Some other Burgess Shale arthropods, including Habelia
, have been reconstructed with a gently curved telson in the living position (Whittington 1981
; Bruton & Whittington 1983
). Although flexible cuticle has been suggested as evidence of a freshly moulted individual in Marrella
, it is rare (one in 25
000+ specimens; García-Bellido & Collins 2004
specimens with a bent telson (b
) are far too common to be considered as freshly moulted individuals. Besides, they are not moults because of the soft-part preservation.
One possible function for bearing a straight telson is accelerated recovery from an overturned posture. In order to explore some possible explanations for the behavioural differences between the straight and bent modes of Burgessia telsons, righting experiments on Limulus were conducted (e). When a Limulus is overturned, it arches its dorsal sclerites and quickly rolls to the side, bending the telson and moving the gills at the same time to right itself. One specimen (d) of Burgessia is preserved in a comparable posture. Specimens in the category B (f) are often preserved with a telson bending more than 40° from the body axis.
Based on comparative anatomy of other well-preserved arthropods in the Burgess Shale deposit, there is evidence to support the proposition that Burgessia
could use its telson to aid in overturn recovery. In some well-preserved specimens of Marrella
(figure S3 in the electronic supplementary material), there are highly reflective bands traversing the lateral and median spines, and such features were interpreted as fluid-filled canals (Whittington 1971
) or part of the nervous system (García-Bellido & Collins 2006
). A similar feature is present in the telson of at least four Burgessia
). This feature is preserved as a silvery film, and similar preservation can also be found in some Burgessia
), which are composed mainly of graphite (see Butterfield et al. 2007
). Thus, it is not an external feature. Based on this line of evidence, the Burgessia
telson probably became straight by stiffening when the animal injected body fluid (i.e. haemolymph) into the caudal spine by changing the hydrostatic pressure. Although there is no exact analogue existing among extant arthropods, haemolymph control is critical for sudden stiffening (in milliseconds) of spider legs during jumping (Parry & Brown 1959
). In addition, Burgessia
probably could manoeuvre its telson via muscular control in the same way as Limulus
). However, conclusive evidence to support this interpretation is required.
As indicated by both the preserved material and mechanics of motion, lateral flexibility (curved or sinuous) is the relaxed state. Both the dorsoventral flexibility and stiffening require active control of the telson; thus, they are considered as active states. Sudden stiffening could be achieved by changing hydrostatic pressure, and the dorsal–ventral motion was probably under muscular control. Second, some Burgessia
specimens are interpreted here as preserving signs of escape response/overturn recovery based on modern analogues (e
). Observations consistent with such interpretation include: (i) a straight/stiff telson (f
) or a strongly curved telson bent notably dorsal-ward (h
), (ii) appendages fully extended in laterally compressed specimens (j
), (iii) a carapace angled from the body axis, suggesting intense body motion (h
), and (iv) dorsoposteriorly projecting antennae, indicating an upside-down posture (i
). Finally, the telson flexibility suggests that the composition of the Burgessia
exoskeleton is different from that of Limulus
. The closest analogues are basal panarthropods (i.e. onychophorans/lobopodians; Brusca & Brusca 1990
), which also have a flexible cuticle and can change their morphology by hydrostatic, haemocoelic-based forces. Thus, Burgessia
cuticle is probably so thin that hydrostatic pressure is the only way to stiffen its telson. By contrast, this function was superseded by thickening and stiffening of the telson cuticle among extant marine arthropods.