Wapitiodus aplopagus, Mutter & Blanger & Neuman, 2007

Mutter, Raoul J., Blanger, Keith De & Neuman, Andrew G., 2007, Elasmobranchs from the Lower Triassic Sulphur Mountain Formation near Wapiti Lake (BC, Canada), Zoological Journal of the Linnean Society 149 (3), pp. 309-337 : 313-323

WAPITIODUS APLOPAGUS SP. NOV.

Holotype specimen: TMP 97.74 .10

Type stratum: From within the Vega-Phroso Siltstone Member of the Sulphur Mountain Formation, Wapiti Lake, British Columbia.

Age: Probably?early Smithian.

Etymology: ‘aplo-‘, Ancient Greek for ‘single’; ‘pago-’, Ancient Greek for ‘peak’; refers to the single cusp on anterior teeth.

Referred specimens: UALVP 17932, UALVP 46527–29 (thin sections UALVP 46528- T 1, T 2, T 3).

Diagnosis: Body rather stout, medium-sized species (1.2–1.5 m in total body length) with broad anterior and slender posterior fin spine; tooth crowns with inconspicuous apical crenulations (ridges) and vestigial cusplets; on anterior teeth, ridges not reaching the longitudinal crest; on posterior teeth, ridges meet the longitudinal crest; tooth roots with concave bases.

Description

The holotype specimen TMP 97.74.10 is relatively complete and well preserved in part and two counterparts. The head, including the jaws, other parts of the visceral skeleton and parts of the neurocranium, is fairly well preserved ( Fig. 3 ). Several elements of the branchial arches (bb) are preserved but damaged. The slender scapulacoracoids (scc) are present although damaged. The metapterygium (mpt) is the only element of the pectoral fin visibly preserved, and the other basal elements and radials are missing in the holotype. Specimen UALVP 46529 shares anterior fin spine morphology and complements the description of the pectoral fin (see below and Fig. 8 ). Both dorsal fins, the pelvic fin and anal fins are preserved. There are traces of the vertebral column posteriad and the caudal fin is missing. The overall body shape is relatively short and stout.

The two counterparts are less complete comprising only imprints of the scapulacoracoid (scc), anterior dorsal fin, pelvic fin and a partial vertebral column.

The neurocranium (nc; Figs 3 , 4 ) is slightly obliquely crushed in the latero-occlusal view. In the shortened dorso-rostral area, the ethmoid area is either only partly preserved or crushed and houses a narrow depression, interpreted as the cerebral fontanelle (cf; Fig. 4 ), and lies posteriorly to the ovalshaped nasal capsule. The well-developed right postorbital process and the supraorbital crest (scr) are seen in lateral view ( Figs 3 , 4 ). Anteriorly, the supraorbital crest is adjoined by the orbitonasal lamina (ol) and projects as a rather moderately developed ectethmoid process (ecp). The ethmoid articulation (eta), although partly obscured, is also broadly based and seems to have been rigid. The supraorbital process (porp) is well developed but quite narrow, and the palatoquadrate (pq) articulates broadly with its posterior wall via a wide articulation facet. Extending behind the supraorbital crest, just above the hyomandibular articulation (ahyo), is a raised border that delimits the extensive lateral otic process (lop) dorsally. The endolymphatic fossa is quite narrow but it is possible that this is the result of postmortem distortion. However, the endolymphatic fossa is discernible, extending far anteriad ( Fig. 4 ; ef ext). The right hyomandibula (hyo) articulates nearly mesial-horizontally with the ventral facet of the lateral otic process.

The visceral skeleton: The hyomandibula (hyo) is short and substantially broadened in its distal shank, is partly covered by the palatoquadrate and curves posteroventrad with its proximal shank. Slightly displaced, the hyomandibula originally articulated broadly with the large and well-calcified ceratohyal (chy), which is almost completely covered by the right Meckel’s cartilage (mc). The left palatoquadrate (pq sin) and left Meckel’s cartilage (mc sin) are partly covered by the right jaw, but provide complementary information on tooth morphology by showing sufficiently well-preserved aspects of the lower and upper jaw tooth files (see below).

