Nuucichthys rhynchocephalus

Reconstruction of Nuucichthys rhynchocephalus（圖片來源：Lerosey-Aubril R and Ortega-Hernández J，採用 CC BY 4.0 授權）

Age

Cambrian（Drumian）

504.5-500.5 Ma

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Taxonomy

Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

Genus: Nuucichthys

Species: Nuucichthys rhynchocephalus

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Morphological description

Nuucichthys rhynchocephalus is a soft-bodied vertebrate with a torpedo-shaped body approximately four times longer than its maximum height, and bearing a ventral notch at mid-length. The total preserved length of the fossil specimen (excluding the caudal process and eyes) is 3.24 cm, with a maximum height of 0.8 cm.

The anterior head region is elongated and clearly differentiated, measuring approximately 1.3 times longer than wide, and bears a pair of large eyes projecting anterolaterally. The branchial chamber is large, occupying about one quarter of the anteroventral body, with a keel present along the ventral margin. Up to seven branchial arches may be present dorsally.

The body possesses approximately 25 myomeres that are straight or slightly curved posteriorly. The anterior myomeres are particularly thick; the first five are short, widely spaced, and nearly parallel. In the middle region, the myomeres form a zigzag pattern due to the offset arrangement of left and right half-myomeres, gradually disappearing posteriorly. The posterior myomeres are small and densely packed.

At the posterior end, a short spiniform process extends posterodorsally at an angle of approximately 35°. No caudal fin is present. Internally, an elongated ellipsoidal organ interpreted as a liver is preserved, and posterior to it lies a narrow band that may represent the intestine, terminating at a possible anus.

The holotype specimen is catalogued as UMNH.IP.6084.

Fossil of Nuucichthys rhynchocephalus；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）（圖片來源：Lerosey-Aubril R and Ortega-Hernández J，採用 CC BY 4.0 授權）

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Etymology

Nuucichthys: "Núu-ci" is the self-designation used by Indigenous peoples of Utah and Colorado, meaning "the people", combined with the Greek ichthys meaning "fish".

rhynchocephalus: from Greek rhyncho- meaning "snout" or "beak", and -cephalus meaning "head".

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Biological description

Nuucichthys rhynchocephalus lived in the sedimentary environment of the Drumian Marjum Formation during the Cambrian. It is interpreted as a planktonektic organism inhabiting the water column. The absence of fins suggests limited swimming ability, likely relying on lateral body undulations for locomotion.

The elongated anterior head region, along with the position and orientation of the eyes, indicates that it was not a benthic organism but instead occupied mid-water environments. The large branchial chamber and multiple branchial arches suggest a well-developed respiratory system. The short caudal spine may have contributed to improved swimming efficiency.

Based on morphological similarities with Emmonsaspis and Metaspriggina, it was likely a microphagous suspension feeder. The exceptional preservation of soft tissues, including eyes, myomeres, and internal organs, indicates that decay after death likely did not exceed a few weeks.

Phylogenetically, it belongs to the vertebrate stem group and is positioned closer to the crown group than Pikaia, Yunnanozoon, and Myllokunmingia. It forms a polytomy with Emmonsaspis, Metaspriggina, and a lineage including conodonts and crown-group vertebrates. Its discovery fills a geographic and stratigraphic gap in the fossil record of early vertebrates in western Laurentia and confirms that these early vertebrates were widely but sporadically distributed across the continent.

Phylogenetic tree（圖片來源：Lerosey-Aubril R and Ortega-Hernández J，採用 CC BY 4.0 授權）

（Author: Shui-Ye You）

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Reference

Lerosey-Aubril R and Ortega-Hernández J. (2024). A long-headed Cambrian soft-bodied vertebrate from the American Great Basin region. The Royal Society.

39.27, -113.28

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