Systematic Palaeontology
Euarthropoda Lankester, 1904 [21] (see discussion in ref. [22]).
Artiopoda Hou & Bergström, 1997 [1].
Xandarellida Chen, Ramsköld, Edgecombe & Zhou in Chen et al., 1996 [23].
Constituent taxa
Cindarella eucalla Chen, Ramsköld, Edgecombe & Zhou in Chen et al. 1996 [23], Luohuilinella deletres Hou, Williams, Sansom, Siveter, Siveter, Gabbott, Harvey, Cong & Liu 2018 [24], Luohuilinella rarus Zhang, Fu, & Dai, 2012 [25], Phytophilaspis pergamena Ivantsov, 1999 [26], Sinoburius lunaris Hou, Ramsköld & Bergström, 1991 [20], Xandarella mauretanica Ortega-Hernández et al., 2017 [27], Xandarella spectaculum Hou, Ramsköld & Bergström, 1991 [20].
Emended diagnosis
Artiopodans with broad semicircular head shield with stalked lateral eyes originating ventrally. Eyes are accommodated in either anterolateral notches in the head shield, or dorsal exoskeletal bulges. Head shield covers uniramous antennae and four or more pairs of biramous appendages. Natant hypostome with elongate suboval outline, situated far behind anterior margin of head shield. Head shield extended posteriorly to cover multiple thoracic tergites. First thoracic tergite variably reduced. Thoracic tergites with variable patterns of dorsoventral mismatch relative to number of biramous appendage pairs. Endopods slender and with small or no endites. Fused pygidium variable in size. Pygidium with broad median spine in most forms. Modified from ref. [28].
Remarks
The new data on Sinoburius lunaris leads us to propose a more accurate diagnosis for Xandarellida [23] that better reflects the morphology of its constituent taxa. This represents an update from the emended diagnosis provided more recently by Ortega-Hernández et al. [27], which took into consideration aspects of the ventral morphology based on Ramsköld et al. [28]. We regard the original diagnosis of the higher taxon Petalopleura Hou and Bergström, 1997 [1] as insufficiently specific given its lack of focus on characters that would effectively differentiate these euarthropods from other artiopodans. The phylogenetic analysis of Edgecombe and Ramsköld [6] supported Petalopleura based on the overlap of the anterior trunk tergites by the head shield, and the presence of an axial spine on the posterior trunk tergite. The revised morphology of S. lunaris indicates that an axial spine is in fact absent, and thus cannot be used to support this clade (see below). Given these observations, we consider that the utility of Petalopleura as a higher systematic unit is somewhat limited, and that its purpose is better embodied by Xandarellida as redefined here.
The recent description of the genus Luohuilinella [24, 25], and the suggestion that Xandarella may be present in the middle Cambrian of Morocco [27], have increased the diversity of Xandarellida to a total of seven species. However, these taxa also embody a significant degree of morphological variability [29], as exemplified by the drastically divergent patterns of dorsal exoskeletal tagmosis observed between Luohuilinella and Cindarella relative to Phytophilaspis [26]. To further complicate matters, our observations demonstrate that some characters previously considered to broadly define the group, such as dorsal eye slits or an axial spine on the posterior part of the body [see characters 7 and 23 respectively in ref. [6], are expressed in fewer than half of these taxa. Instead, we find that more reliable indicators of affinities within Xandarellida include a particular combination of characters rather than specific autapomorphies. For example, the posterior articulation of the head shield with a reduced thoracic tergite appears to be synapomorphic and the only character observable in all representatives of Xandarellida, even if this feature is also found in taxa outside this group (e.g. Zhiwenia coronata [30], Tremaglaspis vanroyi [31]. The occurrence of dorsoventral segmental mismatch between tergites and biramous limb pairs is also expressed in all members of Xandarellida that feature both appendicular and exoskeletal preservation, namely Cindarella, Sinoburius, and Xandarella [1, 6, 28]. All xandarellids also share the presence of ventral stalked eyes, although they may also be accommodated dorsally in an exoskeletal bulge (see char. 6 in ref. [6]. Although Phytophilaspis has been regarded as having dorsal sessile eyes [26, 32], published photographs of the type material suggest that the eyes are not sharply defined within the head shield (see Fig. 2a-d in ref. [26]. Instead, the smooth transition between the convex ocular structures and the head shield of Phytophilaspis closely resembles the raised exoskeletal bulges that typify Xandarella and Sinoburius, and thus is suggestive of the ventral origin of the eyes (see discussion below). Finally, where the hypostome is known in xandarellids it is consistently natant, elongate, and situated a considerable distance away from the anterior margin of the head shield, as observed in Cindarella [28], Luohuilinella [24], Phytophilaspis [26], Sinoburius (Figs. 1, 2 and 3), Xandarella spectaculum [7] and likely also Xandarella mauretanica [27].