The palatoquadrate and the Meckel’s cartilage are fairly well preserved. The anteriormost section of the palatoquadrate is obscured by the preserved section of the neurocranium. At the front of the orbit the ectethmoid process (ecp) articulates with the palatoquadrate at the ethmoid articulation (eta). The palatoquadrate (pq) then extends posteriorly under the postorbital process (porp). The narrow quadrate flange (qf) of the right, deep palatoquadrate (pq dex) curves posteroventrad to articulate with the Meckel’s cartilage. There are depressed areas on the lateral side of the palatoquadrate that may have been the adductor fossae. The left palatoquadrate (pq sin) is also preserved and is visible slightly ventral to the right upper jaw.

The lower jaw is deep and less well preserved than the upper jaw, and the right Meckel’s cartilage (mc dex) articulates with the right palatoquadrate (pq dex) via the narrow quadrate flange. The two halves of the lower jaw may have moved postmortem, sliding antero-posteriorly to each other, and both rami are only partly preserved ( Fig. 4 ). We interpret the hindmost portion of the lower jaw to represent the articulation region of the right ramus (mc dex), which almost perfectly superimposes the left ramus (mc sin) over much of its surface. The hyomandibular (hyo)– ceratohyal (chy) articulation is preserved in situ behind the articulation of the palatoquadrate (pq) with the Meckel’s cartilage (mc).

Dentition: There are several partially preserved tooth files on both the palatoquadrate and Meckel’s cartilage, in which we observe a pronounced monognathic heterodonty ( Figs 4 , 5 ). Although none of the anterior teeth is completely preserved and parts of the crowns are embedded in the matrix, the teeth appear to be fairly symmetrical. In addition, most teeth exposed are not precisely preserved in situ, but many of them can be assigned to the appropriate jaw, and tentatively, to the respective tooth file ( Table 1). Blunt teeth occur in the lateral-distal (posterior) region, and unicuspid acuminate teeth occur in the mesial-symphysial (anterior) region ( Fig. 5 ). At least five tooth files per jaw half share noncuspid posterior teeth. Either five or six anterior tooth files in the upper jaw, and up to six tooth files in the lower jaw, show unicuspid teeth. All these teeth are devoid of side cusplets and have a large, originally acuminate, central cusp. However, in the lower tooth files. The base of the root is remarkably concave and the tip of the cusp is acuminate. On the lateral side of the crown, reaching down slightly from the longitudinal crest, is an unornamented area below which the crenulations then occur and bifurcate toward the base. The root is very shallow in comparison with the crown. There is no evidence of tooth files transitional in morphology between the anterior and posterior files.

The posterior teeth are blunt, as described above; the crown of these teeth is mesiodistally very elongate, the cusp is either absent or faint, and posterior teeth lack lateral cusplets altogether. There are basally pronounced bifurcating crenulations. The root is roughly equal to the crown in depth and possesses a number of irregular foramina scattered randomly on the root.

jaw, close to the symphysis and in the position of tooth families number 7 and 10, there is one tooth, respectively, with a tiny side cusp ( Fig. 5 ; Table 1).

The arrangement of tooth files in the lower and upper jaw is not necessarily identical, but according to the position of the teeth as preserved in the lower and upper left jaw, the composition of the one jaw half can be reconstructed ( Table 1).

The teeth in the anterior files show a broad and acuminate cusp with crenulations reaching the apex. All teeth are apically broken off and the length of the cusp can therefore not be assessed in the respective Pectoral girdle and fin: The two scapulacoracoids (scc) are damaged and the two elements are partly superimposed on top of each other ( Fig. 3 ). The right scapulacoracoid (scc) measures 140 mm in length. The widest point measures 23 mm. The scapulacoracoid is slender but has a broader section, probably at the point of articulation with the pectoral fin. The pectoral fin is no longer articulated, and most of it is not visibly preserved. No radials are visibly preserved, but may be revealed through further preparation. There is a larger cartilaginous element that is most likely to be the metapterygium ( Fig. 3 , mpt; Fig. 6 ). The metapterygium is elongate and thin at the proximal end, but expands dorsally into a bulbous club shape. This shape may be due to distortion. It is 91 mm in length and the bulbous section is 22 mm wide, whereas the narrow section is only 13 mm wide. There is no evidence of additional elements that were probably present.

