New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA

Cheloniellids comprise a small, distinctive group of Paleozoic arthropods of whose phylogenetic relationships within the Arthropoda remain unresolved. A new form, Xus yus , n. gen, n. sp. is reported from the Waukesha Lagerstatte in the Brandon Bridge Formation (Silurian: Telychian), near Waukesha, Wisconsin, USA. Exceptionally preserved specimens show previously poorly known features including biramous appendages; this is the first cheloniellid to show large, anterior raptorial appendages. We emend the diagnosis of Cheloniellida; cephalic appendages are uniramous and may include raptorial appendages; trunk appendages are biramous.

Here we report a new arthropod with large raptorial appendages that suggests inclusion as a 'great appendage arthropod' in the broad sense, but which is also referable to the order Cheloniellida (Figs. 1,2).A large number of specimens from the Waukesha Lagerstätte in the Brandon Bridge Formation (Silurian) of southeastern Wisconsin offer the opportunity to describe previously unknown or little-known features in the group.Cheloniellids are a group of nonbiomineralized or lightly skeletonized Paleozoic arthropods characterized by a wide, ovoidal carapace, forward curved or angled posterior cephalic margin and pleural tips that are anterolaterally directed at the front and become increasingly posterolaterally directed toward the rear (e.g., [19][20]).This new form is among the best preserved fossil cheloniellids and allows for a rediagnosis of the group.

Material and Methods
This study is based principally on five of the better preserved specimens collected from the Waukesha Lagerstätte at the Waukesha Lime and Stone Company quarry near Waukesha, Wisconsin.Specimens are deposited in the University of Wisconsin Geology Museum, Madison, Wisconsin, USA (UWGM).
Specimens were photographed with a Canon EOS Rebel T3i Digital SLR with a Canon MP-E 65 mm macro lens and full spectrum lighting.Specimens, where noted, were whitened with ammonium chloride to enhance contrast.Images were z-stacked using Adobe Photoshop CC and measurements were made using ImageJ.
Fossils occur either in thinly laminated, fissile, organic-rich, argillaceous dolomudstone (termed flinz; see [26]) or thinly laminated, non-fissile, dolomudstone (termed fäule; see [26]).All but one specimen reported here is preserved in flinz.Specimens from flinz layers are preserved by a coating of authigenic phosphate overlying a thin dark organic film and are generally more detailed [24][25].UWGM 2345 is in a fäule layer and is preserved by a thin coating of organic film.
Sedimentary and microbial structures in the Brandon Bridge Formation indicate that organisms are parautochthonous, having been washed in from nearby areas and accumulated in .CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprint small sedimentary traps along a tidally influenced shoreline [24][25].Organisms were neither consumed nor readily decayed, thus increasing their chances of preservation.High salinity or other environmental factors were evidently limiting to most macroscopic organisms.Limited bioturbation indicates that this environment was devoid of most burrowers and grazers that allowed a diverse microbial biota, including microbial mats, to develop and flourish.Microbialmediated processes, such as authigenic mineralization, facilitated the preservation of much of the Waukesha Biota [24][25] including non-biomineralized and lightly skeletonized organisms (compare [27][28][29][30][31][32]).

