Lupopsyrus pygmaeus Bernacsek & Dineley, 1977

Hanke, Gavin F. & Davis, Samuel P., 2012, A re-examination of Lupopsyrus pygmaeus Bernacsek & Dineley, 1977 (Pisces, Acanthodii), Geodiversitas 34 (3), pp. 469-487 : 472-478

publication ID	https://doi.org/ 10.5252/g2012n3a1
persistent identifier	https://treatment.plazi.org/id/038D5604-FFDC-044D-FCF1-0737F7B8FDD3
treatment provided by	Marcus
scientific name	Lupopsyrus pygmaeus Bernacsek & Dineley, 1977
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Treatment

Lupopsyrus pygmaeus Bernacsek & Dineley, 1977 ( Figs 1-6 View FIG View FIG View FIG View FIG View FIG View FIG )

HOLOTYPE. — NMC 22715 ( Bernacsek & Dineley 1977: text-fig. 3A, pl. 1).

REFERRED MATERIAL. — NMC 22700B, C, 22718, 22719, 22700D-F, 22701C, D, 22716, 22717, 22720, 22745. — UALVP 19260, 32420, 32442, 32456, 32458, 32474, 32476, 32480, 32482, 39065, 39067, 39079-

39082, 39121, 41493, 41629, 41632, 41665, 41931, 41939, 41945, 42000, 42002, 42008, 42012, 42013, 42027, 42046, 42061, 42113, 42142, 42150, 42173, 42208, 42274, 42518, 42524, 42529, 42530, 42533, 42538, 42453-42455, 42544, 42597, 42605, 43064, 43091, 43092, 43094, 43095, 43256, 43409, 43456, 45154, 45155.

LOCALITY AND AGE. — Lupopsyrus specimens are recovered from talus below a Lower Devonian (Lochkovian) horizon between 430-435 m in the MOTH locality section (as measured in 1996), central Mackenzie Mountains, N.W. T., Canada (see Hanke et al. 2001b: figs 1, 2); approximately 411 m in the section measured by the Geological Survey of Canada ( Gabrielse et al. 1973). The Devonian fish layer in the MOTH locality section is equivalent to GSC locality 69014 in section 43 of Gabrielse et al. (1973) and locality 129 in the UALVP catalogue system. Although previous authors have suggested habitats ranging from intertidal lagoons to deep-water shelf settings, recent sedimentological, ichnological and taphonomic study suggests an oxygen-poor, intra-shelf topographic low below storm wave base ( Zorn et al. 2005) on a shelf that fringed western Laurussia (combined Laurentia and Baltica; Li et al. 1993).

REVISED DIAGNOSIS. — Acanthodians with two longitudinal rows of enlarged keeled scutes situated along the posterior half of each side of the body and caudal fin axis; largest keeled scutes located below the second dorsal fin base; three dermal opercular gill plates per side; pectoral, pelvic, anal and dorsal spines with widely separated ribs with fine nodular ornamentation; single prepectoral spine positioned over lateral end of each procoracoid; prepelvic fin spines with unornamented blade-like posterior lamina; circumorbital scales identical to head scales; head and body scales have crown with prominent median keel and one lateral flange per side; central keel and lateral flanges of each scale terminating posteriorly in a single point; scales are monodontode with mesodentine, Stranggewebe-like crown histology.

DESCRIPTION

Much of the dermal covering over the head of Lupopsyrus pygmaeus is preserved on UALVP 41493 ( Fig. 1 View FIG ), and other new Lupopsyrus specimens in the University of Alberta collections. The head of L. pygmaeus as preserved is short at 10 % of body length, but the rostrum and jaws either have lost scale cover in all specimens, or lacked scale cover in life and therefore, the full snout length cannot be estimated ( Figs 1 View FIG ; 2B View FIG ); the jaws may also have fallen clear of the carcass in all specimens during decay. The dorsal surface of the head is covered with small scales which are similar in structure to body scales. These head scales differ slightly from those on the body in that the posterior apex of the crown of each scale is short, blunt and does not extend posterior to the basal rim surrounding the pulp canal. The gradual transition between head and body scales occurs over the branchial chamber; enlarged head scales and/or tesserae are absent ( Figs 1 View FIG ; 2 View FIG ).