The pelvic girdle (pvg) is also poorly preserved. There are two visible basal elements, which could be the proximal elements of the pelvic girdle in specimen TMP 97.74.10 (see also UALVP 46529 described below). A second pair of cartilages appears to join the proximal elements distally. There are traces of several radials. The anteriormost three slender elements may have crossed the body–fin junction. No more than 12 rather slender/oblong elements were originally articulated with the (unpreserved) pterygial elements of the pelvic fin. In the area that would have contained the metapterygium, there is a vague shape that could be a crushed piece of cartilage with dermal denticles. The terminal clasper complex has been damaged and is hard to make out; only the extreme distal ends are easily discernible ( Fig. 3 ).

Dorsal fins and fin spines: The fin spine of the anterior dorsal fin is fairly well preserved, although the surface is mostly broken off ( Fig. 7A ). The preserved section is 105 mm in length. The spine is slender, relatively much broader than the posterior one in lateral view, and is inserted deeply into the vertebral column at an angle of 72° to the longitudinal axis of the body. It has been split, destroying the detail of the external ornamentation except for a few imprints of tubercles in the lateral wall. There are no visible denticles projecting from the posterior wall. The basal cartilage extends 47 mm along the spine and 48 mm behind its base. The fin webbing is only preserved on the dorsal side of the fin spine with its tip missing. The preserved section of webbing appears to extend to 32 mm behind the posterior end of the basal cartilage.

The posterior dorsal fin spine ( Fig. 7B ) is much more slender than the anterior spine, and the fin is slightly less complete. The posterior section of the fin webbing is missing. The fin spine is preserved to both ends and is 114 mm in length. The spine is inserted deeply into the vertebral column at an angle of 73° to the longitudinal axis of the body. Like the anterior spine, the dorsal one has suffered from damage to the external layers that prevents the examination of its original ornamentation. No recurved posterior denticles are visibly preserved.

Although no tubercles can be observed on the lateral walls of the spines, the posterodorsal portion of the posterior spine shows a couple of circle-like structures (partial tubercles), which hint at an original covering of tubercles also preserved in UALVP 46528 and UALVP 46529 (see below and Fig. 8 ). In addition there is an anterior ridge, which is Nemacanthus -like but partly tuberculate and only fragmentarily preserved in the holotype and in UALVP 46529 ( Fig. 8 ) .

The posterior basal cartilage is triangular in outline with a concave ventral margin and extends 67 mm along the spine and 54 mm behind its base. There are between six and eight radials, each set at an angle of about 50° to the longitudinal axis of the body and at 110° to the basal cartilage. The distal ends of the radials are missing, but they appear to increase in length posteriad. The most posterior one is the largest.

The webbing on the dorsal fin extends anteriad 20 mm beyond the base of the fin spine. The posterior section of the webbing is incompletely preserved. The preserved section extends for 6 mm above the top of the fin spine and 84 mm behind its base.

Axial skeleton: The preserved section of the vertebral column shows 31 interdorsal elements, each reclining posteriad at an angle of 33° to the longitudinal axis of the body. They appear to become gradually smaller posteriad. No haemal processes could be observed in the posterior half of the body.

See also the description of specimen UALVP 46534 ( Wapitiodidae gen. et sp. indet.) below.

Squamation: The body shape is quite well outlined by the shagreen of denticles, which cover almost the entire trunk and fins. Considerable variation can be found in the denticles, depending on their respective position on the body ( Fig. 9 ).

None of the denticles show the four-pronged extensions described in Schaeffer & Mangus (1976) as being allegedly typical for Palaeobates ( von Meyer, 1851; see also the description and discussion below).

The size of all denticles observed ranges between 0.2 and 1 mm, and is usually between 0.4 and 0.6 mm. The denticles consist of a pedicle and a variably shaped platform ( Fig. 9A–F ). The pedicles are covered by sediment and a pedicle-platformconstriction (or ‘neck’) could not be observed. The crowns or platforms of the pedicles share the same principle morphology, and usually possess side wings with a variable number of antero-posteriorly running ridges across their exterior face. Interestingly, denticles covering the ventral side of the body and the tip of the jaws are anteriorly blunt and posteriorly richly ornamented ( Fig. 9A–B ). Denticles covering the dorsal trunk can be characterized as possessing a principle cusp and side wings ( Fig. 9C ). In the shagreen covering the dorsal fins, the morphology of the crowns is most variable. The principle cusp and the side wings may either be divided in several ridges or be blunt, and along the anterior rim of the fins above the fin spines the crowns are entirely blunt and smooth, and sometimes discshaped ( Fig. 9D–F ).

