A new aglaspidid arthropod from the Upper Cambrian of Tasmania
2010; Wiley; Volume: 53; Issue: 5 Linguagem: Inglês
10.1111/j.1475-4983.2010.00974.x
ISSN1475-4983
AutoresJavier Ortega‐Hernández, Simon J. Braddy, J. B. Jago, Peter Baillie,
Tópico(s)Geological and Geophysical Studies
ResumoPalaeontologyVolume 53, Issue 5 p. 1065-1076 Free Access A new aglaspidid arthropod from the Upper Cambrian of Tasmania JAVIER ORTEGA-HERNÁNDEZ, JAVIER ORTEGA-HERNÁNDEZ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK; e-mail jo314@esc.cam.ac.uk Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK; e-mail s.j.braddy@bristol.ac.ukSearch for more papers by this authorSIMON J. BRADDY, SIMON J. BRADDY Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK; e-mail s.j.braddy@bristol.ac.ukSearch for more papers by this authorJAMES B. JAGO, JAMES B. JAGO Barbara Hardy Centre, School of Natural and Built Environments, University of South Australia, Mawson Lakes, South Australia 5095, Australia; e-mail jim.jago@unisa.edu.auSearch for more papers by this authorPETER W. BAILLIE, PETER W. BAILLIE School of Earth and Environment, University of Western Australia, Nedlands, Perth, Western Australia 6009, Australia; e-mail peterb@arach.net.auSearch for more papers by this author JAVIER ORTEGA-HERNÁNDEZ, JAVIER ORTEGA-HERNÁNDEZ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK; e-mail jo314@esc.cam.ac.uk Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK; e-mail s.j.braddy@bristol.ac.ukSearch for more papers by this authorSIMON J. BRADDY, SIMON J. BRADDY Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK; e-mail s.j.braddy@bristol.ac.ukSearch for more papers by this authorJAMES B. JAGO, JAMES B. JAGO Barbara Hardy Centre, School of Natural and Built Environments, University of South Australia, Mawson Lakes, South Australia 5095, Australia; e-mail jim.jago@unisa.edu.auSearch for more papers by this authorPETER W. BAILLIE, PETER W. BAILLIE School of Earth and Environment, University of Western Australia, Nedlands, Perth, Western Australia 6009, Australia; e-mail peterb@arach.net.auSearch for more papers by this author First published: 16 September 2010 https://doi.org/10.1111/j.1475-4983.2010.00974.xCitations: 13AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Abstract: The aglaspidid arthropod, Australaglaspis stonyensis gen. et sp. nov., is described from the Idamean (Upper Cambrian) of Stony Point, Montagu, north-western Tasmania. The dorsal exoskeleton comprises a semicircular cephalon with a well-defined marginal rim, acute genal angles, oval eyes, subtriangular glabella area and a subtrapezoidal hypostome. The trunk bears eleven somites with well-developed pleural spines that progressively curve backwards. Paired postventral plates cover the last trunk tergites and the base of the tailspine, which is long and characterized by a medial cleft. Bilobed ventral impressions indicate the presence of homopodous appendages, forming a food groove along the trunk. The fossil assemblage and its mode of preservation suggest that Australaglaspis originally possessed a phosphatic cuticle, but chemical traces of it have been lost because of regional metamorphism and weathering. Australaglaspis appears to be closely related to the North American aglaspidid Chraspedops modesta Raasch, 1939, based on its medially cleft tailspine. A glaspidida comprise a group of problematic arthropods found mainly in the Cambrian, although they also include a few Ordovician representatives. Initially regarded as chelicerates (Raasch 1939), the phylogenetic position of aglaspidids has been a subject of controversy, being placed at various positions within the 'arachnomorph' arthropods (Weygoldt and Paulus 1979a, b; Hou and Bergström 1997; Selden and Dunlop 1998; Wills et al. 1998; Cotton and Braddy 2004; Hendricks and Lieberman 2008). Some studies, however, have suggested that aglaspidids are more closely related to trilobites, based