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Sequence of post-moult exoskeleton hardening preserved in a trilobite mass moult assemblage from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco

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Drage H. B., Vandenbroucke T. R. A., Van Roy P., Daley A. C.

Acta Palaeontologica Polonica

2019

tom 64

nr 2

Euarthropods have a tough exoskeleton that provides crucial protection from predation and parasitism. However, this is restrictive to growth and must be periodically moulted. The moulting sequence is well-known from extant arthropods, consisting of: (i) the long inter-moult stage, in which no changes occur to the hardened exoskeleton; (ii) the pre-moult stage where the old exoskeleton is detached and the new one secreted; (iii) exuviation, when the old exoskeleton is moulted; and (iv) the post-moult stage during which the new exoskeleton starts as soft, thin, and partially compressed and gradually hardens to the robust exoskeleton of the inter-moult stage. Trilobite fossils typically consist of inter-moult carcasses or moulted exuviae, but specimens preserving the post-moult stage are rare. Here we describe nine specimens assigned to Symphysurus ebbestadi representing the first group of contemporaneous fossils collected that preserve all key stages of the moulting process in one taxon, including the post-moult stage. They were collected from a single lens in the Tremadocian part of the Fezouata Shale Formation, Morocco. Based on cephalic displacement and comparison to other trilobite moults, one specimen appears to represent a moulted exoskeleton. Four specimens are typical inter-moult carcasses. Four others are wrinkled and flattened, with thin exoskeletons compared to inter-moult specimens, and are considered post-moult individuals. These S. ebbestadi specimens illuminate the preservation and morphology of the post-moulting stage, characterised by strong anterior-posterior exoskeleton wrinkling, as well as overall body flattening and reduced visibility of thoracic articulations. Being found in the same lens, these specimens likely represent the first preserved in-the-act mass moulting event. The displayed sequence of moulting suggests the moulting process in trilobites was comparable to modern arthropods, and conserved within euarthropod evolutionary history.

Reply to Comment on “ Aysheaia prolata from the Utah Wheeler Formation (Drumian, Cambrian) is a frontal appendage of the radiodontan Stanleycaris ” with the formal description of Stanleycaris

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Pates S., Daley A. C., Ortega-Hernandez J.

Acta Palaeontologica Polonica

2018

tom 63

nr 1

As part of a comprehensive examination of all radiodontans from Cambrian localities in the USA, Pates et al. (2017a, b) and Pates and Daley (2017) revised the taxonomic affinities of several described specimens. This included the reinterpretation of two putative lobopodians, one from the Wheeler Formation (Utah, USA) and one from the Valdemiedes Formation (Spain), as frontal appendages of the radiodontan genera Stanleycaris and Caryosyntrips respectively. In their comment, Gámez Vintaned and Zhuravlev (2018) disagree with these conclusions and raise three topics for discussion: (i) anatomical features they suggest support a lobopodian affinity for “Mureropodia”; (ii) the identity of Caryosyntrips as a radiodontan, and the assignment of certain specimens to this genus; and (iii) the nomenclatural status of Stanleycaris hirpex as an invalid taxon. For (i), we dispute that the anatomical features put forward by Gámez Vintaned and Zhuravlev (2018) are biological and conclude that a lobopodian affinity for Mureropodia is untenable. In response to (ii), we provide further evidence supporting a radiodontan affinity for Caryosyntrips, and those specimens ascribed to this genus. Finally, we concur with (iii) Stanleycaris as an invalid taxon according to the International Code on Zoological Nomenclature (ICZN), and have rectified the situation by providing a valid systematic description.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

2 Daley A. C.

1 Drage H. B.

1 Ortega-Hernandez J.

1 Pates S.

1 Van Roy P.

1 Vandenbroucke T. R. A.

1 2019

1 2018

Wyniki wyszukiwania

Sequence of post-moult exoskeleton hardening preserved in a trilobite mass moult assemblage from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco

Reply to Comment on “ Aysheaia prolata from the Utah Wheeler Formation (Drumian, Cambrian) is a frontal appendage of the radiodontan Stanleycaris ” with the formal description of Stanleycaris