The orbits of L. pygmaeus lack ossified sclerotic plates and are highlighted by an area of silveryblack material within the ring of micromeric circumorbital scales (note that terminology follows Burrow et al. [2011]). The dorsal and posterior margin of the orbit has scales which are identical to normal head scales, and there are no enlarged circumorbital scales ( Figs1 View FIG ; 2 View FIG ). There are no scales anterior and ventral to the orbit in any of the available specimens, therefore the lachrymal and labial portions of the head are unknown and may have lacked scales in life.

The jaws, branchial arches and endocranium are not mineralized. Teeth are absent and there is no indication of jaw extent or shape. The position of the otic capsule, and therefore, the position near the posterior end of the braincase is indicated by small masses of sandy, otic material, which Sahney& Wilson (2001) interpret as otic statoconia. When scale cover is complete, these masses appear only as paired bulges ( Figs 2B View FIG ; 4B View FIG ), but where scales are lost, the light coloured sandy material is exposed. Many L.pygmaeus specimens have heads preserved as dorsoventral compressions, and the masses of otic material are well-separated ( Figs 2B View FIG ; 4B View FIG ). This preservation suggests that the braincase and head of L. pygmaeus was fairly broad rather than laterally compressed; a broad, depressed braincase is characteristic in several clades of gnathostomes ( Janvier 1996) and likely is a primitive feature for gnathostomes.

Sensory lines preserved on very few L. pygmaeus specimens run between scales ( Figs 1 View FIG ; 2 View FIG ). The supramaxillary sensory canal and preopercular sensory canals converge posteriorly across the cheek anterior to the hyoidean gill plates ( Fig. 1 View FIG ); the latter canal meets the main sensory line dorsally. The supraorbital sensory canal traces converge posteriorly, but its entire course cannot be determined in the available specimens ( Fig. 2A, B View FIG ). One specimen shows the trace of the infraorbital canal posteroventral to the orbit ( Fig. 2B View FIG ). There also are traces of short, paired, converging, middle pit lines, and posterior pit lines preserved near the level of the otic region of the braincase ( Fig. 2A View FIG ). Behind the posterior pit lines are a pair of gaps in the scale cover which may indicate the position of the external endolymphatic duct openings ( Fig. 2A View FIG ). The occipital cross-commissure is seen as a short canal trace leading dorsally from the main lateral canal level with the origin of the pectoral fin spine ( Fig. 1B View FIG ).

The branchial chamber of Lupopsyrus pygmaeus is not well-preserved in any specimens, but appears compact relative to the orbito-otic region; the extent of the branchial chamber is estimated from the position of the otic material and the position of the pectoral girdle ( Fig. 2B View FIG ). There is no evidence of gill openings, single or multiple, but the presence of three dagger-like dermal plates located in an arc over the opercular region indicates that a single opercular flap was present rather than a series of narrow separate opercular flaps ( Figs 1 View FIG ; 2A, B View FIG ). The middle plate of the operculum is larger than the dorsal and ventral plates, and each plate has a single longitudinal keel, surrounded by small tubercles. This keel is serrated, and its summit is near the mid-length of the plate. The underside of each opercular plate possesses a shallow trough which continues along the entire length of the plate; the basal rim is tear-drop shaped, widest anteriorly, and tapers posteriorly.

The dorsal fins and spines were described in detail by Bernacsek & Dineley (1977), although they did not mention that the anterior dorsal fin spine had a shallow insertion area ( Fig. 1 View FIG ). This basal portion presumably anchored the spine into the epaxial musculature. Lupopsyrus pygmaeus lacks enlarged scales around the base of each fin spine.

The anterior and posterior dorsal fin spines support fin webs which possess a convex trailing margin extending posterior to the apex of the fin spine, and also have irregularly-arranged scales ( Figs 1 View FIG ; 3A View FIG ). The distal half of each dorsal fin web appears to be detatched from the dorsal fin spine ( Fig. 3A View FIG ), but this may be an artefact of preservation or preparation. Fin-web scales are minute, and apart from size, are identical to typical body scales. There is a gradual size transition between typical body scales and small fin web scales ( Fig. 3A, B View FIG ).

