Phylum EUARTHROPODA Lankester, Reference Lankester1904

Subphylum CHELICERATA Heymons, Reference Heymons1901

Family Sanctacarididae nom. nov.

Type genus. Sanctacaris Briggs & Collins, Reference Briggs and Collins1988, designated herein.

Other genera. Utahcaris Conway Morris & Robison, Reference Conway Morris and Robison1988, and Wisangocaris barbarahardyae Jago, Garcia-Bellído & Gehling. in press.

Diagnosis. Arthropods with a highly vaulted and bulbous cephalon possessing expansive lateral margins and ventral eyes situated close to the anterolateral cephalic margin, a trunk composed of 11 trunk segments, the most posterior of which lack extensive pleurae, and a paddle-shaped telson with lateral spines towards the posterolateral margin.

Remarks. As part of a recent restudy of Sanctacaris, Legg (Reference Legg2014) conducted an extensive phylogenetic analysis which resolved this taxon as the oldest crown-group chelicerate. To this analysis Utahcaris and Wisangocaris were added herein using the coding provided in the online supplementary materials (S1 and S2). This analysis resolved Wisangocaris and Utahcaris as successive sister-taxon of Sanctacaris, within the stem-lineage of Euchelicerata (Fig. 3), the clade composed of modern horseshoe crab, arachnids, their most recent common ancestor, and all its descendants. Given the morphological similarities and close relationships of Utahcaris, Wisangocaris and Sanctacaris, as resolved in the phylogenetic analysis, these taxa are assigned to a new family, Sanctacarididae, for which the more completely known taxon Sanctacaris is designated type genus.

Figure 3.

The phylogenetic position of Utahcaris orion as resolved in this study. An abbreviated consensus tree of 35 Most Parsimonious Trees of 146.67015 steps (CI = 0.497; RI = 0.867), produced using New Technology Search Options in TNT v. 1.1. (Goloboff, Farris & Nixon, Reference Goloboff, Farris and Nixon2008), with implied character weighting (k = 3).

Genus Utahcaris Conway Morris & Robison, Reference Conway Morris and Robison1988

Type species. Utahcaris orion Conway Morris & Robison, Reference Conway Morris and Robison1988 (by monotypy).

Diagnosis. Sanctacaridid with a cephalic shield that is subtriangular in dorsal aspect, a trunk composed of 11 segments, lacking paired carinae, and a two-segmented abdomen differentiated from the more anterior tergites by a lack of extensive pleurae (emended from Conway Morris & Robison, Reference Conway Morris and Robison1988, p. 30).

Remarks. In its original description, Utahcaris was compared to both Sanctacaris, and the leanchoiliid megacheirans Actaeus Simonetta, Reference Simonetta1970, Alalcomenaeus Simonetta, Reference Simonetta1970, and Leanchoilia Walcott, Reference Walcott1912. However, besides the number of trunk tergites, and the presence of a paddle-shaped telson in Alalcomenaeus, Utahcaris shares few features with leanchoiliids, instead bearing more similarity to Sanctacaris. For instance, leanchoiliids lack an abdomen; instead their posterior tergites bear reduced pleurae which become progressively more flexed towards the posterior of the animal. It should be noted that although Sanctacaris has at times been allied with megacheiran arthropods (e.g. Budd, Reference Budd2002), based mainly on the erroneous interpretation of its frontal appendages (see discussion in Legg, Reference Legg2014), few recent phylogenetic analyses have supported this assignment, instead resolving megacheirans outside crown-group Euarthropoda and resolving Sanctacaris within the crown-group amongst vicissicaudates (Legg, Sutton & Edgecombe, Reference Legg, Sutton and Edgecombe2013) or within crown-group chelicerates (Legg, Reference Legg2014).

With viccissicaudates, Utahcaris shares a posteriorly differentiated abdomen (Ortega-Hernández, Legg & Braddy, Reference Ortega-Hernández, Legg and Braddy2013). It differs from most, however, in possessing two segments rather than three, as seen in Sidneyia Walcott, Reference Walcott1911 (Bruton, Reference Bruton1981), or one, as seen in Emeraldella Walcott, Reference Walcott1912 (Stein & Selden, Reference Stein and Selden2012), cheloniellids (Ortega-Hernández, Legg & Braddy, Reference Ortega-Hernández, Legg and Braddy2013) and Sanctacaris (Briggs & Collins, Reference Briggs and Collins1988; Legg, Reference Legg2014). Utahcaris also differs from the latter, and Wisangocaris, in lacking paired tergal carinae. It is this combination of features which confirms its status as a separate taxon.

