Isoxys longissimus Simonetta and Delle Cave, 1975

García-Bellido, Diego C., Vannier, Jean & Collins, Desmond, 2009, Soft-part preservation in two species of the arthropod Isoxys from the middle Cambrian Burgess Shale of British Columbia, Canada, Acta Palaeontologica Polonica 54 (4), pp. 699-712 : 708-711

Isoxys longissimus Simonetta and Delle Cave, 1975

Fig. 7 .

1975 Isoxys longissimus sp. nov.; Simonetta and Delle Cave 1975: 6–7, pl. 5: 7, pl. 54: 3 A, B.

1991 Isoxys longissimus Simonetta and Delle Cave, 1975 ; Delle Cave and Simonetta 1991: fig. 20G.

Type material: Simonetta and Delle Cave (1975: 6) designated USNM

189170 as the holotype of Isoxys longissimus . This specimen ( Fig. 7A )

is represented by its part and counterpart and comes most probably from

Walcott’s “35k” locality, which corresponds to the Phyllopod Bed on

Fossil Ridge ( Fig. 1 ).

Material.—One laterally compressed specimen showing remains of soft−bodied features ( ROM 57908; Fig. 7E ), and four headshields ( ROM 57909, 57910, 57911, 57919; Figs. 7B–D, F ), also laterally compressed. ROM 57910, 57911 were recovered from the Raymond Quarry Shale Member ( Fig. 1B ), at layer + 10.4 m and from the “ Tuzoia layer”, about 3 m below the top of this member, respectively. ROM 57908 and ROM 57909 are both from the same horizon within the Emerald Lake Oncolite Member, more precisely the bed known as the “Upper Ehmaniella layer” of the Collins Quarry ( UE; Fig. 1B ). The three localities where Isoxys longissimus was collected lie adjacent to the contact between the Burgess Shale Formation and the escarpment of the Cathedral Limestone Formation, on the west slope of Fossil Ridge, between Wapta Mountain and Mt. Field, in Yoho National Park ( Fig. 1B ; locs. 1b, 1c, 1e).

Emended diagnosis.—Headshield with elongated, hemielliptical lateral outline tapering posteriorly and extremely long cardinal spines; anterior one ca. 0.75L1 with thick basal part almost parallel to dorsal margin extending into a much thinner distal section directed slightly upwards; posterior one at least as long as L1, straight or slightly curved downwards. Angle between anterior spine axis and anterior margins between 75 ° and 90 °; angle between posterior spine axis and posterior margin between 30 ° and 45 °. L1:H ca. 3. Anterior part of dorsal margin slightly convex. Spherical eyes present, protruding beyond the anteroventral margin of the headshield. Homonomous series of more than 10 trunk appendages. Telson protruding beyond the posteroventral margin of headshield.

Description.— Isoxys longissimus has extremely long cardinal spines and an unusually high L1:H ratio approaching 3. L2 reaches 105 mm in the holotype ( Simonetta and Delle Cave 1975: 6; Fig. 7A ), and ranges from 40 to 60.5 mm in the ROM specimens. The posterior spine of the holotype is more than three times the length of the anterior spine, and exceeds L1. A close inspection of the specimens shows that the tips of the cardinal spines are often buried under the matrix (e.g., anterior spine of holotype), making their measurement difficult without careful preparation. Although always very high (ca. 3), the L1:H shows significant variations due to the effect of the orientation of the specimens to bedding and to the frequent wrinkling and folding of the headshields. Hmax lies at 0.5L (amplete outline, Fig. 7B ) or slightly anterior to it (preplete outline, Fig. 7C, D ). Each cardinal spine has a relatively strong basal part that extends into a long foil−like projection. The anterior spine is slightly curved upwards, the posterior one straight or tilted slightly downwards. The dorsal margin is convex with maximum convexity in the anterior half of headshield ( Fig. 7B, C ). The angle between the anterior spine axis and the anterior margin is between 75 ° and 90 °, that between the posterior spine and the dorsoventral margin between 30 ° and 45 °. No micro−ornament is visible on the external surface of the headshield. The soft anatomy of Isoxys longissimus is known from a single specimen ( Fig. 7E ). A rounded feature in front of the headshield and under the anterior spine is interpreted as an eye. The succeeding (possibly 10 to 15) segments bear a series of appendages but no details of their fine structure is revealed. The posteriormost appendages are shorter and stick out of the shield. The body ends into a small pointed telson−like feature. Ill−defined black areas may suggest the presence of digestive features such as midgut glands. Discussion.— Isoxys longissimus is unique among Isoxys by its extremely long cardinal spines and elongated shape. Comparable exoskeletal features occur in I. paradoxus from the lower Cambrian Chengjiang biota, which has unusually long spines ( Hou et al. 2004) although shorter and stouter than those of I. longissimus . Isoxys curvirostratus also from the lower Cambrian of China ( Vannier and Chen 2000) has a slightly recurved anterior spine that recalls that of Isoxys longissimus . Resemblances with Isoxys volucris from the lower Cambrian Buen Formation of Greenland ( Williams et al. 1996) should be also noted. The dorsal outline of the headshield of I. volucris forms a small cusp−like projection behind the basal part of the anterior cardinal spine. This feature also occurs in some specimens of Isoxys longissimus (e.g., Fig. 7C ). The two species are armed with the same type of slender, probably very fragile spines that occur in no other congeneric species.