The pectoral fin spine was described in detail by Bernacsek & Dineley (1977) and no new information can be added in our account. However, details of the pectoral dermal plate armour require further clarification. Bernacsek& Dineley (1977) mentioned that a single lorical plate, with a single, median spine was present on the holotype of Lupopsyrus pygmaeus . They also indicated that the lorical plate was poorly preserved, as is evident in the photographs published with the species description. A median lorical plate is not known in any University of Alberta Lupopsyrus specimens ( Figs 2A, B View FIG ; 4 View FIG ), although in one, a small pile of displaced scales positioned posterior to the pectoral girdle creates the impression of a small, elevated node ( Fig. 4B View FIG ). It may be possible that the “lorical plate” described by Bernacsek & Dineley (1977) was a similar mass of scales.Specimens examined at the Canadian Museum of Nature lack both lorical plates and median “lorical” spines.

The margins of the prepectoral spines cannot be distinguished in the figures presented by Bernacsek& Dineley (1977), and they suggest that the prepectoral spines are completely fused to what they described as compound pinnal plates. The University of Alberta specimens show that the prepectoral spines have a distinct basal rim which denotes the perimeter of each spine ( Figs 2A View FIG ; 4 View FIG ), and these spines sit over the lateral end of procoracoid bones (see below). The prepectoral spines are curved, have a broad basal cavity, and possess longitudinal ribs which are ornamented with fine nodes ( Figs 2A View FIG ; 4 View FIG ).

Bernacsek & Dineley (1977) wrote that L. pygmaeus had compound pinnal plates, and went so far as to suggest a homology between the first and second pinnal plates of Climatius reticulatus . In fact, the bones that subtend both prepectoral spines and contact the anterior-most base of the pectoral spines are smooth, unornamented, and have a surface texture similar to that of the perichondrally-ossified scapulocoracoids, not like the dermal tuberculated plates which are characteristic of climatiid pectoral dermal armour ( Miles 1973; Watson 1937; Denison 1979). In L.pygmaeus , these bones extend anteromedial to the scapulocoracoid and meet at the ventral midline ( Figs 2A, B View FIG ; 4 View FIG ). We interpret these smoothly ossified structures to be procoracoids, based on structure and similarity of position to procoracoids of other acanthodians. The procoracoids have a small fossa on the lateral margin ( Fig.4 View FIG ) which may have served as the point of articulation for the proximal end of the pectoral fin spine. A revised reconstruction of the pectoral girdle of Lupopsyrus pygmaeus , based on UALVP 32476, 39079 and 39080 ( Figs 2 View FIG ; 3 View FIG ) is presented in Figure 5 View FIG . Any apparent ornamentation on the procoracoids ( Fig. 4A View FIG ) consists of overlying scales which had settled on the bone during decay and preservation of the carcass.

Two specimens (UALVP 39080 and 41493) show that a pectoral fin web was present posterior to the pectoral fin spine.The fin web is covered with minute scales, and the trailing edge of the fin probably was convex ( Fig. 1 View FIG ). The fin web is not attached to the fin spine in UALVP 41493; additional specimens will be needed to determine whether the detatchment of the pectoral fin web in UALVP 41493 is a taphonomic artefact.

Little can be added to the descriptions of the prepelvic and pelvic fin spines. However, since the material available to Bernacsek & Dineley (1977) was poorly preserved, they could not determine which prepelvic spine had a posterior, flat lamina. Based on the material at the University of Alberta, all prepelvic spines of Lupopsyrus pygmaeus possess this flat, unornamented, trailing lamina, which is most prominent on the posterior-most prepelvic spine pair ( Fig. 1 View FIG ).