Description of paratypes: Specimen UALVP 46528 is a partially preserved anterior dorsal fin with a partially preserved fin spine. The fin spine is 53 mm in length (but the lower portion is missing), 100 mm wide and is inserted into the vertebral column at an angle of 40° in relation to the horizontal axis of the body. The external surface is poorly preserved. A thin section through the central portion reveals that the lateral walls of the spine were originally covered with fairly large apically flattened tubercles. In cross-section, the fin spine is laterally flattened, moderately triangular to quadran- gular in shape and is not vaulted to the convex posterior wall ( Fig. 8A–D ). The basal cartilage in this specimen is partially preserved. The webbing extends over 11 mm behind the spine and exhibits the same elevation as seen in specimen UALVP 46527, indicating that its shape is not defined by the matrix. As in all other specimens, no posterior denticles are visible in the posterior wall, but one of the thin sections through the spine reveals a single displaced recurved tubercle, reminiscent of posterior denticles in hybodont fin spines, and that could have been shifted from its original position ( Fig. 10 ). However, smaller recurved tubercles may also occasionally occur elsewhere near either the insertion site of the spine in primitive sharks or as very small tubercles along the anterior rim ( R Mutter, pers. observ.).

Specimen UALVP 46529 consists of an anterior dorsal fin spine, two pectoral fins, an imprint of one scapulacoracoid, a preserved section of vertebral column back to the pelvic girdle, which is preserved in weathered traces, and the outline of the body shape delimited by dermal denticles ( Fig. 11 ). The fin spine is 63 mm long and 13 mm wide. As the specimen is preserved in dorsal view, it is impossible to measure at what angle the spine was inserted. Although it is damaged, there is an imprint of the external structure that shows a series of fine striations and circles descending the length of the spine.

The preserved parts of both pectoral fins are composed of a series of at least 14 radials (in the right pectoral) that splay out like finger-shaped projections. The radials appear to be thin at the base and then thicken towards the middle section before tapering at the end. The radials also appear to be jointed, consisting of a proximal and a longer distal series. Originally, there were at least 14 radials present in each fin. Radial seven is the longest with the neighbouring ones becoming, respectively, smaller.

The preserved lengths of the radials are: (1) 17 mm, (2) 27 mm, (3) 34 mm, (4) 45 mm, (5) 52 mm, (6) 59 mm, (7) 62 mm, (8) 57 mm, (9) 53 mm, (10) 47 mm, (11) 38 mm, (12) 21 mm, (13) 19 mm and (14) 9 mm. The fin webbing extends for at least 34 mm beyond the longest radial and 39 mm behind radial 14 in the right pectoral fin.

The scapulacoracoid is only preserved as a vague imprint of the distal portion that is 49 mm in length. It is very slender (2 mm wide) and represents only the outline of one side of the element. The vertebral column extends over 27 mm anterior to the fin spine and 114 mm posterior to it. The neural arches with processes are preserved but are very vague in detail.

The preserved parts of the pelvic girdle (?and clasper complex) appear to be composed proximally of one oblong/slender and three cuboid elements, whereas the distal portion consists of shortly jointed smaller and more slender elements that may represent the radials of the pelvic fin ( Fig. 11 ).

Although poorly preserved, some of the dermal denticles seem to have a similar structure to those seen in specimens UALVP 46527 and TMP 97.74.10, whereas others at the rear of the specimen have a moderately long central projection flanked by two shorter lateral projections.

Specimen UALVP 46527 is a partially preserved vertebral column with both dorsal fins. The remains include interdorsals, a set of disarticulated fragments located ventrally to the interdorsals (most likely to be the remains of the basiventrals and the ribs) and the remains of the pelvic girdle, although these have been badly damaged and are only identified as the pelvic girdle because of their position antero-ventral to the posterior dorsal fin. The anterior fin has a partially preserved fin spine and webbing but no basal cartilage. The posterior fin is preserved including spine, radials, basal cartilage and webbing. The preserved section is 320 mm long, indicating that the total length of the shark was about 900 mm.

