New insights into the appendage morphology of the Cambrian trilobite-like arthropod Naraoia compacta

Abstract

For understanding the life habits of extinct organisms, functional morphology is of crucial importance. In arthropods, the morphology of the appendages in particular gives insights into the mode of life. Yet, in the fossil record of arthropods mainly dorsal structures are preserved, as they are often more sclerotised or even mineralised. Exceptions are, for example, the species of Naraoia from the Cambrian Burgess Shale (about 508 million years old), of which the appendages are also preserved. We present here new details of the appendages of Naraoia compacta, which were made visible with modern imaging methods that were not yet available during former investigations. According to our re-study, the appendage shows a division into basipod, endopod and exopod. The basipod-body joint has not been visible to earlier investigators, and both basipod and endopod are significantly more strongly adorned than previously interpreted. The most important difference to earlier studies regards our interpretation of the exopod morphology. According to our new data, the exopod is composed of a series of similar, small paddles attached to a shaft, and not feather-like in appearance. This morphology has a strong impact on the swimming behaviour of N. compacta, as the paddles can be closed to form a functionally single large paddle during the power stroke, and will be opened during the recovery stroke to reduce the water drag. In this way, our new data provide important insights into the life habits of this long extinct species. •

Phylogenetic response of naraoiid arthropods to early–middle Cambrian environmental change

“…2007). However, this was not observed during this study, nor in other recent work (Haug & Haug 2016). Endopod preservation in the specimens of N. spinosa and M. longicaudata examined in this study was insufficient for detailed description of such features.…”

“…Obdeltoid paddles bearing spinose setae have been described along the length of the exopod shaft in this species. Such obdeltoid paddles were previously described in N. compacta specimens USNM 57868, 83945B, 189216, 189221, 189222 and 199815 (Haug & Haug 2016). These features have been confirmed here in eight specimens (USNM 83945B, 83945D, 189202, 189221, 199815, 189216 and 235843) including the lectotype, USNM 57687 (Fig.…”

“…Naraoiids are argued to have adopted a (nekto‐)benthic mode of life and, like Olenellina and Archaeocyatha, were generally restricted to low palaeolatitudes (Whittington 1977; Zhang et al . 2007; Haug & Haug 2016). Dissimilar to olenellines and archaeocyathids, which predominantly inhabited shallower marine environments (inner shelf to the upper outer shelf), Cambrian naraoiids occur in sediments that accumulated below maximum storm wave base, deposited in outer shelf environments near the margins of carbonate platforms (e.g.…”

Unusual preservation of an Ordovician (Floian) arthropod from Peary Land, North Greenland (Laurentia)

“…2f). While such lamellae have been described in the exopod of Naraoia compacta Walcott, 1912 from the Burgess Shale Lagerstätte by Haug and Haug (2016), there is little morphological similarity between the two structures or other indication that the petaloids reflect exopods or other parts of limbs.…”

Moving legs: A workflow on how to generate a flexible endopod of the 518 million‐year‐old Chengjiang arthropodErcaicunia multinodosausing3D‐kinematics (Cambrian, China)

“…Research on Cambrian arthropods from famous fossil sites like the Burgess Shale (Briggs, Erwin, & Collier, 1994; Caron & Jackson, 2008) unveiled delicately preserved species, for which accurate anatomical studies had been conducted the past years (Haug, Briggs, & Haug, 2012; Haug & Haug, 2016; Haug, Maas, Haug, & Waloszek, 2011; Haug, Maas, & Waloszek, 2009; Haug, Waloszek, Haug, & Maas, 2010; Vannier, Aria, Taylor, & Caron, 2018; Zhang, Maas, Haug, Siveter, & Waloszek, 2010). Among Cambrian fossil sites, there are vertiginous Konservat‐Lagerstätten like the unique Chengjiang Lagerstätte (Hou et al, 2017), containing impressive arthropods suitable for in‐depth 3D research (Liu, Scholtz, & Hou, 2015; Zhao et al, 2017), due to their excellent preservation (Gabbott, Hou, Norry, & Siveter, 2004).…”