The pelvic fin web has a convex distal margin which extends beyond the apex of the fin spine, and overlaps the origin of the anal fin spine ( Fig. 1 View FIG ). The anal fin web also has a convex posterior margin, is sub-triangular, and extends posterior to the distal tip of the anal fin spine. Both the pelvic fins and anal fin are covered with scales which are identical to those of the dorsal fins, and the transition between typical body scales and those of the pelvic and anal fins is gradual ( Fig. 3B View FIG ).

The epicercal heterocercal caudal fin is elongate with only a slight dorsal deflection relative to the body axis ( Figs 1 View FIG ; 3C View FIG ). The hypochordal lobe is elongate and the entire tail superficially resembles those of several extant slow swimming sharks such as the frilled shark, hexanchids and scyliorhinids (see examples in Castro 1983; Compagno et al. 2005). The posterior end of the caudal fin axis extends beyond the hypochordal lobe of the fin ( Fig.3C View FIG ). The leading edge of the hypochordal lobe is covered with scales which are similar to those on the rest of the fin ( Fig.3D View FIG ); this condition is in contrast to that of other acanthodians where slightly enlarged scales reinforce the leading edge of the caudal fin.Body scales on the caudal fin axis grade into typical caudal fin scales with no abrupt change in scale size, and these irregularly-arranged scales decrease in size along the caudal axis toward the posterior tip ( Figs 1A View FIG ; 3C View FIG ).

The scales of L. pygmaeus change little in size over the body ( Fig. 1 View FIG ), although as mentioned above, smaller scales are found on the fin webs, the posterior portions of the caudal fin axis, and on the head ( Figs 1A View FIG ; 2 View FIG ; 3 View FIG ). Bernacsek & Dineley (1977) described the external ornamentation of the scales of L. pygmaeus , but their photographs did not show the fine details of scale structure and pattern. All scales are ornamented with a central crest and a pair of lateral flanges which possess smooth edges ( Fig.6 View FIG A-D) and merge posteriorly to form the trailing tip of each scale. The pulp cavity extends into the posterior portions of the central crest ( Fig. 6C View FIG ). Lupopsyrus scales have a developed neck, but no neck canals were identified in the scales which were sectioned. Internally, the scales of L. pygmaeus are simple monodontode structures ( Fig. 6G View FIG ). The scale crowns appear to be mesodentinous with parallel cell spaces resembling that of Stranggewebbe; the tissue around the basal rim of the neck also may be cellular ( Fig. 6G View FIG ), but the poor preservation of histological detail prevents detailed comparison. The underside of the scale neck forms a concave, rhombic rim which lacks traces of Sharpey’s fibres or cellular basal tissue and the pulp cavity remains open ( Fig. 6A, B View FIG ). The apparent lack of Sharpey’s fibres is problematic given that MOTH fishes have poor histological preservation. It is also impossible to determine whether the scales of L. pygmaeus were aligned in life, although the rhombic shaped neck and basal rim ( Fig. 6B View FIG ) suggests that scales could have aligned in oblique rows.

Similar monodontode scales lacking basal tissue are known in thelodonts and certain chondrichthyans as well as modern elasmobranchs (Karatajute-Talimaa 1973, 1992, 1998; Turner 1991; Hanke & Wilson 2004; Märss et al. 2002). Bernacsek& Dineley(1977) suggested that the scales of L. pygmaeus were simple as a secondary specialization rather than a primitive feature relative to the complex, layered scales of other acanthodians (and compound scales of some putative chondrichthyans).

Little can be added to the description of the flank scutes of Lupopsyrus pygmaeus ( Fig.6E, F View FIG ). The scutes are largest at the level of the second dorsal fin and on the caudal peduncle ( Figs 1 View FIG ; 3C View FIG ). This region corresponds to the region where scales are first added during ontogeny in acanthodid species ( Zidek 1985, 1988), and may indicate a similar origin for scale development in L. pygmaeus . There is no evidence that the scutes conducted the main sensory canal as discussed by Bernacsek & Dineley (1977), but the scutes may have functioned to direct water along the caudal peduncle and reduce turbulence as do scales of extant sharks ( Reif 1978; Reif & Dinkelacker 1982; Dean & Bhushan 2010).