Systematic paleontology
Phylum Arthropoda von Siebold, 1848 [33] Class Uncertain Order Cheloniellida Broili, 1932 [34] Emended diagnosis.Small to medium-size, biramous, arthropods having wide, ovoidal, flattened, non-biomineralized or lightly skeletonized dorsal exoskeleton.Cephalon short, with suture separating procephalon from gnathocephalon; eyes present; trunk wider than cephalon, with 8 to 13 tergites; trunk normally with narrow, convex medial area and nearly flat plural areas; distal ends of first few tergites directed anterolaterally, becoming increasingly posterolaterally directed rearward.Terminal tergite cylindrical or round, bears anus, and encompassed by previous tergite.Caudal assembly consists of an elongate, forked, posteriormost segment bearing anus; elongate medial spine may articulate with it.Cephalic appendages .CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprint uniramous, consisting of long paired antennae and five pairs of endopods, all except the first endopod bear ganthobases.One appendage pair may be modified to raptorial appendages.Trunk appendages biramous, with one pair per segment, consisting of an endopod and shorter, brushlike exopod, not present on terminal tergite. Discussion.Cheloniellids, as reviewed by Dunlop [20], range from the Early Ordovician (Tremadoc) to the Early Devonian (Emsian).Prior to this paper, six monospecific genera were described and referred to the group: Triopus drabowiensis [35] (Ordovician of the Czech Republic), Duslia insignis [36] (Ordovician of the Czech Republic and Morocco), Cheloniellon calmani [34] (Devonian of Germany), Neostrabops martini [37] (Ordovician of the USA), Pseudarthron whittingtoni [38] (Silurian of Scotland), and Paraduslia talimaae [20] (Devonian of Russia).In addition, an undescribed cheloniellid, has been illustrated from the Ordovician of
Cheloniellon has a reduced head segment, which we regard as a procephalon, with a larger segment (gnathocephalon) bearing gnathobases.
Cheloniellids have a wide, ovoid trunk with between 8 (Pseudarthron) and 12 (Xus n. gen.) tergites.Tergites are narrow medially and wider laterally.The anterior 2-3 pleural tips are directed forward, and the rest become increasingly posterolaterally directed toward the rear.
Cheloniellon has an expanded second to last tergite ( [50], fig 11a).Some forms have a marginal spiny fringe around the entire organism (Duslia, Moroccan form and Xus n. gen.).
The terminal tergite is either cylindrical or round and partly surrounded by the previous tergite.Both Stürmer & Bergström [50] and Cotton & Braddy [49] reported a segment behind the terminal tergite that they referred to as a telson in cheloniellids.Cotton & Braddy [49] described a 'faint tergite boundary' on the terminal tergite indicating the presence of a small telson in Duslia ( [51], plate 57, fig.4).We consider this to likely be taphonomic in origin, though it could be construed as evidence of a median spine.Similarly, Cheloniellon was reconstructed with a large telsonic segment, behind the insertion of the furcae ( [50], figs 1, 2).However, figured specimens do not appear to reflect this reconstructed morphology (compare [50], figs 10,11).[50] and Cotton & Braddy [49], we find no evidence of a telson behind the terminal tergite in cheloniellids.

Unlike Stürmer & Bergström
Cheloniellids display a wide range of morphological variation in the postabdomen.Genus XUS n. gen.
Etymology.From Latin, X, hunter, and US, extraordinary (masculine), in reference to the raptorial appendages.
Diagnosis.As for the species.
. CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprint Type species.Xus yus n. sp. Diagnosis.Exoskeleton small, wide, ovoidal, nearly flat except for convex eyes and axis with spiny marginal fringe.Cephalic shield short and wide; anterior margin rounded, thickened, upturned; posterior margin angled forward.Lateral eyes large, located midway from anterior margin; paired suture extending from anteromedial area to eyes; trunk with 11-12 tergites each with a thickened rim; first two tergites directed anterolaterally, third tergite directed laterally, remaining tergites become increasingly posterolaterally directed; terminal tergite round with centrally located anus; caudal apparatus articulates to terminal tergite and is composed of paired lateral spines and separate longer medial spine; cephalic appendages consisting of small paired setal appendages and large paired, laterally oriented raptorial appendages; trunk appendages biramous?, hooked endopod and possible brush-like exopod; appendages present on all but last tergite.Etymology.From Latin, yus, a three-pronged spear, in reference to the caudal assembly.).This condition entered into the original interpretation of this unusual arthropod ( [22], fig 21).The positions of the pleural margins in some specimens can be approximated by the more easily identifiable microbial decay halo (see [53]), which lies just beyond the ends of the tergites (Figs 1F, 1L).The cuticle of the pleural regions may have been quite thin in life, which leads to some difficulty in interpretation.Sufficiently well-preserved material is necessary for correct interpretation of morphology and affinities.Without the thin lateral areas of the carapace, this animal resembles a branchiopod crustacean (as noted by Mikulic et al. [21][22]).