on the possession of an antenniform first appendage (Briggs et al. 1979), and to other 'antennate' arthropods within the mandibulate stem-lineage (Scholtz & Edgecombe 2005; Van Roy 2006) than to chelicerates. Aglaspidids are notable for their particularly scarce fossil record, so the recognition of any new representative is a matter that deserves close attention. The occurrence of Cambrian aglaspidids and aglaspidid-like arthropods, until now, has been restricted largely to the Northern Hemisphere, most notably North America (Raasch 1939; Hesselbo 1989, 1992; Waggoner 2003), the Czech Republic (Chlupáč 1963, 1965, 1999; Rak et al. 2009), Siberia (Černyšev 1945; Repina and Okuneva 1969; Andreeva 1957), Sweden (Størmer 1956; Bergström 1968, 1971) and China (Fortey and Theron 1995; Zhang and Shu 2005). Previous reports of aglaspidids from the Southern Hemisphere are only those of Quilty (1971), from the late Middle Cambrian, and Jago and Baillie (1992), from the Upper Cambrian of Tasmania. Quilty (1971, p. 21) commented on the affinities of his fossil that 'the closest taxon is probably Beckwithia Resser, although if the Tasmanian specimen belongs to that genus, it is a different species from any so far included in Beckwithia'. The aglaspidid affinities of this fossil, however, were questioned by Jago and Baillie (1992), and more recently by Van Roy (2006), whom regarded it as a problematic arthropod. Jago and Baillie (1992), therefore, reported the presence of the first possible aglaspidid from Australia, briefly describing the single specimen available at that time, and comparing it to AglaspisHall, 1862. This study comprises a detailed description of the aglaspidid arthropod previously reported by Jago and Baillie (1992), from the Upper Cambrian (Idamean) of north-western Tasmania. Since the initial report, more material has been recovered from the outcrop and, despite their generally poor preservation, it is now possible to identify these fossils as a new genus within the Order Aglaspidida. Materials and methods Seventeen specimens were used for the taxonomic description, and an additional one for chemical analysis. All the described material is deposited in the School of Earth Sciences, University of Tasmania (UTGD), under the reference numbers UTGD125504 – UTGD125519. Specimens were photographed using a Nikon D80 digital camera with an AF Macro Nikkor 60-mm lens. Specimen UTGD 125518 was gold-coated for SEM-EDX (energy-dispersive X-ray spectroscopy, 15.0 kV, WD = 23.5 mm). Geological setting The fossils were found near Stony Point, Montagu (Jago and Baillie 1992, locality details), within an Upper Cambrian siliciclastic turbidite deposited within a submarine basin–floor fan complex (Baillie and Jago 1995). The succession occurs within the upper part of the Neoproterozoic to Cambrian Smithton Basin, details of which are provided by Lennox et al. (1982), Brown (1989), Seymour and Baillie (1992), Baillie and Jago (1995) and Calver et al. (2004). Based on lithology and the presence of fragmentary dendroid graptolites, Jago and Baillie (1992) suggested that the material belongs to the same succession from which Rickards et al. (1990) described an Idamean dendroid graptolite fauna, from Scopus, about 7 km south-east of Stony Point. Late Middle Cambrian trilobites have been described from Christmas Hills, 20 km to the south of Stony Point (Jago 1976; Jago and Bentley 2007). Preservation The fossils are preserved as light brown-grey compressions within a finely laminated dark-grey siltstone. In common with other Tasmanian Cambrian faunas, the specimens described herein are tectonically distorted. The matrix is abundant in silicates and contains small nodules rich in sodium chloride, the latter because of the outcrop being exposed near the sea. The material shows clear signs of weathering as evidenced by orange-coloured stains on the exposed surfaces, including some of the specimens (e.g. TEXT-FIG. 1. , TEXT-FIG. 2. ; EXPLANATION OF PLATE 1 , EXPLANATION OF PLATE 2 ). Figure TEXT-FIG. 1. Open in figure viewerPowerPoint Australaglaspis stonyensis gen. et sp. nov. A, Holotype (UTGD 125519) complete articulated specimen with postventral plates (photograph courtesy of P. Van Roy). B, Interpreted camera lucida drawing of holotype. Abbreviations: tsp = tailspine; mc = medial cleft; gl = glabella; mr = marginal rim; ob = occipital band; pvp = postventral plates. Scale bar represents 5 mm. Figure TEXT-FIG. 2. Open in figure viewerPowerPoint Australaglaspis stonyensis gen. et sp. nov. A, Paratype (UTGD 125515) cephalon with well-preserved eyes, glabella region, occipital band and marginal rim; B, Paratype (UTGD 125511) cephalon with articulated trunk segments; C, Paratype (UTGD 125513) cephalon with well-preserved genal angles, marginal rim and occipital band; D, Paratype (UTGD 125516) articulated trunk with base of tailspine; E, Paratype (UTGD 125517) ventral imprint of almost complete specimen. Scale bar represents 5 mm. Figure EXPLANATION OF PLATE 1 Open in figure viewerPowerPoint Figs 1–5. Australaglaspis stonyensis gen. et sp. nov. 1, Paratype (UTGD 125504) dorsal exoskeleton with fragmentary cephalon, hypostome, trunk and tailspine with medial cleft; 2, Paratype (UTGD 125508) complete dorsal skeleton; 3. Paratype (UTGD 125506a) ventral imprint of trunk with bilobed ridge; 4, Paratype (UTGD 125506b) counterpart; 5, Paratype (UTGD 125510) ventral view of almost complete specimen. Scale bar represents 5 mm. Figure EXPLANATION OF PLATE 2 Open in figure viewerPowerPoint Figs 1–6. Australaglaspis stonyensis gen. et sp. nov. 1, Paratype (UTGD 125505b) ventral view of trunk and tailspine; 2, Paratype (UTGD 125505a) counterpart showing tailspine with medial cleft; 3. Paratype (UTGD 125512) ventral imprint of incomplete trunk with tailspine; 4, Paratype (UTGD 125514) exoskeleton of almost complete individual; 5, Paratype (UTGD 125507) ventral imprint of complete trunk showing imprints of the coxae; 6, Paratype (UTGD 125509) almost complete articulated trunk and tailspine with well-preserved medial cleft. Scale bar represents 5 mm. The fossil assemblage includes abundant unidentified dendroids preserved as dark carbonized compressions, fragmentary inarticulate brachiopods and a single polymerid trilobite pygidium. The pygidium is preserved as an internal mould, a style of preservation that resembles the polymerid and agnostid trilobite faunas of the Middle Cambrian Christmas Hills (Jago 1976; Jago and Bentley 2007). Similar fossil assemblages have been used as evidence that aglaspidid arthropods possessed a mineralized cuticle (Raasch 1939; Briggs and Fortey 1982), based on the preferential preservation of phosphatic fossils (e.g. graptolites, inarticulate brachiopods) and dissolution of calcitic ones (e.g. trilobite cuticle). The assemblage at Stony Point therefore provides strong indirect evidence that the fossils were originally phosphatic. Analysis of specimen UTGD 125518 did not indicate significant amounts of phosphate when compared to the better-preserved aglaspidids from North America (e.g. Briggs and Fortey 1982). The combination of the low-grade metamorphism in the region and intensive weathering because of the coastal location of the outcrop are probably responsible for leaching the mineralized component of the fossils. Despite their poor state of preservation, most specimens are represented by articulated trunks and tailspines; almost complete individuals are relatively common. There is no sign of isolated tergites or other fragments, suggesting in situ burial or minimal transport of the material. In most specimens, the body outline is highlighted by its contrast with the darker matrix. It is preserved in faint relief and can be hard to distinguish from the metamorphic lineation if not viewed under low-angle illumination (e.g. EXPLANATION OF PLATE 1 , EXPLANATION