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. 2024 Jul 24;11(7):240350.
doi: 10.1098/rsos.240350. eCollection 2024 Jul.

A long-headed Cambrian soft-bodied vertebrate from the American Great Basin region

Affiliations

A long-headed Cambrian soft-bodied vertebrate from the American Great Basin region

Rudy Lerosey-Aubril et al. R Soc Open Sci. .

Abstract

The fossil record suggests that chordates might have been minor components of marine ecosystems during the first major diversification of animal life in the Cambrian. Vertebrates are represented by a handful of rare soft-bodied stem-lineage taxa known from Konservat-Lagerstätten, including Myllokunmingia and Yunnanozoon from the Stage 3 of South China, and Emmonsaspis and Metaspriggina from Stage 4-Drumian deposits of northeast USA and British Columbia. Here, we describe the first soft-bodied vertebrate from the American Great Basin, a region home to a dozen Cambrian Konservat-Lagerstätten. Found in the Drumian Marjum Formation of Utah, Nuucichthys rhynchocephalus gen. et sp. nov. is characterized by a finless torpedo-shaped body that includes a snout-like anterior head bearing anterolateral eyes, approximately 25 thick myomeres, a large branchial chamber with a keel and approximately seven putative dorsal bars and a spiniform caudal process. Using Bayesian inference, our analysis recovers Nuucichthys within the vertebrate stem, closer to the crown than Pikaia, Yunnanozoon and Myllokunmingia, where it forms a polytomy with its Laurentian relatives, Emmonsaspis and Metaspriggina, and a scion consisting of conodonts and crown-group vertebrates. Based on the eye orientation and absence of fins, we tentatively reconstruct Nuucichthys as a pelagic organism with limited swimming abilities (planktonektic).

Keywords: Laurentia; Marjum Formation; Miaolingian; exceptional preservation; planktonekton; vertebrate origin.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Nuucichthys rhynchocephalus gen. et sp. nov. from the Drumian Marjum Formation of the House Range of Utah, USA.
Figure 1.
Nuucichthys rhynchocephalus gen. et sp. nov. from the Drumian Marjum Formation of the House Range of Utah, USA. (a,b) General views of the holotype (UMNH.IP.6084), which was photographed dry with direct light (a) or immersed in dilute ethanol with cross-polarized light (b). (c) Interpretative drawing combining details of (a,b). Abbreviations: ah, anterior head region; an, anus; bbd, dorsal branchial bar; bc, branchial chamber; cp, spiniform caudal process; ey, eye; in, intestine; ke, keel; li, liver; my, myomere; ph, posterior head region; sh, shelly fragment; ta, tail; tr, trunk region.
Nuucichthys rhynchocephalus gen. et sp. nov. from the Drumian Marjum Formation of the House Range of Utah, USA.
Figure 2.
Nuucichthys rhynchocephalus gen. et sp. nov. from the Drumian Marjum Formation of the House Range of Utah, USA. (a,b,d–f) Detailed views of the anterior region of the body (a), eye (b), possible liver (d), myomeres (e), and posterior trunk region (f) in specimen UMNH.IP.6084; specimen dry (a,b,f) or immersed in dilute ethanol (d,e) illuminated with direct light. (c,g) Interpretative drawings of (a,f). Abbreviations: ah, anterior head region; an, anus; bbd, dorsal branchial bar; bc, branchial chamber; cp, spiniform caudal process; ey, eye; in, intestine; ke, keel; my, myomere; ph, posterior head region; sh, shelly fragment; ta, tail.
Nuucichthys rhynchocephalus gen. et sp. nov. and its phylogenetic position within deuterostomes.
Figure 3.
Nuucichthys rhynchocephalus gen. et sp. nov. and its phylogenetic position within deuterostomes. (a) Reconstruction of the animal in its putative living environment (credit: Franz Anthony). (b) Simplified topology recovered from the analysis of a modified version of Tian et al.’s dataset [3] through Bayesian inference (see electronic supplementary material, figure S1, for complete tree). Numbers at the nodes indicate posterior probability values. Note that the phylogenetic placement of vetulicolians remains highly debatable, but the exclusion of these organisms from our analysis resulted in essentially the same tree topology (electronic supplementary material, figure S2).
Diversity of Cambrian non-biomineralizing stem-group vertebrates.
Figure 4.
Diversity of Cambrian non-biomineralizing stem-group vertebrates. (a,b) Nuucichthys rhynchocephalus gen. et sp. nov., Drumian Marjum Fm., Utah, USA. (a) Holotype UMNH.IP.6084, general view (dry, direct light). (b) Morphological reconstruction; note that the presence of dorsal branchial bars in this taxon is uncertain. (c,d) Metaspriggina walcotti Simonetta & Ineson [37], Wuliuan Burgess Shale Formation, British Columbia, Canada. (c) ROMIP 65347, general view (dry, polarized light); this specimen is incompletely preserved anterodorsally, resulting in the absence of the anteriormost myomeres and branchial bars. (d) Morphological reconstruction. (e,f) Emmonsaspis cambrensis (Walcott) [42], Cambrian Stage 4 Parker Slate Fm., Vermont, USA. (e) USNM 15314a (specimen 1), general view (dry, polarized light); this specimen only preserves the trunk. (f) Morphological reconstruction. (g,h) Myllokunmingia fengjiaoa Shu et al. [in 31], Cambrian Stage 3 Chiungchussu Fm., Yunnan, China. (g) RCCBYU 10200a, general view (dry, low-angle direct light); note that some details of the anterior head are not visible in this specimen. (h) Morphological reconstruction. (i,j) Yunnanozoon lividum Hou et al. [43], Cambrian Stage 3 Chiungchussu Formation Fm., Yunnan, China. (i) RCCBYU 10310a, general view (dry, direct); note that the caudal process is missing in this specimen. (j) Morphological reconstruction; note that the number and distribution of the myomeres are tentative. The reconstructions in (b,d,f,h,j) only illustrate some discriminating anatomical features discussed in the text, as they would be preserved in fossils (i.e. including decay changes, such as the shrinkage of myomeres), rather than the living organisms. Images in (c,e,i) were mirrored to facilitate comparison with the new fossil, and those in (g,i) were first published in [38] and [44], respectively. Credits: J.-B. Caron for (c,e) and D. J. Siveter for (g,i).
Palaeobiogeography and biostratigraphy of Cambrian soft-bodied vertebrates of Laurentia.
Figure 5.
Palaeobiogeography and biostratigraphy of Cambrian soft-bodied vertebrates of Laurentia. (a) Distribution of Konservat-Lagerstätten yielding soft-bodied vertebrate fossils in Laurentia; number in parentheses in (a) refers to the number of specimens known for each occurrence according to [4,59] and the present study; background map modified from [60]. (b) Biostratigraphic distribution of the Cambrian soft-bodied vertebrates of Laurentia; numbers in (b) refer to the numbering of the deposits in (a); durations of geochronologic units and approximate ages of their boundaries are from [61].

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