The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group

BMC Evolutionary Biology

Joachim T Haug³  Joanne Kluessendorf¹  Donald G Mikulic⁴  Derek E G Briggs²  Carolin Haug³ 

[1] Weis Earth Science Museum, University of Wisconsin - Fox Valley, Menasha, 54952, WI, USA;Yale Peabody Museum of Natural History, Yale University, New Haven, 06520, CT, USA;Department of Biology II and GeoBio-Center, LMU Munich, Großhaderner Str. 2, Martinsried-Planegg, 82152, Germany;llinois State Geological Survey, Champaign, 61820, IL, USA

关键词: Remipedia;    Predatory crustaceans;    Solnhofen lithographic limestones;    Wisconsin;    Waukesha;   

Others  :  1118009
DOI  :  10.1186/s12862-014-0159-2

 received in 2014-03-05, accepted in 2014-07-11,  发布年份 2014

【 摘 要 】

Background

Thylacocephala is a group of enigmatic extinct arthropods. Here we provide a full description of the oldest unequivocal thylacocephalan, a new genus and species Thylacares brandonensis, which is present in the Silurian Waukesha fauna from Wisconsin, USA. We also present details of younger, Jurassic specimens, from the Solnhofen lithographic limestones, which are crucial to our interpretation of the systematic position of Thylacocephala. In the past, Thylacocephala has been interpreted as a crustacean ingroup and as closely related to various groups such as cirripeds, decapods or remipeds.

Results

The Waukesha thylacocephalan, Thylacares brandonensis n. gen. n. sp., bears compound eyes and raptorial appendages that are relatively small compared to those of other representatives of the group. As in other thylacocephalans the large bivalved shield encloses much of the entire body. The shield lacks a marked optical notch. The eyes, which project just beyond the shield margin, appear to be stalked. Head appendages, which may represent antennulae, antennae and mandibles, appear to be present. The trunk is comprised of up to 22 segments. New details observed on thylacocephalans from the Jurassic Solnhofen lithographic limestones include antennulae and antennae of Mayrocaris bucculata, and endites on the raptorial appendages and an elongate last trunk appendage in Clausocaris lithographica. Preserved features of the internal morphology in C. lithographica include the muscles of the raptorial appendage and trunk.

Conclusions

Our results indicate that some ‘typical’ thylacocephalan characters are unique to the group; these autapomorphies contribute to the difficulty of determining thylacocephalan affinities. While the new features reported here are consistent with a eucrustacean affinity, most previous hypotheses for the position of Thylacocephala within Eucrustacea (as Stomatopoda, Thecostraca or Decapoda) are shown to be unlikely. A sister group relationship to Remipedia appears compatible with the observed features of Thylacocephala but more fossil evidence is required to test this assertion. The raptorial appendages of Thylacocephala most likely projected 45 degrees abaxially instead of directly forward as previously reconstructed. The overall morphology of thylacocephalans supports a predatory mode of life.

【 授权许可】

   
2014 Haug et al.; licensee BioMed Central Ltd.

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