OF PLATE 2 ). Convex structures such as the marginal rim, glabella and eyes are mostly preserved as faint but well-delineated reliefs ( TEXT-FIG. 2. , TEXT-FIG. 5. ). Figure TEXT-FIG. 5. Open in figure viewerPowerPoint Interpreted camera lucida drawings of Australaglaspis stonyensis gen. et sp. nov. A, UTGD 125511 (Text-fig. 2B) showing well-preserved cephalic structures; B, UTGD 125516 (Text-fig. 2A) showing cephalon and anterior portion of the trunk. Abbreviations: tsp = tailspine; mc = medial cleft; gl = glabella; mr = marginal rim; ob = occipital band. Scale bar represents 5 mm. Morphological reconstruction The body has a length/width ratio of approximately 7:4, excluding the tailspine. Based on the size of the specimens, the total length of individuals varies from 13.5 to 21 mm (Table 1). The cephalon is semicircular in outline. The lateral margins have acute genal angles that do not develop into spines. The cephalon has a well-developed marginal rim, forming an occipital band near the trunk. The posterior margin of the cephalon is straight. The axial area shows signs of a raised subtriangular glabella that occupies a large portion of the cephalon in several specimens ( EXPLANATION OF PLATE 1 , EXPLANATION OF PLATE 2 ; TEXT-FIG. 2. , TEXT-FIG. 3. , TEXT-FIG. 4. , TEXT-FIG. 5. ). The lateral eyes are oval, positioned dorsally in front of this glabella region and in contact with the marginal rim (Pl. 1, fig. 2; TEXT-FIG. 2. , TEXT-FIG. 3. , TEXT-FIG. 5. ). The hypostome is subtrapezoidal in shape, similar to that of Aglaspis spinifer (Briggs et al. 1979; Hesselbo 1992), but the nature of its attachment is obscured by post-mortem displacement and absence of the anterior portion of the cephalon in UTGD 125504 (Pl. 1, fig. 1; Text-fig. 3A). However, its resemblance to the hypostome of Aglaspis and possible close relationship between these taxa suggests a similar mode of attachment. Table 1. Measurements (mm) of described material. Specimen Cephalon length Max width Trunk length Tailspine length UTGD 125504 n/a 5 9.5 5.5* UTGD 125505a n/a 7 10.5 6* UTGD125505b n/a 7 n/a 10 UTGD125506a 4.8 5.4 10 n/a UTGD125506b n/a 5.4 10 5* UTGD125507 n/a 6 10 4* UTGD125508 4 6 9 4* UTGD125509 n/a 4.8 7.9 5* UTGD125510 2.5 5 6 5 UTGD125511 3.5 8 n/a n/a UTGD125512 n/a 6 8 7 UTGD125513 5 9 n/a n/a UTGD125514 5 6 9 n/a UTGD125515 3 8.7 n/a n/a UTGD125516 n/a 9 15 n/a UTGD125517 4 5.2 10.4 n/a UTGD125519 4.1 9 9.5 6.5* n/a denotes missing region. *Fragmentary structure. Figure TEXT-FIG. 3. Open in figure viewerPowerPoint Interpreted camera lucida drawings of Australaglaspis stonyensis gen. et sp. nov. A, UTGD 125504 (Pl. 1, fig. 1) showing eleven tergites, hypostome and incomplete tailspine with medial cleft; B, UTGD 125508 (Pl. 1, fig. 2) showing complete morphology and cephalic structures; C, UTGD 125505a (Pl. 2, fig. 2) showing eleven tergites and tailspine with medial cleft. Abbreviations: hyp = hypostome; tsp = tailspine; mc = medial cleft; gl = glabella; mr = marginal rim; ob = occipital band. Scale bar represents 5 mm. Figure TEXT-FIG. 4. Open in figure viewerPowerPoint Interpreted camera lucida drawings of Australaglaspis stonyensis gen. et sp. nov. A, UTGD 125506a (Pl. 1, fig. 3) showing bilobed ventral impression along the trunk and underside of tergites; B, UTGD 125507 (Pl. 2, fig. 5) showing ventral imprints of the coxae and food groove; C, UTGD 125516 (Text-fig. 2D) showing almost complete articulated trunk and base of tailspine. Abbreviations: tsp = tailspine; mc = medial cleft; gl = glabella; mr = marginal rim; ob = occipital band; bas = basipod. Scale bar represents 5 mm. Jago and Baillie (1992) described the trunk as probably comprising about 12 tergites, with rather pointed pleurae. Close examination of the new fossils shows that the trunk is actually composed of eleven tergites with well-developed pleurae that increase in curvature posteriorly (Pl. 1, figs 1–2; TEXT-FIG. 1. , TEXT-FIG. 