Sinoburius Hou, Ramsköld & Bergström, 1991 [20].
Type species
Sinoburius lunaris Hou, Ramsköld & Bergström, 1991 [20].
Emended diagnosis
Head shield large relative to total body length, crescentic in outline, with paired exoskeletal bulges accommodating ventral stalked eyes situated mediolaterally. Head shield covers a pair of antennae and four pairs of biramous appendages. Antennae short, and proximally bear an antennal scale. First and second biramous limbs with elongate stenopodous exopods, morphologically distinct from trunk exopods. First endopod pair greatly reduced. Thorax consisting of seven freely articulating tergites. First thoracic tergite with reduced pleurae and partly covered by head shield. Biramous appendages consist of gnathobasic basipodite, endopod with seven podomeres without endites, and setae-bearing exopod composed of slender shaft with small distal lobe. Segmental mismatch between thoracic tergites and number of biramous appendages. Pygidium with well-developed median spine, and two pairs of smaller lateral spines. Pygidium covers at least four appendage pairs and a conical tail piece. Modified from ref. [1].
Remarks
Sinoburius is among the least studied Chengjiang artiopodans, which partly stems from the rarity of this taxon and its restricted stratigraphic occurrence compared to some of the more common components of this biota [1, 6, 20, 33], such as the nektaspidids [10]. Since its original description [20], the morphology of Sinoburius was slightly revised by Hou and Bergström [1] based on a few additional specimens, but no further details of the ventral anatomy were available given the preservation of the fossils. Likewise, Edgecombe and Ramsköld [6] addressed the dorsal morphology of this taxon based on newly figured specimens and discussed specific aspects of its exoskeletal organization in the context of Xandarellida, but were again limited by the material available. Here, we provide a more accurate diagnosis of Sinoburius based on fluorescence microscopy and new three-dimensionally preserved appendicular data obtained through X-ray based computer tomography [17,18,19].
Sinoburius lunaris Hou, Ramsköld & Bergström, 1991 [20].
1991 Hou, Ramsköld & Bergström, pp. 403, Fig. 4 [20].
1996 Chen et al., pp. 165, Figs. 214, 215 [23].
1997 Luo et al., pp. 102, pl. 2, Fig.4 [33].
1997 Hou & Bergström, pp. 83–86, Figs. 77–79 [1].
1999 Luo et al., pp. 50, 51, pl. 7, Fig. 1 [34].
1999 Edgecombe & Ramsköld, p. 264, Fig. 1 [6].
1999 Hou et al., pp. 125, Figs. 179–181 [35].
2003 Bergström & Hou, pp. 326, Fig. 3c [29].
2004 Hou et al., pp. 174, 175, Figs. 16.61, 16.62 [36].
2004 Chen, pp. 266, Figs. 414, 415 [37].
2017 Hou et al., pp. 202, 203, Figs. 20.32, 20.33 [7].
Diagnosis
As for genus.
