3D animation of Leanchoilia superlata.
Animation by Phlesch Bubble © Royal Ontario Museum
Megacheirans are basal true arthropods with a frontal appendage pointing upward and made of multiple claws (the cheira, or “great appendage”). Leanchoilia and other leanchoiliidae characteristically also bear long filaments on their frontal appendages, called flagellae, and most likely used as sensory devices. Megacheirans are generally considered to be among the first true arthropods (that is, arthropods with both articulated bodies and appendages), and possibly the earliest representatives of the extended chelicerate lineage (Aria, 2022).
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.”
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 (Simonetta, 1970; García-Bellido & Collins, 2007); L. protagonia from the Walcott Quarry (Simonetta, 1970; Briggs et al., 2008).
Other deposits: L. illecebrosa and L. obesa from the Lower Cambrian Chengjiang biota (Liu, Hou & Bergstrom, 2007; He et al., 2017); L.? protagonia, and L.? hanceyi from the Middle Cambrian of Utah (Briggs et al., 2008).
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); L. illecebrosa and L. obesa from the Lower Cambrian Chengjiang biota (Liu et al., 2007; He et al., 2017).
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 digestive system 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. Haug et al. (2012) later provided a new revision of the fossil material for L. superlata. Due to a long history of research and its abundance in both the Burgess Shale and Chengjiang biota, Leanchoilia frequently continues to feature in studies of early arthropod evolution as a model for Leanchoiliidae, megacheirans, and more broadly basal true arthropods. Tomographic studies of juvenile specimens from China have revealed information about the development of appendages and important oral structures (Liu et al., 2014, 2016, 2020). Poorly-preserved specimens from China were used more recently to argue for the presence of detailed brains (Lan et al., 2021), but this was refuted (Aria et al., 2023). Additional tomographic investigation of an adult Leanchoilia illecebrosa specimen revealed the presence of additional lamellate gills at the base of the limbs (Liu et al., 2021).
Leanchoilia was a relatively large predator. The body lacks any biomineralization. Like other leanchoiliid megacheirans, it is characterized by flagellate frontal appendages (cheirae) made of three long claws, and a body divided into two regions (tagmata): the cephalon, covered by a single shield, and the segmented trunk. At the front of the head, leanchoiliids bore a pair of large short-stalked lateral eyes and a pair of smaller, mushroom-shaped median eyes. The megacheiran appendages were of similar simple morphology throughout the body, reflecting the typical arthropod biramous limb: sub-cylindrical basis with teeth for mastication (basipod), relatively strong walking legs (endopods), and paddle-like, semi-rigid flaps (exopods) fringed with lamellae. Pleurae (lateral extensions of the segments) were characteristically sub-horizontal and serrated. The trunk was composed of 11 segments, and expressed two strong parallel carinae on its dorsum. The tail is a single element called a telson, here spinose and the shape of spear’s tip (lanceolate).
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 only two specimens in the Walcott Quarry.
Like other leanchoiliid megacheirans, Leanchoilia used its combined sensing and grasping frontal appendages to detect and catch prey items. Food caught was brought under the body where it might have been rudimentarily masticated between the bases of limbs (basipods), before being channeled back to the mouth. As a leanchoiliid, Leanchoilia also had large digestive glands atop its gut, suggesting it was either storing food for extended periods of time, or compensating rough mastication with additional enzymes. The megacheiran body appendages, made of relatively strong walking legs (endopods) as well as paddle-like, semi-rigid flaps (exopods), would have allowed for both comfortable locomotion on the sea floor and swimming. The exopods likely served for gas exchanges (like breathing) as well, but recent studies also showed that megacheirans and other Cambrian arthropods possessed dedicated gills (Liu et al., 2021).