2. , TEXT-FIG. 3. - TEXT-FIG. 6. ). The pleurae bear short spines that become more pronounced on the posterior half of the trunk (Pl. 1, figs 1, 2; TEXT-FIG. 1. , TEXT-FIG. 3. , TEXT-FIG. 6. ). The dorsal surface of the trunk is undifferentiated with no signs of trilobation. Some specimens, however, display an axial region expressed ventrally as an elevated, bilobed ridge along the mid-line of the trunk ( EXPLANATION OF PLATE 1 , EXPLANATION OF PLATE 2 ; Text-fig. 4A, B), resembling a miniature Cruziana. This was possibly produced by the food groove of the animal during burial. A few specimens even preserve delicate paired structures along this region that are interpreted as imprints of the basipods (Pl. 2. fig. 5; Text-fig. 4B), indicating that these arthropods probably had numerous, homopodous legs (Text-fig. 6). The posterior-most portion of the trunk bears a pair of medially fused subtrapezoidal postventral plates ( TEXT-FIG. 1. , TEXT-FIG. 6. ). These structures cover the base of the tailspine, the entire ventral side of the eleventh tergite and, most likely, part of the tenth. The tailspine is long and relatively wide, comprising about one third of the total length of the animal, with a well-developed medial cleft ( EXPLANATION OF PLATE 1 , EXPLANATION OF PLATE 2 ; TEXT-FIG. 3. - TEXT-FIG. 6. ). Because of the preservation, it is not possible to identify any traces of the dorsal sculpture characteristic of aglaspidids. Figure TEXT-FIG. 6. Open in figure viewerPowerPoint Reconstruction of Australaglaspis stonyensis. Attachment of hypostome, limb number and morphology are hypothetical. Scale bar represents 5 mm. Systematic palaeontology Terminology. Aglaspidid descriptive terminology follows that of Briggs et al. (1979); as such, terms such as 'cephalon', 'trunk' and 'tailspine' are favoured over 'prosoma', 'opisthosoma' and 'telson', respectively, as the latter imply a chelicerate affinity. We use the term 'hypostome', following Cotton and Braddy (2004), Fortey and Rushton (2003) and Van Roy (2006), rather than 'labrum' as used by Briggs et al. (1979) and Hesselbo (1992). Order AGLASPIDIDA Raasch, 1939 Remarks. The diagnosis of Aglaspidida has been the subject of a long-lasting debate. Hall (1862) was the first to highlight the absence of facial sutures, progressive curvature of the tergites and distinctive exoskeletal sculpture. Raasch (1939) interpreted aglaspidids as merostomes (i.e. aquatic chelicerates). He defined the group on the presence of a phosphatic cuticle and possession of postventral plates, which have been widely recognized as two of the most important synapomorphies of Aglaspidida (e.g. Briggs and Fortey 1982; Hesselbo 1992). Other workers proposed their own definitions based on this initial chelicerate alignment, albeit with minor variations (Størmer 1955; Raw 1957; Weygoldt and Paulus 1979a, b). Re-study of Aglaspis spinifer by Briggs et al. (1979) prompted removal of aglaspidids from the chelicerates, based on the presence of an antenniform first appendage and identification of four or five pairs of cephalic appendages, as opposed to the six pairs inferred by previous workers (e.g. Bergström 1971). These observations were later confirmed by Hesselbo (1992), whom also noted a pair of distinct cuticular outgrowths, or 'anterior tergal processes' on the tergite margins of a number of aglaspidids. Later definitions were based largely on the aforementioned characters, but also included the possession of genal spines, eleven trunk segments, a tailspine and the mode of feeding (e.g. Hou and Bergström 1997; Cotton and Selden 1997; Selden and Dunlop 1998; Cotton and Braddy 2004; Zhang and Shu 2005). The most recently described representatives (Fortey and Rushton 2003, 2009; Van Roy 2006) were assigned to Aglaspidida based on a partial set of these characters (e.g. mineralized cuticle eleven trunk segments, absence of facial sutures, postventral plates, tailspine). These