Proclivity of nervous system preservation in Cambrian Burgess Shale-type deposits

Abstract

Recent investigations on neurological tissues preserved in Cambrian fossils have clarified the phylogenetic affinities and head segmentation in pivotal members of stem-group Euarthropoda. However, palaeoneuroanatomical features are often incomplete or described from single exceptional specimens, raising concerns about the morphological interpretation of fossilized neurological structures and their significance for early euarthropod evolution. Here, we describe the central nervous system (CNS) of the short great-appendage euarthropod Alalcomenaeus based on material from two Cambrian Burgess Shale-type deposits of the American Great Basin, the Pioche Formation (Stage 4) and the Marjum Formation (Drumian). The specimens reveal complementary ventral and lateral views of the CNS, preserved as a dark carbonaceous compression throughout the body. The head features a dorsal brain connected to four stalked ventral eyes, and four pairs of segmental nerves. The first to seventh trunk tergites overlie a ventral nerve cord with seven ganglia, each associated with paired sets of segmental nerve bundles. Posteriorly, the nerve cord features elongate thread-like connectives. The Great Basin fossils strengthen the original description-and broader evolutionary implications-of the CNS in Alalcomenaeus from the early Cambrian (Stage 3) Chengjiang deposit of South China. The spatio-temporal recurrence of fossilized neural tissues in Cambrian Konservat-Lagerstätten across North America (Pioche, Burgess Shale, Marjum) and South China (Chengjiang, Xiaoshiba) indicates that their preservation is consistent with the mechanism of Burgess Shale-type fossilization, without the need to invoke alternative taphonomic pathways or the presence of microbial biofilms.

Keywords: Alalcomenaeus; Burgess Shale-type preservation; Marjum Formation; Pioche Formation; palaeoneurobiology.

Figure 1.

Nervous system preservation in Alalcomenaeus sp. from the Stage 4 Pioche Formation. (a) MCZ IP-197956a, part with preserved soft tissues. (b) Detail of CNS in the head and trunk. (c) Detail of trunk exopods with marginal setae. (d) MCZ IP-197956b, counterpart with well-preserved paddle-shaped tailspine and marginal setae. cn, connectives; ey, stalked eye; exp, trunk exopod; hs, head shield; ms, marginal setae; of, oesophageal foramen; sgn, segmental ganglia; Tn, trunk tergites; VNC, ventral nerve cord. (Online version in colour.)

Figure 2.

Nervous system preservation in Alalcomenaeus sp. from the Drumian Marjum Formation. (a) KUMIP 204782 photographed under cross polarized light. (b) Detail of anterior half photographed with ultraviolet illumination. Cn, cephalic appendage nerves; ey, stalked eye; hyp, hypostome; hs, head shield; of, oesophageal foramen; opn, optic nerve; sgn, segmental ganglia; Tn, trunk tergites.

Figure 3.

Interpretative diagram of preserved morphology in Alalcomenaeus. (a) MCZ IP-197956a part. (b) MCZ IP-197956b counterpart. (c) KUMIP 204782. Cn, cephalic appendage nerves; cn, connectives; ey, stalked eye; exp, trunk exopod; hyp, hypostome; hs, head shield; ms, marginal setae; of, oesophageal foramen; opn, optic nerve; sgn, segmental ganglia; Tn, trunk tergites; VNC, ventral nerve cord. (Online version in colour.)

Figure 4.

Morphological reconstruction of internal anatomy in the leanchoiliid Alalcomenaeus. (a) Dorsal view. (b) Organization of CNS. (c) Lateral view showing spatial relationship between the CNS and the overlying gut tract. Note that the morphology of the great appendages and the endopods is conjectural and based on those of A. cambricus [44]. cn, connectives; Cn, cephalic appendage nerves; ey, stalked eye; of, oesophageal foramen; opn, optic nerve; SGA, short great-appendage; sgn, segmental ganglia. (Online version in colour.)

Figure 5.

Stratigraphic distribution of Burgess Shale-type (BST) deposits with CNS preservation during the Cambrian. Note that symbols are not necessarily neuroanatomically accurate for all taxa. Based on data on the present study and [–30,37,38]. Cases where only the optic nerves and neuropils are preserved are not included in the diagram (see the electronic supplementary material, table S1 for more information). (Online version in colour.)

Figure 6.

Preservation of internal structures in Opabinia regalis from the Wuliuan Burgess Shale. (a) USNM 155600, complete individual preserved in lateral view. Arrowheads point to lateral triangular extensions that most likely correspond to the lobopodous ventral limbs [62,63]. (b) USNM 205258, complete individual preserved in dorsal view. bl, body lobes; ey, stalked eye; fa, frontal appendage; g, gut tract; set, setae; tl, tail lobes.