Description
Completely articulated specimens range between 7.2 to 8.04 mm in length (sagittal), measured from the anterior margin of the cephalon to the tip of the median spine on the pygidium (Figs. 1, 2, 4, 6). The dorsal exoskeleton consists of three distinct tagmata, comprising a semicircular head shield, a thorax with freely articulating tergites, and a fused pygidium (Figs. 1, 4, 6). The head shield has a pronounced crescentic outline resulting from the semicircular anterior margin, coupled with the extensive lateral genal angles (Fig. 1a, b). The head shield (including the genal angles) is proportionately large compared to the trunk, representing approximately half of the entire body length (sag.), and is at least 1.5 times wider (transverse) than the trunk (Figs. 1, 4, 6). The genal angles are broadly triangular and terminate in a short posterior-facing genal spine (Figs. 1a, 4a, 6a). The tips of the genal spines reach posteriorly to the level of the third trunk tergite (Figs. 1a, b, 4a, b, 6a, b). The posterior cephalic margin is anteriorly reflexed on its median region, accentuating the crescentic shape of the head shield (Figs. 1, 4, 6). The head shield features a pair of ovoidal dorsal exoskeletal bulges (se char. 9 in ref. [6] situated slightly anterior relative to the midline (trans.) of the head (Figs. 4a, b, 6a, b), and that accommodate a pair of ventral stalked eyes (Figs. 4c, d, 6c). There is no evidence of ecdysial sutures or other dorsal structures. The axial region of the head shield is slightly elevated, corresponding to the bulk of the underlying body, but not developed into a morphologically discrete well-defined glabella with furrows or lobes. The elevated sagittal area narrows anteriorly into an acute termination that conveys a bullet-like appearance to the axial region (Figs. 1a, b, 4a, b, 6a, c).
The ventral side of the head shield features a thin doublure along the margins, and an elongate natant hypostome with an overall ovoidal shape (Figs. 2b, 3a, b, 4d). The hypostome occupies approximately one third of the underside of the head, and is separated from both margins of the head by a similar third both anteriorly and posteriorly (Fig. 2b). Most of the hypostome surface consists of a flattened plate with an elongate teardrop-like appearance with an elevated lateral margin rim. The marginal rim of the hypostome is further developed on the anterior tip, with both sides converging into an abruptly elevated triangular structure resembling an inverted furcula (Figs. 2b, 3a). The cephalic appendages include a pair of antennae, followed by four pairs of biramous limbs (Figs. 1c, d, 2b). The antennae originate close to the anterior margin of the hypostome, as typically observed in the deutocerebral first limb pair observed in various Cambrian euarthropods [38]. The antennae are short, and consist of five podomeres that decrease in size distally as indicated by the presence of constrictions along their length and the presence of regular transverse breakages (Figs. 3a, 4d, e). The antennae do not face forwards as typically observed in artiopodans [1, 6, 10, 24, 27, 30, 39], but rather are splayed at either side due to the strong lateral flexure of the third to fifth podomeres. We regard the preserved orientation of the antennae in YRCP 0011 as close to life position (Fig. 4d, e), whereas the antennae in YKLP 11407 appear to be bent backwards due to compression during burial as evidenced by the gap between the antennae and the body (Fig. 3a). The basal podomere is robust and cup-shaped, whereas the more distal podomeres become progressively smaller in overall size until terminating in an acute tip (Fig. 4e). In both YKLP 11407 and YRCP 0011, each short antenna is associated with an anterior-facing straight spine-like structure that originates from the second podomere (Figs. 3a, 4d, e). The spine-like structure is slightly longer than the third antennal podomere. The distal tip of the spine-like structure is located close to the anterior margin of the head shield, but does not project beyond it (Figs. 2b, 4c, d). The first pair of biramous post-antennal appendages occupy a para-oral position, and are located at either side of the hypostome (Fig. 2b), in accordance with a tritocerebral segmental origin [38]. The endopod in the first limb pair is greatly reduced in size, and consists of only five podomeres without endites (Fig. 2b). In contrast, the corresponding exopod is elongate and stenopodous; it consists of at least a dozen well-defined podomeres that taper in size distally and lack any indication of preserved setae or lamellae. At least the distal half of the exopod extends beyond the margins of the head shield (Figs. 1c, d, 2b, 6). The second pair of biramous limbs occupies a post-oral position. The endopod has a more conventional construction and size. Although it is incompletely preserved, its size suggests that it consists of approximately seven podomeres without endites and a terminal claw. The preservation of the distal podomeres indicate that the second endopod is mostly covered by the head shield, with only the terminal claw extending beyond the margins of the dorsal exoskeleton in some specimens (Fig. 2b). The corresponding exopod is similar to that of the first biramous limb pair, namely it is elongate, stenopodous, and extends far beyond the head shield margins (Figs. 1c, d, 2b). Like the first pair, the second biramous limbs does not preserve evidence of setae or lamellae on the exopods. The third and fourth biramous appendage pairs in the head are similar to each other (Fig. 5). The endopods consist of approximately seven podomeres without endites, whose terminal claws extend close to the edge of the head shield (Fig. 4c). The exopods are considerably shorter compared to the previous appendages, and resemble the corresponding structures on the trunk appendages (see description below). The basipodite is visible in the third and fourth post-antennal limb pairs of YRCP 0011, and demonstrates the presence of faint gnathobasic endites that face adaxially (Figs. 4c, d, 5).