Examples of long cardinal spines are known in Recent pelagic crustaceans such as Gnathophausia ( Lophogastrida ; Fig. 9A ), the planktotrophic larvae of some malacostracans ( Fig. 9B ), and halocypridid ostracods ( Vannier and Chen 2000). Their exoskeletons show a remarkable development of shield spines that are indeed very similar to the cardinal spines of Isoxys longissimus (compare Figs. 7 and 9 ). This example of exoskeletal convergence between Cambrian and Recent marine arthropods is probably the expression of comparable morphological adaptations to the pelagic lifestyle ( Vannier and Chen 2000; Vannier et al. 2009).

Stratigraphic and geographic range.—Raymond Quarry Shale Member to Emerald Lake Oncolite Member, Burgess Shale Formation, Stage 5, Series 3, Cambrian ( Fig. 1B ); Fossil Ridge, British Columbia, Canada ( Fig. 1A ).

Figures

Fig. 7. Bivalved arthropod Isoxys longissimus Simonetta and Delle Cave 1975. Burgess Shale Formation, middle Cambrian, near Field, British Columbia, Canada (see Fig. 1), general morphology. A. USNM 18170, holotype. B. ROM 57910A. C. ROM 57911A. D. ROM 57909A. E. ROM 57908A, B; E1, part of slightly oblique specimen with soft−body preservation, including large eye, telson and some possible exopods; E2, counterpart; E3, line drawing of specimen. F. ROM 57919A. All laterally compressed specimens. A, low angle light from top left; B–F, polarized light.

Fig. 1. A. Topographic map of the Burgess Shale area near Field, British Columbia (Canada). The numbers indicate localities where specimens of Isoxys acutangulus (Walcott, 1908) and Isoxys longissimus Simonetta and Delle Cave, 1975 were collected by ROM parties. 1. West slope of Fossil Ridge: 1a, Greater Phyllopod bed, Walcott Quarry; 1b, Raymond Quarry; 1c, “persephone layer” (RQ +20 to +23); 1d, “Tuzoia layer” (TZ); 1e, Collins Quarry—“Ehmaniella Zone” (EZ) and “Upper Ehmaniella” (UE); 2, South face of Mt. Field; 3, North shoulder of Mt. Stephen (ESA, ESB); 4, S7 locality; 5, Mt. Stephen Collins Quarry (WS); 6, Mt. Stephen Trilobite Beds (ST); 7, Stanley Glacier. B. Stratigraphic section of the Burgess Shale and Stephen Formations (modified from Fletcher and Collins, 1998). Circles indicate the levels where specimens of Isoxys acutangulus (Walcott, 1908) and Isoxys longissimus Simonetta and Delle Cave, 1975 from the seven localities in A were collected.

Fig. 9. Spiny shields in Recent crustaceans. A. Gnathophausia zoea Willemoes−Suhm 1873 (Malacostraca, Lophogastrida). B. Porcellanid larva (Decapoda). A from http://en.wikipedia.org/wiki/Crustacean; B from www.zooplankton−online.net/gallery.html (courtesy of W.S. Johnson, Goucher College); both used with permission of the copyright holders.