|Habelia optata Walcott, 1912|
Aria & Caron, 2017.
Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile “panchelicerate” body plan.
In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates.
Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheirans Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods—Chelicerata and Mandibulata—and implies that this diversification was facilitated by the expansion of durophagous niches.
Keywords: Arthropoda, Chelicerata, Convergence, Macroevolution, Cambrian, Burgess Shale
Superphylum Panarthropoda Nielsen, 1995.
Phylum Euarthropoda Lankester, 1904.
Clade Arachnomorpha Heider, 1913 (= Arachnata Lauterbach, 1973).
Diagnosis (emended from Størmer, 1944). Euarthropods with the following characters: Cephalic shield encompassing at least four pairs of appendages with well-developed endopods; originally, presence along body of at least one pair of appendages with basipod differentiated into a well-sclerotized gnathal sclerite bearing setae or teeth (“gnathobasic appendage”); third gnathobasic cephalic appendage also part of groundplan; post-cephalic endopods terminating in a trident of claws with various arrangements.
Order Habeliida, ord. nov. Aria and Caron
Type family. Habeliidae Simonetta and Delle Cave, 1975.
Other included taxa. Sanctacarididae Legg and Pates, 2016.
Diagnosis. Arachnomorph arthropods with the following characters: Cephalic shield with sub-triangular, sub-horizontal pleural expansions and with antero-lateral notches accommodating pair of lateral compound eyes with no peduncle; cephalic shield with large mesio-dorsal bulge accommodating stomach; five pairs of anterior, slender and segmented antennule-like exopods likely inserted below the eyes and dorsally to other head appendages; on ventral side of head, reduced pair of appendages inserted anteriormost (presumed in Sanctacarididae), followed by five pairs of appendages composed of gnathobasic basipods increasing in size posteriad and bearing seven-segmented spinose/setose enditic endopods projecting anteriad; trunk bearing paddle-like exopods fringed with thin lamellae.
Remarks. We maintain the family Sanctacarididae erected by Legg and Pates , since 10 trunk segments and a spatulate telson remain diagnostic of Sanctacaris uncata, Utahcaris orion  and Wisangocaris barbarahardyae . The affinity of Messorocaris magna  is less clear, but the peculiar shape of its trunk pleurae may place it in its own family.
Habelia had previously been assigned to the orders Aglaspina by Walcott and Emeraldellia by Størmer . Given the lack of cladistic support for these taxa, which would be para- or polyphyletically nested within Arachnomorpha, the lack of redescription for Molaria, and the fact that their diagnoses should be extensively revised in light of the new data gathered on aglaspidids and Emeraldella, we have not reused Aglaspina or Emeraldellia herein.
Diagnosis. Habeliidan euarthropods with the following characters: Body elongate, 19-segmented, divided into three distinct tagmata: cephalon (or “prosoma”) of seven segments (or eight somites) and trunk (12 segments) composed of a five-segmented thorax (or “mesosoma”) and eight-segmented post-thorax (or “metasoma”); trunk tagmatization based on discrete limb differentiation between thorax and post-thorax; posteriormost cephalic appendage (7th) similar to thoracic appendages, all characterized by a cheiromorph morphology: large undifferentiated basipods, well-developed seven-segmented endopods without endites, and paddle-like exopods fringed with oblanceolate lamellae; telson elongate.
Remarks. We hereby establish a diagnosis for the family Habeliidae, as the original publication of the taxon was not associated with one ; we also formalize diagnoses and descriptions for Habelia optata hereafter. The genus Thelxiope was also included in Habeliidae by Simonetta and Delle Cave; however, the presence of eight post-cephalic tergites and a pygidium would rather seem to indicate a relationship with Mollisonia [54, 55]. Thelxiope is therefore removed from Habeliidae.
Genus Habelia Walcott, 1912.
Type species. Habelia optata Walcott, 1912.
Diagnosis. Habeliid arthropod with the following characters: Post-ocular lateral and postero-lateral cephalic margins as well as pleural margins of trunk segments adorned with triangular spines; cuticular surface of cephalon and posterior portion of trunk segments richly adorned with small blunt spines/tubercles; cephalic gnathobases with elongate proximal “arm”; gnathobasic teeth differentiated antero-posteriorly (slender and long to short and stout); cephalic endopods with setal brush on podomeres 5 and 6; five-segmented thorax bearing strong biramous appendages with robust, clawed endopods and long basipods; very long (subequal to slightly greater than head and trunk length) bipartite telson, with a long, dentate proximal portion adorned with lateral spines, and a short distal portion about 1/3rd as long as proximal portion.
Cédric Aria and Jean-Bernard Caron. 2017. Mandibulate Convergence in An Armoured Cambrian Stem Chelicerate. BMC Evolutionary Biology. 17:261. DOI: 10.1186/s12862-017-1088-7
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