TY - JOUR
T1 - The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group
AU - Haug, Carolin
AU - Briggs, Derek E. G.
AU - Mikulic, Donald G.
AU - Kluessendorf, Joanne
AU - Haug, Joachim T.
N1 - We thank Klaus Westphal, Richard Slaughter and Carrie Eaton, Geology Museum, University of Wisconsin, for loan of material. Waukesha Lime and Stone Company permitted and assisted with the initial study in their quarry. Erik Tetlie prepared preliminary camera lucida drawings of the Waukesha specimens. The manuscript benefitted from the comments of Fred Schram, Seattle, and one anonymous reviewer. Part of this work was funded by National Science Foundation grant DEB 92–01048 to D. G. Mikulic (who publishes with the permission of the Director of the Illinois State Geological Survey) and J. Kluessendorf. We thank Günter Schweigert, SMNS Stuttgart; Roger Frattigiani, Laichingen; and Michael Fecke, Langenberg, for providing specimens of Jurassic thylacocephalans for study. We acknowledge those who programmed the freely available software used in this study, such as OpenOffice, CombineZM/ZP, Microsoft Image Composite Editor, GIMP 3.0, and Blender. Most of the research was carried out while JTH and CH were based at Yale. JTH was supported by a Feodor Lynen postdoctoral research fellowship from the Alexander von Humboldt-Foundation (AvH) and by Yale University. He is currently funded by the German Research Foundation (DFG) under HA 6300/3-1.
PY - 2014/8/22
Y1 - 2014/8/22
N2 - 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.
AB - 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.
KW - ISGS
KW - Remipedia
KW - Waukesha
KW - Solnhofen lithographic limestones
KW - Predatory crustaceans
KW - Wisconsin
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U2 - 10.1186/s12862-014-0159-2
DO - 10.1186/s12862-014-0159-2
M3 - Article
SN - 1471-2148
VL - 14
SP - Article 159
JO - BMC Evolutionary Biology
JF - BMC Evolutionary Biology
IS - 1
M1 - 159
ER -