Utahcaris orion Conway Morris & Robison, Reference Conway Morris and Robison1988

Figures 1 and 2

Holotype. KUMIP 204784, by original designation.

Paratype. KUMIP 204785, designated herein.

Diagnosis. As for genus.

Description. The majority of new information regarding this taxon was retrieved from KUMIP 204785 (Fig. 2). KUMIP 204784 (Fig. 1) and KUMIP 204785 (Fig. 2) measure 87 mm, and 44 mm, from the anteriormost tip of their cephalic shield to the posteriormost tip of their telson, respectively.

Although incompletely preserved in the holotype, the cephalic shield appears subtriangular in dorsal aspect, with a pointed anterior margin, a gently rounded posterolateral margin and a straight posterior margin with a poorly developed medial emargination (Fig. 1). The centre of the cephalic shield of KUMIP 204784 possesses a slight convexity which may be due to the underlying phosphatized gut content, although it is thought to be a genuine feature based on comparisons with KUMIP 204785, which, although lacking noticeable relief, appears to have a bulbous cephalic shield (Fig. 2); this bulbous region is flanked on either side by subtriangular pleural extension (Fig. 2). An enlarged, circular compound eye is present under the anterolateral margin of KUMIP 204785 (Fig. 2). Although very poorly preserved, the remnants of cephalic appendages can be observed in KUMIP 204785 (Fig. 2).

Eleven trunk segments can be observed in KUMIP 204784 (Fig. 1). Tergites show little overlap and a slight convexity, although the exact extent of both is difficult to determine based on the available specimens, both of which show considerable post-mortem deformation. The trunk of KUMIP 204784 shows noticeable torsion, and the majority of anterior tergite pleura are missing; however, lateral pleura can be observed on the posterior thoracic tergite, namely tergites 8 and 9 (Fig. 1). These pleura are posteriorly flexed with a subtriangular posterolateral margin (Fig. 1). The pleura of the posterior two trunk somites are extremely reduced, with tergite 11 lacking subtriangular pleural extensions altogether (Fig. 1). The two posteriormost somites are further distinguished from the anterior ones in being more elongate, with tergite 11 nearly twice as long as tergite 1. Poorly preserved appendages are present beneath trunk segments 4–7 of KUMIP 204785 (Fig. 2). The presence of gnathobases and the number of podomeres could not be determined.

A simple gut, lacking accessory glands, runs through the length of the trunk, terminating in an anus between segment 11 and the telson (Fig. 1). The paddle-like telson of Utahcaris is long, accounting for 20% of total body length in KUMIP 204784, and 31% of total body length in KUMIP 204785, subcircular and possesses a strongly curved posterior margin (Figs 1, 2). The posterolateral margins are characterized by subtriangular margins which extend into the telson interior (Fig. 1). A second pair of less prominent spines is present halfway between the last trunk tergite and the more posterior telson spines (Fig. 1).

Discussion. The description of Utahcaris presented herein differs from the original of Conway Morris & Robison (Reference Conway Morris and Robison1988) in a number of key aspects. Specifically, the original describers found no evidence for lateral eyes or appendages, which, although poorly preserved, give additional insight into the ecology of this taxon (see below). The bulbous nature of the cephalon, with its subtriangular posterolateral extension is recognized for the first time, and along with the anterolateral position of the ventral eyes, further strengthens similarities to the slightly younger Sanctacaris uncata. Given the sister-taxon relationship of these taxa, and Wisangocaris, as resolved in the phylogenetic analysis herein, this means that these taxa represent the oldest currently named chelicerates.

Remarks. Our restudy of Utahcaris revealed a number of previously unrecognized features of KUMIP 204785, specifically regarding the morphology of the cephalon, including its bulbous nature and the presence of ventral eyes under the anterolateral margin, and the presence of appendages, which are otherwise lacking from the holotype.