A giant stem-group chaetognath

Abstract

Chaetognaths, with their characteristic grasping spines, are the oldest known pelagic predators, found in the lowest Cambrian (Terreneuvian). Here, we describe a large stem chaetognath, Timorebestia koprii gen. et sp. nov., from the lower Cambrian Sirius Passet Lagerstätte, which exhibits lateral and caudal fins, a distinct head region with long antennae and a jaw apparatus similar to Amiskwia sagittiformis. Amiskwia has previously been interpreted as a total-group chaetognathiferan, as either a stem-chaetognath or gnathostomulid. We show that T. koprii shares a ventral ganglion with chaetognaths to the exclusion of other animal groups, firmly placing these fossils on the chaetognath stem. The large size (up to 30 cm) and gut contents in T. koprii suggest that early chaetognaths occupied a higher trophic position in pelagic food chains than today.

Fig. 1.. Holotype (MGUH 34286) of T. koprii gen. et sp. nov.

(A to C) Entire specimen. (D and E) Jaw apparatus in the anterior region of trunk. (A) Wavelength-dispersive x-ray spectrometry (WDS) map of carbon on the specimen surface. (B) Polynomial texture mapping (PTM) visualization using specular enhancement, illuminated from top left. (C) Interpretative drawing. (D) Carbon map of jaw apparatus indicating some indistinct enrichment of carbon within it. (E) PTM image illuminated from top left of jaw apparatus. (F) Interpretative drawing of jaw apparatus based on tracing of multiple illumination angles (see movie S1 for visualization of this). Bp, basal plate; Lb, lateral bars; Jw, jaw; Ja, jaw apparatus; G, gut; Tm, transverse muscles; Fr, fin rays; Ps, posterior structure; Lm, longitudinal muscles.

Fig. 2.. Additional specimens of T. koprii gen. et sp. nov.

(A) MGUH 34287, the largest preserved individual imaged with high dynamic range (HDR) based on multiple images taken with different incident illumination angles while submerged in water. (B) Interpretative drawing. (C) MGUH 34288, another very large individual preserving less detail in low angle illumination. (D) HDR image. (E) Interpretative drawing. (F) MGUH 34289, laterally preserved specimen. (G) Interpretative drawing. An, antennae; Cr, caudal region; Hd, head; Mu, muscles; Mgc, mineralized gut contents; G, gut; Tr, trunk.

Fig. 3.. Musculature in T. koprii gen. et sp. nov.

MGUH 34295, small specimen preserving phosphatized musculature in association with the ventral ganglion revealing both dorsal and ventral configuration. (A) Whole specimen in low angle illumination. (B) Close-up of phosphatized musculature, revealing musculature from both dorsal and ventral surfaces due to unequal splitting of the mineralized material. (C) Interpretative drawing, highlighting longitudinal musculature (red), transverse/circular musculature (purple), and the gut (green). Hatched lines indicate unequal splitting of the phosphatized musculature. (D) Close-up of the mineralized musculature in epirelief; stippled lines trace transverse musculature. (E) Close-up of impressions of musculature from the opposite surface in hyporelief, revealing faint impressions of transverse musculature (stippled lines) underneath the longitudinal musculature.

Fig. 4.. Digital 3D model of T. koprii gen. et sp. nov.

Reconstruction showing internal and external anatomy (red, musculature; blue, ventral ganglion; black, jaw apparatus; green, gut). (A) Lateral view. (B) Dorsal view. (C) Ventral view. (D) Ventral view excluding musculature.

Fig. 5.. Ventral ganglion comparisons and phylogenetic relationships.

(A and B) T. koprii gen. et sp. nov. MGUH 34290 and an interpretative drawing highlighting the presence of a paired set of bilobed structures (arrowed), mineralized by phosphate interpreted as lateral neuron somata of a ventral ganglion. (C and D) A small undescribed chaetognath from Sirius Passet, MGUH 34299, preserving the ventral ganglion (arrowed) as paired phosphatized structures. See also fig. S10 for more details of this specimen and details of the grasping spines and additional specimens (figs. S10 and S11). Taphonomic model for preservation of the ventral ganglion is shown in fig. S9. (E to G) Confocal laser scanning microscope images of the extant chaetognath Sagitta sp. (E) Histochemical labeling of nuclei (blue) and α-tubulin (green). (F) Same view as in (E), with only immunolocalization nuclei (blue). (G) Magnified view of the ventral nerve center and the lateral neuron somata (arrowed) enriched in nuclei (blue). (H) Summary of phylogenetic analysis (fig. S13 for full analysis) placing T. koprii on the chaetognath stem. Schematic reconstructions at the tips indicate relative association of the jaw apparatus, pedal ganglion in rotifers, and ventral ganglion.

Fig. 6.. Isoxys predation and evolution of food web tiering.

(A to C) T. koprii gen et sp. nov. MGUH 34291, specimen with several specimens of the bivalved arthropod Isoxys volucris in its gut. (A) Specimen photographed with reflective light. (B) Specimen photographed with low angle light illuminated from top left. (C) Interpretative drawing (color guide: green, gut; dark yellow, Isoxys specimens). (D) Schematic presentation of the preserved and inferred components of the pelagic food web in Sirius Passet during the early Cambrian in comparison to modern pelagic food webs, highlighting the downward shift of arthropods and chaetognaths that took place during the Paleozoic as jawed vertebrates evolved to dominate the upper tiers in the food chain.

Fig. 7.. Reconstruction of T. koprii gen et sp. nov. in the pelagic ecosystem preserved in Sirius Passet.

Other taxa shown in the foreground are Kiisortoqia, Siricaris, Kerygmachela, Pauloterminus, Kleptothule, and Isoxys. Further in the background is two radiodonts: Tamisiocaris and an amplectobeluid. Artwork by Robert Nicholls/BobNichollsArt.