Isoxys acutangulus ( Walcott, 1908 )

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 : 702-708

Isoxys acutangulus ( Walcott, 1908)

Figs. 2–6.

1908 Anomolocaris (?) acutangulus sp. nov.; Walcott 1908: 13, pl. 2: 5. 1928 Anomalocaris ?? acutangula Walcott, 1908; Walcott 1928: 320. developed midgut glands; A 2, counterpart; A 3, specimen with high reflection of carbonaceous surfaces; A 4, line drawing of specimen. B. ROM 57900; B 1, specimen with well preserved posterior end, including telson with flaps; B 2, line drawing of specimen. C. ROM 57903A; C 1, specimen showing both endopod and exopod of body appendages; C 2, line drawing of specimen. D. ROM 57913A; D 1, specimen with marginal setae on exopods; D 2, line drawing of specimen. All laterally compressed specimens and polarized light except A 3. Midgut glands in gray tone.

1

1975 Isoxys acutangulus ( Walcott, 1908) ; Simonetta and Delle Cave 1975: 6, pl. 5: 6, pl. 54: 3, 4, 6–9 [non 1 (= Burgessia bella )].

1986 “shrimp”; Collins 1986: 37, fig. 4.

1991 Isoxys acutangulus ( Walcott, 1908) ; Delle Cave and Simonetta 1991: fig. 20F.

1991 “shrimp”; Delle Cave and Simonetta 1991: fig. 23D.

1994 Isoxys acutangulus ( Walcott, 1908) ; Briggs et al. 1994: 149, pl. 102.

2001 Isoxys acutangulus ( Walcott, 1908) ; Donovan and Lewis 2001: fig. 1F.

Type material: The type−material consists of two specimens from the Mt. Stephen Trilobite Beds ( Fig. 1A , loc. 6,), which occur within the Campsite Cliff Shale Member (ST, Fig. 1B ), Burgess Shale Formation ( Fletcher and Collins 1998). These are USNM 56521 (part and counterpart) and USNM 56521B. No holotype was designated in the original description by Walcott (1908: 13).

Material.—295 catalogued specimens in the ROM collections, 41 of them with preserved soft parts ( Table 1). 62 and 29% of specimens were collected from the Walcott Quarry Shale Member and the Raymond Quarry Shale Member, respectively. 83% of the specimens with soft parts are from Raymond Quarry.

Emended diagnosis.—Pleural folds of headshield with an hemielliptical lateral outline tapering posteriorly and short cardinal spines; anterior one ca. 0.2L1 (L1= shield length excluding spines), slightly droopy; posterior one very short; angle between anterior spine axis and anterior margin between 90 ° and 110 °, between posterior spine axis and posterior margin ca. 40 °; L1:H ca. 2. Cardinal spines of each pleural fold joined to form a strong anterior and posterior rostrum. No sculpture or lobation on lateral flaps. Dorsal margin slightly convex. Very narrow latero−admarginal ridge. Large spherical eyes (diameter up to ca. 0.1L1) protruding on both sides of anterior rostrum and directed slightly anteroventrally. Ocular segment bulbous, eye stem very short. First pair of appendages adjacent to ocular segment, uniramous, long, curved with a serrated inner margin; consists of a proximal, possibly 2−segmented peduncle followed by a 4−segmented claw−like unit (3 podomeres bearing a conical tooth−like outgrowth + a terminal subchelate element). Following 13 appendages biramous with a simple endopod and a large elliptical exopod bearing numerous marginal setae. Size of exopods decreasing towards the telson. Telson bearing lateral flaps. Tubular midgut flanked with at least 8 pairs of bulbous digestive glands.

Description.—The total length of the headshield (L2) of the laterally preserved specimens varies from ca. 10 to almost 40 mm (H between 4 to 16 mm), L1:H being almost constant at about 2. The outline of juveniles and presumed adults is virtually identical with the anterior and posterior cardinal spines equalling ca. 0.2L1 and 0.15L1, respectively. In most laterally preserved specimens, Hmax lies at 0.5L1 (amplete outline) or slightly anterior to it (preplete outline). Dorso−ventrally compacted specimens show a symmetrical headshield with no splitting area (i.e., groove) between the right and the left flaps. The cardinal spines of the two pleural folds are fused into a single pointed rostrum at both ends of the shield ( Fig. 5A ). This indicates the absence of a true dorsal hinge as, for example, in ostracods. The headshield of Isoxys acutangulus was obviously thin, flexible and folded dorsally, resulting in two lateral pleural folds. These folds may have been joined by a relatively resistant ligament of the same composition as the headshield itself which would explain the lack of external suture. By contrast, headshields with the two folds splayed open on the sediment surface (“butterfly” orientation; Vannier et al. 2007) are clearly split into two halves along their entire dorsal line including spines (e.g., Briggs et al. 1994: fig. 102). These dorso−ventrally compressed specimens lack soft parts and are most probably molted shields, the dorsal split being possibly an artifact produced during the molting process and emphasized later on after burial. The dorsal outline is slightly convex with a maximum elevation lying anteriorly at ca. 0.3–0.4L1 (e.g., Figs. 2C, 4B ). The ventral outline is tapering regularly beyond the mid−length line towards the posterior cardinal angle. No micro−ornament is visible on the external surface of the headshield. The dorsal attachment of the headshield to the body was probably very narrow ( Fig. 3A, B, D ) and limited to the head part. Indeed, most of the body—i.e., beyond the segment bearing the frontal appendages—seems to have been free and not attached to the shield. This is shown at various stages of the decay process ( Fig. 3B, D ).