The spine of the anterior dorsal fin is only partially preserved. The upper section is present but the lower section (probably just over half the spine) is missing. The (fragmentary) preserved section is 30 mm in length and 9 mm in width. The spine is deeply inserted into the vertebral column at an angle of ∼53° to the longitudinal axis of the body and it has been split, thereby destroying the detail of the external ornamentation. Either outlines or fragments of the original tubercles are still discernible. The basal cartilage is not preserved. The fin webbing is only preserved on the dorsal side of the fin spine and begins just above the base of the preserved section (41 mm above the vertebral column). The preserved section of the webbing appears to extend at least 9 mm, but then is obscured by the matrix. There is an elevation at the posterior end of the preserved section of the webbing.

The posterior fin is more complete. The fin spine is 79 mm in length and 15 mm wide at its widest point, and is inserted fairly low in the vertebral column at an angle of 61° to the longitudinal axis of the body. Like the anterior spine, the dorsal one has suffered from damage to the external layers preventing the examination of the original tuberculate ornamentation. There are no posterior denticles, and the basal cartilage is only partially preserved (the posterior section is missing).

There are four radials preserved at an angle of ∼49° to the longitudinal axis of the body and at 78° relative to the basal cartilage. The radials are only partially preserved – the top end is missing – but appear to increase in length posteriad. From the anteriormost to the posteriormost radial the preserved sections measure 8, 11, 16 and 18 mm, respectively. The second radial appears to be thicker than the others, but it could be branched and it is possible that two radials are superimposed on each other.

The webbing on the dorsal fin extends anteriorly, but is obscured by the matrix so it is impossible to examine exactly how far. The posterior section of the webbing is incompletely preserved. The preserved section extends over 9 mm above the top of the fin spine and 78 mm behind its base.

The preserved section of the vertebral column comprises 23 interdorsal elements – each reclining posteriad at an angle of 26° to the longitudinal axis of the body. The interdorsals appear to become gradually smaller posteriad and range in length between 42 and 29 mm.

The section ventral to the interdorsals has a jumble of preserved elements – probably ribs – but they are broken into smaller pieces and are too badly preserved to make out any morphological detail, and the preserved section of the pelvic fin is also too badly preserved for description.

The dermal denticles exhibit a similar degree of variation in the number of ridges in the platform as observed in holotype TMP 97.74.10. Most denticles are badly weathered, but in the mid-trunk region several well-preserved but slightly displaced denticles show the same basic structure ( Fig. 12 ). As a tendency, the platforms of the denticles possess blunt anterior borders, no side wings and fewer ridges in these denticles that lie along the border of fins. In non-exposed areas, the platforms are winged, conspicuously ridged and quite acuminate anteriorly (see also Fig. 9 ).

Specimen UALVP 17932 is a posterior dorsal fin preserved along with the fin spine and part of the vertebral column. The fin spine is 81 mm long and is 6 mm wide at its maximum width, and is inserted deeply into the vertebral column at an angle of 77 ° to the longitudinal axis of the body. The exterior surface of the spine has been damaged obscuring the original ornamentation. No posterior denticles are visibly preserved on fin spines.

The visible fin webbing extends 74 mm posteriad. The webbing continues dorsad from the apex of the spine along the same angle for 34 mm, and the overall depth of the fin is 101 mm (from the base of the basal cartilage). There are at least nine visible supporting radials increasing in length posteriad and lying at the same angle of 48° to the longitudinal axis of the body. The shortest visible radial is 7 mm in length, whereas the longest is 25 mm in length.

The anterior section of the specimen shows interdorsals, but the posterior section has a darker area (possibly caused by water damage) that obscures the details of the vertebral column. In the preserved area there are 17 interdorsal elements, posteriorly inclined, which are all approximately 21 mm long, although they seem to decrease in size posteriad. The interdorsals are inclined at an angle of 50° to the longitudinal axis of the body.

