Ischnacanthiformes Berg, 1940

Voichyshyn, Victor & Szaniawski, Hubert, 2012, Acanthodian jaw bones from Lower Devonian marine deposits of Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 879-896 : 883-886

Order Ischnacanthiformes Berg, 1940 Family Ischnacanthidae Woodward, 1891 Genus Podoliacanthus nov.

Type species: Podoliacanthus zychi sp. nov.; see below.

Etymology: Referring to Podolia, where fossils were first found, and Latin acanthus, thorn.

Species included.— Besides the type species, Podoliacanthus sp. 1 , P. sp. 2, and P. sp. 3, all are from Early Devonian of Podolia .

Diagnosis.—Small ischnacanthiforms with jaw bones from about 3 mm to 7 mm long. Conical teeth of the lateral tooth row are generally subcircular−triangular in parabasal section. The teeth, which increase in size anteriorly, have narrow posterior flanges and variable additional dentiform structures attached to their base or lower third of the tooth on the medial side. These structures look as small robust denticle(s) and/or short vertical ridge. Lingual ridge of jaw bone is sloped down anteriorly and bears a tooth row or tooth field which terminates posterior to the lingual ridge end. The lingual tooth row consists of groups of denticles, with one principal (central) denticle and usually two (or rarely more) smaller ones. The denticle groups of the lingual row have a common elongate base that is surrounded by several small pores of vascular canal system. Inter−tooth pits of the lateral tooth row are quite clearly defined, rather deep and posses or not pores of vascular canals on their surface. Inter−tooth flanges comprise three to seven denticles that decrease in height/number posteriorly. Posterior part of the jaw bone has either postero−lateral and postero−medial processes, or only a postero−lateral process.

Remarks.—If we assume that we are dealing with adult remains, dentigerous jaw bones from other genera differ in being significantly larger in size. Most also lack the additional denticles medially placed at the tooth base. No other genera with jaw bones having a lingual tooth row show the same denticle groupings as in Podoliacanthus gen. nov. In contrast, Atopacanthus (Middle Devonian of Russia, Spitsbergen, and USA) differs in having circular teeth (in parabasal section) in the lateral tooth row, with striated lingual face and smooth labial face, and by lacking inter−tooth denticles; length of the jaw bone of the genus is estimated at 30–80 mm ( Burrow 2004b). Cacheacanthus (Early Devonian of USA) differs in having monocuspid teeth with a D−shaped parabasal section in the lateral tooth row; the jaw bone is up to 50 mm long ( Burrow 2007). Gomphonchus (Late Silurian to Early Devonian of England, Estonia, Greenland, Lithuania, Nova Scotia, Podolia, Severnaya Zemlya, Spitsbergen, Sweden, and Timan− Pechora) differs in having a single row of teeth that are typically circular in parabasal section ( Denison 1979; Burrow 2004a: fig. 2B); the genus was originally based on isolated scales, and there is no certainty that jaws, teeth and scales attributed to this genus are from related fishes (see Hanke et al. 2001b). Grenfellacanthus (Late Devonian of Australia) differs in having large, broad−based teeth in the lateral row with a lunate parabasal section, a wide lingual field of parallel longitudinal rows of regularly spaced small denticles and a separate lingual row of small equidimensional teeth along the posterior half of the bone, and in lacking inter−tooth denticles; the jaw bone is estimated at 90–100 mm of length and is probably the second largest ischnacanthiform after Xylacanthus grandis ( Long et al. 2004) . Ischnacanthus (Late Silurian to Early Devonian of Canada, Great Britain, and USA) differs in having large teeth of the lateral tooth row with subcircular or oval parabasal section, in lacking additional denticles at the base of tooth and a lingual tooth row; the jaw bone length is 14–90 (up to 150?) mm; assuming all specimens are adults total body length is 4 to 16 cm ( Ørvig 1967; Denison 1979; Long 1986; Hanke et al. 2001b; Burrow 2002, 2004a, 2007). Persacanthus (Late Devonian of Canada, Iran, and USA) differs in having subpyramidal teeth, ornamented on their lingual faces with vertical sharp crests; estimated jaw bone length for fishes in this genus is 30–50 mm ( Denison 1979; Reed 1986; Valiukevičius 1992; Hermus and Hanke 2002; Burrow 2004b). Plectrodus (Late Silurian of England) differs by the lack of denticles attached to each tooth of the lateral tooth row ( Denison 1979). Rockycampacanthus (Early Devonian of Australia) differs in having large teeth in both the lateral and lingual tooth rows, and in teeth of the lateral tooth row having two rows of secondary cusps on their lingual face; jaw bone is about 14 mm long ( Long 1986). Taemasacanthus (Early Devonian of Australia) differs in having two rows of large teeth separated by a longitudinal ridge, and teeth (circular in parabasal section) with up to 10 vertical noded ridges; the jaw bone length in different species ranges between 15–36 mm ( Long 1986); Burrow (2002) conditionally referred to this genus jaw bone fragments which are similar in size to Podoliacanthus gen.