Eyes are by far the most frequently preserved organs in Isoxys acutangulus , being present in more than 90% of specimens with soft−part preservation. They consist of a pair of spherical bulbs attached to the front part of the head (ocular segment) through a very short stalk. The diameter of the eye bulb is approximately 0.07–0.1L1. Eyes protrude markedly beyond the shield margins both laterally ( Figs. 2, 3 ) and on each side of the anterior rostrum ( Fig. 5B, C ). The eyes are directed frontwards and slightly downwards, their axis diverging slightly (e.g., Fig. 5B, C ). Microscopic observations did not reveal any details such as ommatidia or retinal features. The external cuticular layer (cornea?) is likely to have been more decay−resistant, which probably contributed to preserving the spherical shape of the eye bulbs.

The frontal appendage of Isoxys acutangulus typically has a serrated outline. It is uniramous, curved and largely protrudes beyond the anteroventral margin of the headshield. It is divided into a possible 2−segmented peduncle−like basal part ( Figs. 2B, 3B ) followed by a 4−segmented section ( Figs. 2B, 3A ). This section consists of 3 segments each bearing a stout tooth−like outgrowth and a small terminal subchelated segment. The various positions in which this appendage is preserved ( Figs. 2A, B, 3A–C , 4B ) indicate that it could flex inwards and fit under the headshield. However, boundaries and possible articulated joints between segments are hardly discernible. The body segment that bore the frontal appendage seems to be adjacent to the ocular segment (e.g., Figs. 2A–C, 3B ). However, no boundaries between these segments are visible, suggesting that head segments may be fused. In one specimen ( Fig. 3C ) the appendage is preserved in an upturned position with the serrated margin directed posteriorly. This unusual orientation probably results from the post−mortem displacement of the decaying body, also apparent in the posterior part of the specimen (last trunk exopods and telson flaps).

The frontal pair of uniramous appendages is followed by a series of 13 pairs of evenly spaced, biramous appendages characterized by a uniform design ( Fig. 6 ). The endopod is slender, slightly curved and displays no particular features or podomere boundaries. In laterally compressed specimens, it is generally concealed under the exopod ( Fig. 2C). Exopods are large, paddle−like and fringed with numerous long setae along their external margins. The size of exopods gradually decreases towards the posterior end of the animal but their overall shape remains unchanged. The exopods are longer than endopods and protrude beyond the ventral margin of the headshield, especially in its posterior half ( Fig. 2A, D). The last 4 pairs of appendages have a more oblique position and seem to be lacking long setae. Their paddle−like exopods form, together with the telson flaps, a symmetrical fan−like structure ( Fig. 5B, C ). In dorsoventrally preserved specimens, exopods are flexed back on each side of the body ( Fig. 5B 2, C 1 ) and overlap each other slightly. The detailed morphology of the basal part of the biramous appendages could not be observed.

The gut of Isoxys acutangulus is thin, cylindrical and runs from the head section to the ventral part of the telson where the anus opens ( Fig. 4B ). How the gut leads to the mouth opening via, e.g., the esophagus, remains unclear. In numerous specimens ( Figs. 2A 3 –A 4, B, 4 , 5C ), the gut is underlined by lobate to subrectangular, typically 3−dimensional, features preserved in calcium phosphate (apatite). Each of the first 8 segments that bear biramous appendages is provided with one pair of such features that have been convincingly interpreted as serial digestive glands ( Butterfield 2002, Vannier and Chen 2002). The glands reach their maximum size in the middle part of the gut. One specimen ( Fig. 2A) displays a series of remarkably preserved glands. All are directed upwards and some of them clearly bifurcate into two lobes. So far, no internal features (e.g., tubules, caeca) could be observed, making the relation of the glands with the gut lumen unclear. One specimen ( Fig. 4B ) shows a diffuse dark area near the anus opening that may result from the squeezing out of fecal material.

Discussion.— Isoxys acutangulus differs from all congeneric species by the relatively short cardinal spines of its headshield (see Vannier and Chen 2000) and does not show any lineated or reticulated micro−ornament as seen, for example, in Isoxys auritus and I. curvirostratus . Isoxys acutangulus is so far the species that displays the best preserved soft−bodied features (e.g., visual and digestive organs, full series of appendages). The eyes of Isoxys acutangulus are almost identical to those of I. curvirostratus ( Vannier and Chen 2000: fig. 4), I. auritus (e.g., Chen et al. 2002: pl. 11: 3; Hu 2005: pl. 5: 2), Isoxys sp. ( Vannier et al. 2009: fig. 2k, l), all from the lower Cambrian Maotianshan Shale, as well as I. communis and Isoxys glaessneri from the lower Cambrian Emu Bay Shale ( García−Bellido et al. 2009: pl. 1: 10, 11, pl. 3: 7, 10). The position of the large forward−facing eyes of Isoxys acutangulus suggests that panoramic vision or light reception was important for this arthropod. Spherical eyes with a comparable size and overall external morphology are frequent in Recent crustaceans (e.g., krill, Fig. 8 ; compare with Fig. 5B, C ). However, the lack of information concerning the visual/optical properties of the eyes does not allow inferring how effectively they may have perceived images and how they could have estimated distances.