There is a large cartilaginous element directly below the fin spine that may have been a portion of the pelvic fin, but it has been too badly damaged to make out the structure. There are also numerous dermal denticles on the specimen that show the range of degree of variation in crown shapes as observed in TMP 97.74.10 and UALVP 46527 ( Figs 9 , 12 ). The necks and bases cannot be observed. However, denticle types with blunt crowns near the edges of the fin and crowns with few (±four) and faint ridges can be identified closer to the centre of the fin.

CLASS CHONDRICHTHYES HUXLEY (1880) SUBCLASS ELASMOBRANCHII BONAPARTE, 1838 SUPERFAMILY? HYBODONTOIDEA OWEN, 1846

Figures

Figure 3. Wapitiodus aplopagus gen. et sp. nov.: photograph (A) and drawing (B) of holotype specimen TMP 97.74.10. See the Appendix for the abbreviations.

Figure 4. Wapitiodus aplopagus gen. et sp. nov.: photograph (A) and drawing (B) of the skull of holotype TMP 97.74.10. See the Appendix for the abbreviations.

Figure 5. Wapitiodus aplopagus gen. et sp. nov.: dentition of holotype TMP 97.74.10. A, close-up of a single tooth from (B); unicuspid teeth may occasionally reveal vestigial cusplets (arrowhead). B, anteriormost tooth files (7–10) with unicuspid teeth. C, lateral and distal (posterior) teeth lacking cusps. D, close-up of (C) showing the absence of a central cusp and the conspicuous transverse crenulations on tooth crowns.

Figure 6. Wapitiodus aplopagus gen. et sp. nov.: the?metapterygium is the only element of the pectoral girdle visibly preserved in the holotype TMP 97.74.10. See the text for a discussion.

Figure 7. Wapitiodus aplopagus gen. et sp. nov.: dorsal fins and fin spines of holotype TMP 97.74.10. Note that the posterior fin spine (A) is much more slender and more elongate than the anterior one (B).

Figure 8. Wapitiodus homalorhizo sp. nov.: structure of fin spine in cross-sections (A–D) through the apical half of specimen UALVP 46528 (thin sections T1–3) and the external view of the apex of the fin spine in specimen UALVP 46529 (E, silicon peel dusted with NH4Cl). A, overview: note the large but secondarily obliterated, partly remodeled cavity and the large tubercles clearly delimited from the core of the fin spine by lines of arrested growth. B, note the couple of displaced and stacked tubercles in the left postero-lateral corner (as indicated in A) at a slightly lower level of the spine (thin section T3). C, note the internal structure in the posterolateral wall of the fin spine that is characteristic of ctenacanthoids. D, attempts at restoration of the cross-section through the apical third. E, note the well-spaced stellate tubercles (arrowhead 2), the absence of recurved denticles in the posterolateral wall (arrowhead 1, but also see Fig. 10 and the description of specimen UALVP 46528) and the partly preserved anterior rim (arrowhead 3).

Figure 9. Wapitiodus aplopagus gen. et sp. nov.: morphologic variation of dermal denticles as found in various body regions in holotype TMP 97.74.10. A, dermal denticles from between the pelvic and anal fins. B, dermal denticles from the tip of the lower jaw. C, dermal denticles from the dorsal trunk area. D, dermal denticles from the tip of the anterior dorsal fin. E, dermal denticles from the mid-area of the posterior dorsal fin. F, dermal denticles from the anterior rim of the fin above the posterior dorsal fin spine. The denticles are not necessarily either in situ or orientated in the same way, see the text for the description. The shark was drawn by Beat Scheffold, PIMUZ.

Figure 10. Wapitiodus aplopagus gen. et sp. nov. An isolated recurved denticle possibly shifted from the posterolateral wall (?) as discovered in a thin section of specimen UALVP 46529.

Figure 11. Wapitiodus aplopagus gen. et sp. nov.: specimen UALVP 46529; photograph (A, dusted with NH4Cl) and sketch (B) of the anterior body preserved in a ventral view with pectoral fins, anterior fin spine and pelvic girdle. See the Appendix for the abbreviations.

Figure 12. Wapitiodus aplopagus gen. et sp. nov. Slightly displaced dermal denticles, probably from the midtrunk region as preserved in specimen UALVP 46527. Anterior is to the right.