studies also added the presence of a hypostome to the character set of aglaspidids, although Briggs et al. (1979) and Hesselbo (1992) had already pointed to the presence of a hypostome-like structure in Aglaspis. Van Roy (2006) noted that most previous definitions of Aglaspidida were largely based on characters that are either plesiomorphic (e.g. tailspine), highly homoplastic (e.g. genal spines) or very susceptible to preservational bias (e.g. absence of cephalic sutures); however, he also emphasized that some of these characters bear great significance (e.g. anterior tergal processes), and proposed a new character set for the group. The provisional definition suggested by Van Roy (2006) includes the possession of a mineralized exoskeleton, absence of cephalic sutures (except for a possible hypostomal suture), anterior tergal processes, four or five pairs of cephalic appendages, first appendage antenniform, homopodous trunk appendages, absence of appendages below the undifferentiated final three or four trunk tergites and possession of post ventral plates. Apart from the anterior tergal processes and absence of appendages under the last three tergites, Van Roy (2006) considered that all the other characters are not exclusive to aglaspidids and that a combination of part of the set must be used to justify the assignment of any arthropod to the Order Aglaspidida. Family AGLASPIDIDAE Miller, 1877Genus AUSTRALAGLASPIS gen. nov. Derivation of name. Conflation of austral, from Latin meaning 'southern', as this is the first aglaspidid described from the Southern Hemisphere, and aglaspis alluding to the affinities of the animal. Type and only species. Australaglaspis stonyensis sp. nov. Diagnosis. Small aglaspidid with slightly elongated trunk composed of eleven somites and a tailspine. Cephalon semicircular, with straight posterior margin, acute genal angles and well-delineated marginal rim that develops into an occipital band. Dorsal eyes oval, in contact with the marginal rim. Subtriangular glabella. Hypostome subtrapezoidal, probably formed by a medial extension of the doublure. Trunk somites with short pleural spines that progressively curve backwards, becoming more pronounced after the fifth tergite. Tailspine spinose, one-third of total body length, with well-developed medial cleft. Medially fused subtrapezoidal postventral plates with elevated ridge. Trunk appendages probably homopodous. Remarks. The studied material is recognized as a new genus within the Order Aglaspidida as it displays numerous characters previously reported in various members of this group, including absence of facial sutures, possession of a hypostome, possible phosphatic mineralized cuticle, a trunk composed of eleven somites with homopodous legs and a terminal tailspine. Particularly important is the presence of postventral plates, which are widely acknowledged as one of the most reliable aglaspidid synapomorphies. Australaglaspis stonyensis sp. nov. EXPLANATION OF PLATE 1 , EXPLANATION OF PLATE 2 ; TEXT-FIG. 1. , TEXT-FIG. 2. , TEXT-FIG. 3. , TEXT-FIG. 4. , TEXT-FIG. 5. , TEXT-FIG. 6. 1992 Aglaspidida gen. and sp. indet. Jago & Baillie, p. 14, fig. 1. Holotype. UTGD125519 (Text-fig. 1A). Complete, articulated individual with postventral plates. Other material. Paratypes, UTGD 124458, 125504–1255118. Diagnosis. As for the genus. Description Holotype UTGD 125519. Complete specimen with signs of ventral and dorsal preservation (Text-fig. 1). Articulated individual deformed by metamorphism. Weak signs of trunk segmentation. Dorsal cephalic structures barely visible. Partially preserved paired postventral plates on the posterior-most portion of the trunk, at the base of the tailspine. Postventral plates show gentle subtrapezoidal outline; fused medially as indicated by elevated ridge. Proximal portion of tailspine concealed below the postventral plates; with well-preserved medial cleft. UTGD 125504. Dorsal