A single specimen of Isoxys from the lower Cambrian of China ( Hu 2005; Hu et al. 2007; Vannier et al. 2009) has a prominent frontal appendage that resembles that of I. acutangulus . However, its outline is straight and its inner margin, instead of being coarsely serrated as in I. acutangulus , bears a series of tiny spines. This small−sized specimen is likely to belong to the juvenile stage of possibly I. auritus ( Vannier et al. 2009) . One specimen of I. volucris from the lower Cambrian of north Greenland has also been described as having a pair of uniramous frontal appendages, yet these bear a pair of long, thin spines per podomere, when preserved (Stein et al. 2008: figs. 1, 2).

The series of 13 biramous trunk appendages bearing large flap−like setose exopods seems to be a recurrent feature now recognized in both lower and middle Cambrian Isoxys species. However, some differences may occur in the pattern and density of these marginal setae. For example, Isoxys sp. from the lower Cambrian of China ( Vannier et al. 2009) has well−defined, stiff, radiating primary setae along the margins of its exopods that may not have exact counterparts in Isoxys acutangulus . A comparable serial pattern of digestive glands is recognized in I. acutangulus , I. curvirostratus , and Isoxys sp. from the Maotianshan Shale of China, and also I. communis from the Emu Bay Shale of Australia ( García−Bellido et al. 2009: pl. 2: 4–6).

Stratigraphic and geographic range.—Kicking Horse Shale Member to Emerald Lake Oncolite Member, Burgess Shale Formation and Waputik Member, Stephen Formation, Stage 5, Series 3, Cambrian ( Fig. 1B ); Fossil Ridge, Mount Stephen and Stanley Glacier, British Columbia, Canada ( Fig. 1A ).

Figures

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. 5. Bivalved arthropod Isoxys acutangulus (Walcott, 1908), Burgess Shale Formation, middle Cambrian, near Field, British Columbia, Canada (see Fig. 1), general morphology. A. ROM 57902A, headshield in dorsal view without soft parts, note the different angle of the contact between the pleural folds and the cardinal spines, which allows for the position of the stalked eyes. B. ROM 57907A, B; B1, part of unique specimen from WS locality, with eyes, telson and telson flaps; B2, counterpart; B3, line drawing of specimen. C. ROM 57904A; C1, specimen showing paired midgut glands and telson flaps; C2, line drawing of specimen. All dorsoventrally compressed specimens and photographed under polarized light; B2, in water. Midgut glands in gray tone.

Fig. 4. Bivalved arthropod Isoxys acutangulus (Walcott, 1908), Raymond Quarry Shale Member, Burgess Shale Formation, middle Cambrian, near Field, British Columbia, Canada (see Fig. 1). A. ROM 57905A, B; A1, part of specimen alternating midgut glands, possibly due to compaction, and narrow doublure; A2, specimen under low angle light from top left; A3, counterpart of specimen under low angle light from top left; A4, line drawing of specimen. B. ROM 57906B; B1, specimen with raptorial appendage, and 8 sets of midgut glands; B2, image showing endopod and exopod details; B3, specimen under low angle light from top left; B4, line drawing of specimen. All laterally compressed specimens. A1, B1, B2, polarized light; B2, in water; A2, A3, and B3 dry. Midgut glands in gray tone.

Fig. 3. Bivalved arthropod Isoxys acutangulus (Walcott, 1908), Raymond Quarry Shale Member, Burgess Shale Formation, middle Cambrian, near Field, British Columbia, Canada (see Fig. 1), general morphology. A. ROM 57898A; A1, specimen showing large stalked eyes and the pair of raptorial frontal appendages; A2, line drawing of specimen. B. ROM 57914A; B1, partially decayed specimen with most of body and appendages rotated towards the front; B2, line drawing of specimen. C. ROM 57899A; C1, specimen with raptorial appendages oriented backwards and possible telson flaps; C2, line drawing of specimen. D. ROM 59871A; D1, considerably decayed specimen showing forward rotation of body and appendage remains; D2, line drawing of specimen. All laterally compressed specimens and polarized light. Midgut glands in gray tone.

Fig. 6. Reconstruction of the bivalved arthropod Isoxys acutangulus (Walcott, 1908), swimming in the water column. Note that the position of the mouth and the attachments of the appendages to the body are conjectural. Not to scale.

Fig. 8. Recent crustacean Euphausia superba Dana, 1852 (krill) from Gerlache Strait, Antarctica. A. Left lateral view. B. Dorsal view. C. Detail of stalked eyes after removing head shield. All light micrographs.