Discussion
Non-biomineralized arthropod cuticle, primarily composed of chitin, is relatively rare in the fossil record [54].In most environments, it is readily digested by microorganisms and macroorganisms [53].Specific environments have been shown to limit biodegraders and the biodegradation of chitin, allowing the chitin to survive long enough to fossilize.Mikulic et al. [21][22], Wendruff [24] and Wendruff et al. [25] discussed paleoenvironmental and depositional conditions leading to exceptional preservation in the Waukesha Lagerstätte.The Waukesha Biota was deposited in a microbially-rich environment that facilitated exceptional preservation [24][25].Macroorganisms including Xus yus were transported into a nearshore environment where breakdown of chitin was inhibited.Non-biomineralized and lightly skeletonized arthropods in the Waukesha Lagerstätte tend to be compressed and have secondary phosphatic overgrowths [24][25]55].Overgrowths are commonly crinkled and tend to distort or obscure morphologic features [24][25]55].
All observed specimens of Xus yus are articulated.Taphonomic experiments demonstrate .CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprint that arthropods can remain at the sediment surface for weeks before complete disarticulation [56][57].Limbs and gills are among the first structures to be lost in arthropods.They begin to separate from the exoskeleton in about one week [56][57][58].Endopods are commonly present, whereas the gilled exopods are commonly lost.Babcock & Chang [56] and Babcock et al. [57] found that the first structures to disarticulate in extant arthropods are gills, followed by limbs. Most specimens retain some walking legs.All but one specimen of X. yus has lost the brush-like structures.If these structures are gills, then the gills were the first body parts lost in X. yus, which would suggest burial within about two weeks of death.Ambiguity in the limb structure of cheloniellids should be noted.Within the cephalon of Cheloniellon, unusual brush-like structures were noted on 'uniramous' appendages by Stürmer & Bergström ([50], fig 5).Similar structures are preserved on a ventrally preserved specimen of Xus yus (Figs 1G, 1I) across the head and trunk.These brush-like elements may represent gilled exopods or possibly structures that supported the marginal frill.We infer that these structures were lost quite readily, whether they functioned as gills or not, and we do not suggest homology or function.
The general morphology of Xus yus indicates affinity with cheloniellids supported by the posterior margin of the head, widely ovoid body, posterolaterally trending tergites and caudal furcae among other features.However, it possesses some unusual morphology that is unique within the chelioniellids including raptorial appendages (Fig 1J ) and a  Originally, X. yus was interpreted by Mikulic et al. [21][22] as bearing affinities to either branchiopod or remipedian crustaceans on the basis of large, specialized cephalic appendages.
Without a large number of differentially preserved specimens, the inferred affinities of X. yus would be different.In general, chelionellids could easily be confused with a number of other Paleozoic arthropods that have grossly similar body forms including the trilobite Burlingia [59] (see also [60][61][62]) and non-biomineralized arthropods, such as Arthroaspis [63] and aglaspid-like arthropods [64].Even some extant arthropods share similar morphology including serolid isopods (compare [65]) and larval water penny beetles (compare [66]).Cheloniellids such as Xus yus also could be confused with great appendage arthropods.Without a preserved pleural region, the raptorial appendages appear as more prominent structures.