The thorax consists of seven freely articulating tergites that overlap widely with each other (Figs. 1, 4, 6). The thorax is approximately as long (sag.) as the distance between the posterior and anterior margins of the head shield. All the tergites are approximately of subequal length (sag.), and possess well-developed pleurae that curve into posterior-facing spines. The first thoracic tergite has reduced pleural regions and is the narrowest (trans.) point of the body; this tergite is also partly concealed by the posterior margin of the head shield in life position. The thorax is widest at the level of the third or fourth tergite depending on the individual, and subtly narrows towards the pygidium. The central portion of the tergites, corresponding approximately to a third of the tergite width (trans.), is slightly raised into a weak axial lobe that continues into the corresponding region of the cephalic shield. The fused pygidium is shorter (sag.) than the thorax. The pygidium bears two pairs of broad lateral spines directed backwards, and its posterior portion consists of a broad median spine with an acute termination (Figs. 1, 4, 6). The length (sag.) of the posterior median spine varies between individuals, and can be either shorter (Fig. 4) or subequal (Figs. 1, 6) relative to the main body of the pygidium (measured without the posterior spine).
The thorax and pygidium bear a series of homonomous biramous appendages that become smaller towards the posterior end of the body (Figs. 1d, 2b, 4c, 6c). Proximally, the biramous appendages possess a differentiated basipodite that bears three ridge-like crescentic endites (Figs. 3b, 4). In the thorax, the endopods consist of seven podomeres without endites, and a single terminal claw. There is some variability in the robustness of the endopods throughout the body, with those near the anterior region being more gracile (Fig. 4) compared to the ones towards the middle portion of the thorax (Figs. 2b, 3b). The exopod is composed of a slender shaft consisting of two or three elongate podomeres, and terminated on a small rounded distal lobe. Both the slender shaft and the distal lobe bear delicate lamellae (Figs. 1b, 2b, 3b, 5), although these are best preserved on the distal portions of the exopod (Figs. 1b, 5b). Most of the trunk tergites are associated with a single pair of biramous limbs. However, segmental mismatch is expressed in the presence of diplotergites (i.e. two appendage pairs associated with one tergite, see ref. 40, 41) that appear to be variably situated along the body. In YKLP 11407, diplotergites are expressed on the fourth and seventh thoracic tergites, whereas all the other tergites feature a single appendage pair (Figs. 1, 2). By contrast, both Hz-f-10-45 (Fig. 6) and YRCP 0011 (Fig. 4) have a diplotergite on the seventh trunk tergite only, although these taxa differ in the presence of four or three biramous limb pairs under the pygidium respectively. The size and podomere number of the endopods decreases gradually towards the posterior end, with some of the posteriormost endopods featuring only five observable podomeres (Fig. 2b). The ventral side of the body shows regular changes in texture that suggest the presence of sternites, but these are not well preserved (Fig. 2b). The pygidium covers at least four pairs of biramous appendages. The reduced podomere count and small size of the endopods under the pygidium resemble limb-buds rather than fully developed biramous appendages. The posterior body termination consists of a conical tail piece without limbs (Figs. 2b, 6c).