” arthropods (Hou and Bergström 1997). Megacheirans have been suggested to either be stem-lineage chelicerates (Chen et al., 2004; Edgecombe, 2010), or the stem-lineage euarthropods (Budd, 2002).
Leanchoilia – from the Scottish name Leanchoil, the name given to a now defunct railway station on the Canadian Pacific Railway southwest of Field in Yoho National Park.
superlata – from the Latin superlata, “exaggerated.”
Holotypes – USNM 57709 (L. superlata),USNM 155651 (L. persephone),USNM(155648) (L. protagonia) in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: L. persephone from Walcott Quarry and Raymond Quarry on Fossil Ridge, as well as other sites on Mount Stephen and Mount Field; L. protagonia from the Walcott Quarry.
Other deposits: L. illecebrosa from the Lower Cambrian Chengjiang biota (Liu et al., 2007); L.? sp. protagonia, and L.? hanceyi from the Middle Cambrian of Utah (Briggs et al., 2008).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
Burgess Shale and vicinity: The Walcott and Raymond Quarries, Fossil Ridge. Additional localities are known on Mount Field and Mount Stephen – Tulip Beds (S7).
Other deposits: L. superlata,, from the Middle Cambrian of Utah (Briggs et al., 2008).
Brief history of research:
Leanchoilia superlata was first described by Walcott in 1912, and later revised by Simonetta (1970), who added the second and third species L. persephone and L. protagonia. L. superlata was later restudied in detail by Bruton and Whittington (1983) and García-Bellido and Collins (2007), who included in their study an analysis of L. persephone. The three-dimensionally preserved gut of Leanchoilia was analyzed by Butterfield (2002). A more detailed description of L. protagonia was provided by Briggs et al. (2008) who also identified L. superlata from the Cambrian sediments of Utah.
The body is convex in cross section and is widest in the posterior part of the trunk. The largest animal recorded is 12 cm long, including appendages. The head shield has a pointed anterior with a distinct upward-curving snout. The lateral edges of the head shield are serrated with short spines. The head bears a pair of frontal appendages, often referred to as “great appendages,” each consisting of three branches terminating in long flexible flagella, followed by two pairs of appendages that are segmented and branch into two (biramous), with large and flat outer gill blades. Two pairs of simple eyes are present below the head shield and the mouth was positioned just behind the base of the great appendages.
The trunk is composed of 11 segments with two dorsal angular peaks (or carinae) along the midline. The trunk also appears serrated, with each segment having short spines along the edges. Each segment bears one pair of biramous appendages similar to the ones on the head. The inner branch of the limb attaches to a small coxa with no spines and has small elongate spines along its podomeres, with the last terminating in small claws. The telson is triangular and is fringed on both sides with 11 long and straight lateral spines. Serially repeated three dimensional structures along the body have been interpreted as mid-gut glands preserved in phosphate. L. persephone and L. protagonia lack a frontal snout and have shorter great appendages, with the latter having an elongate telson with six pairs of long spines. L. persephone and L. superlata have been interpreted as potential sexual variants (see García-Bellido and Collins, 2007)
L. superlata is rare in the Walcott Quarry (0.1% of the community, Caron and Jackson, 2008) but is abundant in the Raymond Quarry, with more than 1,200 specimens known from that site. L. persephone occurs in both localities but represents only a fraction of the number of L. superlata specimens. L. protagonia is extremely rare and is currently known from two specimens in the Walcott Quarry.
Deposit feeder?, carnivorous?
The large gill branches could have been used for respiration as well as for swimming. The lack of strong sclerotization of the limbs and strong hinge joints suggest that this animal was not adapted for walking. The flagellae of the great appendages are thought to be sensory organs. The great appendages would have presumably folded backwards below the trunk while the animal was swimming, and pointed forwards when the animal was resting or feeding. The distal claws and the leg branches could have been used to dig through the superficial level of the mud and to bring particulate matter or small prey items towards the mouth. More recently, the presence of mid-gut glands, eyes, and spines along the limbs have also been interpreted as potential evidence for scavenging or predatory habits. The animal would have probably swum just above the sea bottom in search of food.