exoskeleton (Pl. 1, fig. 1; Text-fig. 3A). Cephalon partially preserved, with occipital band and straight posterior margin. Hypostome subtrapezoidal and with surrounding elevated rim, displaced from original position, covering the posterior-most portion of the cephalon. Trunk slightly disarticulated, composed of eleven segments. Well-developed overlapping pleurae with short spines directed posteriorly. Pleural curvature more pronounced after fifth tergite. Tailspine with well-preserved medial cleft. UTGD 125505a, b. Part and counterpart of almost complete dorsal trunk and articulated tailspine (Pl. 2, figs 1–2; Text-fig. 3C). Tailspine with preserved medial cleft. UTGD 125506a, b. Part and counterpart of dorsal imprint of cephalon with ventral impression of trunk (Pl. 1, figs 3–4; Text-fig. 4A). Specimen articulated, but lacking the last two segments of the trunk and the tailspine. Cephalon semicircular, with acute genal angles and straight posterior margin. Elevated marginal rim surrounding cephalon, forming an occipital band. Axial region bears an elevated subtriangular glabella area. Anterior half of trunk well preserved as a bilobed ventral imprint, interpreted as the food groove. The undersides of some pleurae are visible on the left side and anterior right side of the specimen. UTGD 125507. Ventral side of trunk and part of the tailspine (Pl. 2, fig. 5; Text-fig. 4B). Underside of pleurae barely visible. Food groove preserved as bilobed imprint, with at least ten paired convex bulges representing imprints of the coxae. It is not possible to recognize the boundary between the cephalon and the trunk; as such, the exact number of cephalic appendages is not known. Posterior portion of trunk lacks detail. Tailspine partially preserved, but showing medial cleft as an elevated ridge. UTGD 125508. Complete dorsal exoskeleton (Pl. 1, fig. 2; Text-fig. 3B). Cephalon semicircular, flanked by elevated marginal rim. Straight posterior margin with occipital band. Acute genal angles. Oval eyes placed anteriorly. Subtriangular glabella region dominates axial portion of cephalon. Trunk with articulated segments. Well-developed pleurae with short spines. Posterior tergites more curved than anterior ones. Tailspine poorly preserved. UTGD 125509. Dorsal imprint of almost complete specimen (Pl. 2, fig. 6). Only posterior margin of cephalon, acute genal angles and a portion of the glabella area are visible. Trunk with eleven fully articulated segments. Pleural spines not preserved. Anterior half of tailspine well preserved, displaying marked medial cleft. UTGD 125510. Ventral side of complete specimen (Pl. 1, fig. 5). Specimen slightly compressed longitudinally. Underside of posterior of cephalon margin and pleurae barely visible. Axial region of trunk preserved as an elevated crest that extends from the glabella area to the base of the tailspine. Tailspine incomplete. UTGD 125511. Cephalon and anterior portion of the trunk ( TEXT-FIG. 2. , TEXT-FIG. 5. ). Cephalon outline semicircular, bordered by well-developed elevated marginal rim. Genal angles acute. Both oval eyes preserved, placed anteriorly. Subtriangular glabella region occupies the axial surface of the cephalon, the apex accommodated between the eyes. First two trunk segments well preserved, displaying the pleural spines on the left side. UTGD 125512. Posterior portion of the trunk with articulated tailspine (Pl. 2, fig. 3). Faint imprints of the pleurae are preserved on the left side of the specimen. UTGD 125513. Deformed cephalon with two articulated trunk segments (Text-fig. 2C). Marginal rim, acute genal angles, occipital band and glabella area well preserved. UTGD 125514. Almost complete specimen (Pl. 2, fig. 4). Cephalon retains glabella area, marginal rim and acute genal angles. Segmentation only visible at posterior part of the trunk, displaying backwardly curved tergites. Tailspin
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