Abstract
Trace fossils represent the primary source of information on the evolution of animal behaviour through deep time, and provide exceptional insights into complex life strategies that would be otherwise impossible to infer from the study of body parts alone. Here, we describe unusual trace fossils found in marginal-marine, storm- and river-flood deposits from the Middle Devonian Naranco Formation of Asturias (northern Spain) that constitute the first evidence for infaunal moulting in a non-trilobite euarthropod. The trace fossils are preserved in convex hyporelief, and include two main morphological variants that reflect a behavioural continuum. Morphotype 1 consists of a structure that superficially resembles a Rusophycus with an oval outline that possesses a distinctly three lobed axis with an elevated central ridge and regularly spaced transverse furrows that convey the appearance of discrete body segments. The anterior part is the most irregular region of the structure, and it is not always recorded. Morphotype 2 displays more elongated, tubular morphology. Careful observation, however, reveals that it comprises up to three successive morphotype 1 specimens organised in a linear fashion and partially truncating each other. Trilobate morphology and effaced transverse furrows are locally evident, but the predominant morphological feature is the continuous, elevated ridge. The detailed morphology of morphotype 1 and well-preserved, discrete segments of morphotype 2 closely resemble the dorsal exoskeleton of the enigmatic late Carboniferous euarthropod Camptophyllia, suggesting the possible affinities of the producer. Comparisons with patterns of Devonian phacopid trilobite exuviation suggest that the Naranco Formation trace fossils may have been produced by the infaunal activities of an euarthropod that anchored its dorsal exoskeleton in the firm sediment during the body inversion moult procedure. Our findings expand the phylogenetic and environmental occurrence of infaunal moulting in Palaeozoic euarthropods, and suggest a defensive strategy against predation, previously only known from trilobites preserved in open-marine deposits.
Highlights
The process of ecdysis – in which the exoskeleton is discarded and substituted with a new one in order to accommodate changes in size and/or shape during growth – represents a fundamental biological constraint that has shaped the evolution of euarthropods for more than half a billion years[1,2]
The new ichnofossils from the Naranco Formation consist of bilaterally symmetrical structures with clear antero-posterior orientation, and a distinctive trilobate morphology preserved in convex hyporelief (Fig. 3a–f)
The overall morphology and variability in the preservation of the Naranco Formation trace fossils analysed here are best regarded as the result of different facets of a single anatomy defining a behavioural continuum
Summary
The process of ecdysis – in which the exoskeleton is discarded and substituted with a new one in order to accommodate changes in size and/or shape during growth – represents a fundamental biological constraint that has shaped the evolution of euarthropods for more than half a billion years[1,2]. The studied material comes from the Middle Devonian (Eifelian-earliest Givetian12) Naranco Formation of Asturias in northern Spain (Fig. 1), which mainly consists of intercalations of sandstone and mudstone, with several oolitic ironstone beds[10]. This unit accumulated in a wave-influenced, marginal- and shallow-marine environment that was affected by fluvial discharge during storm floods. Interbedded, relatively thick and normally graded mudstone units tend to be unbioturbated
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