.
CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprintCheloniellid species with preserved posterior regions have caudal furcae attached to the terminal tergite.Cheloniellon possesses long, flexible furcae, whereas Paraduslia, Duslia, the Moroccan form and Xus n. gen.possess short furcae.Species with short furcae are further differentiated by the structure and shape of the furcae, seemingly being either flexible tassels (Paraduslia) or inflexible spines (Duslia, Moroccan form and Xus n. gen).Xus is further differentiated by the presence of a separate, longer medial spine.Appendages are poorly known in cheloniellids.Prior to this description, Cheloniellon was the only described species with preserved appendages.Cheloniellon has six uniramous paired cephalic appendages, including long, narrow antennae and five endopods; the posterior four endopods bear gnathobases ([50], fig 7).An unusual brush-like structure was also noted by Stürmer & Bergström [50] on the second cephalic appendage but it was not included in the reconstruction or diagnosis.The undescribed Moroccan cheloniellid preserves long antennae; no other appendages are apparent ([1], fig 2b; [39], figs 2a, b).Trunk appendages are biramous, based on several frilled, ovoid elements on one specimen of Cheloniellon that are interpreted to be gilled exopods ([50], fig 8).

Holotype.
UWGM 2439 (Figs 1A, 1B, 1C).Paratypes.UWGM 2345 (Fig 1D), UWGM 2436 (Figs 1E, 1F, 1H, 1J, 1K); UWGM 2575 (Figs 1G, 1I); UWGM 2437 (Fig 1L).Additional material.14 specimens.Occurrence.Silurian (Llandovery, Telychian), occurring in a 12 cm layer in the lower Brandon Bridge Formation, Waukesha Lime and Stone Company quarry, north of State Highway 164, Waukesha, Wisconsin, USA.Diagnosis.As for genus.Description.Exoskeleton nearly flat except for eyes and axis and likely weekly mineralized.Length ranging to at least 45 mm.Body width approximately 70% of the body length excluding furcae.Cephalic shield has short, wide, rounded anterior margin with upturned edge; posterior margin angled forward.Lateral eyes large, ovoid located midway between anterior and posterior margins, directed anteromedially.Cephalic shield raised medially between eyes extending .CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprint posteriorly through trunk.Abdomen ovoid, wider than cephalon with 11-12 tergites present.Trunk widest at tergites 5 and 6.Medial region moderately wide with thickened cuticle.Pleural region cuticle thin, commonly poorly preserved.Tergites appear wider laterally and thinner medially.Anterior two pleural tips directed forward, third tergite is directed laterally and the rest become increasingly posterolaterally directed toward the rear.Terminal tergite is circular and bears anus medially.Caudal apparatus composed of two parts, a forked sclerite with stout, bowed posteriorly directed spines and a moderately long, stout median spine.Forked sclerite abuts and wraps around edges of the terminal segment.Lateral spines situated ventrally, whereas the median spine is dorsally situated.Two pairs of appendages observed on the cephalic shield.Small paired setae bearing appendages and raptorial appendages extending anterolaterally, consisting of approximately five elongate podomeres.Base of the raptorial appendage attaches adjacent to inferred position of the mouth.Distal podomere articulates at 90° to preceding podomere; terminates in a sharp, stout, tip.Trunk limbs extend just beyond the medial region and consist of sharply hooked endopod.Brush-like structures may represent exopods.Discussion.Xus yus is one of the more common organisms reported by Mikulic et al. [21-22] from the Waukesha Lagerstätte.It is known from more than 20 specimens.UWGM 2439 (Figs 1A, 1B) and UWGM 2345 (Fig. 1D) are preserved in dorsal view, and all others are in ventral view.Some specimens are partly split through the exoskeleton (Figs 1A, 1B, 1E, 1I, 1L).Studied specimens are predominately incomplete and show varied preservational conditions including the loss of cephalic appendages (Fig 1L), differential preservation of medial and pleural regions of .CC-BY 4.0 International license under a not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available The copyright holder for this preprint (which was this version posted September 7, 2018.; https://doi.org/10.1101/407379doi: bioRxiv preprint the abdomen (Fig 1E, 1I), and secondary authigenic, early mineralization (Fig 1E).Often specimens do not preserve the pleural regions well, giving the appearance of a more slender organism (compare Fig 1E to 1D medial spine (Fig 1K).
peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.It is made available