http://dx.doi.org/10.4202/app.2011.0079

nov. Xylacanthus (Late Silurian to Early Devonian of Canada and Spitsbergen) differs in having coarsely striated monocuspid teeth that are (sub)circular in parabasal section, and low densely spaced denticles forming the lingual tooth row; estimated length of the lower jaw is from 57 to 350 mm ( Ørvig 1967; Denison 1979; Long 1986; Gagnier and Goujet 1997; Hanke et al. 2001b; Long et al. 2004). X. minutus from the Late Lochkovian (or Pragian according to Hanke et al. 2001b) of Spitsbergen has additional medial denticles on the tooth base comprising one or two very small denticles situated behind the principal medial vertical stria of the teeth in the lateral tooth row ( Gagnier and Goujet 1997: fig. 2). These denticles, in comparison with those of Podoliacanthus gen. nov., are very small (1/9 of the teeth height) and probably of different shape, appearing to be tiny tubercles rather than denticles. Youngacanthus (Lochkovian of China) differs in having teeth in the lateral tooth row of triangular parabasal section formed by three well developed ridges at their anterior, posterior and medial margins; the teeth lack medially attached denticles; the lingual field is randomly covered with small blunt tubercles; estimated total length of jaw bone is about 10 mm ( Wang 1984; Long 1986; Burrow 2004a). Zemlyacanthus (Lochkovian of Severnaya Zemlya) differs in having thick tricuspidate teeth with 2–3 sharp vertical keels; the numerous tiny low denticles (rather tubercles) of the lingual row are randomly and densely distributed and appear to have more numerous vertical ridges than the teeth ( Valiukevičius 1992: fig. 5D, pls. 4: 3, 9: 1a, b); the jaw bone is about 30 mm long ( Valiukevičius 1992).

The genera Cambaracanthus and Cavanacanthus from the Lower Devonian of Australia are considered synonyms of Taemasacanthus since their species are based on probably incomplete jaw bones which are similar to the jaws of Taemasacanthus erroli ( Burrow 2002) . In contrast with Podoliacanthus gen. nov., these forms possess vertical denticle rows surrounding each tooth of the lateral tooth row. Length of their jaw bones is about 15–22 mm ( Lindley 2000) corresponding to of the size in Taemasacanthus ( Long 1986) .

An isolated jaw bone SMNH P4229 ( Ørvig 1967: pl. 3: 1, 2) from the Upper Silurian of Oesel ( Estonia) was assigned to Nostolepis sp. following Gross (1957), but Denison (1976) determined that it must derive from an ischnacanthid acanthodian. It bears some resemblance to the material described here in having medial “side−cusps” which, according to Ørvig (1967: 147), are “not infrequently found in Nostolepis ”. However, this medial cusps comprises three or four denticles arranged in a vertical row ( Ørvig 1973: text−fig. 1A, B) resembling such (more numerous) rows in Taemasacanthus . Also the specimen SMNH P4229 has a lingual row of denticles that are thick, low, randomly arranged, with vertical striations and not grouped as in the lingual tooth row of Podoliacanthus gen. nov. All the denticles of the specimen from Oesel, independent of their position, seem to have blunt, probably, worn tips. The teeth, at least the biggest and non−broken one, have vertical keels (three keels form a triangular parabasal section of the tooth) and a similar size to those of Zemlyacanthus menneri ( Valiukevičius, 1992). The SMNH P4229 specimen is also much longer than the jaw bones of Podoliacanthus gen. nov., being 9.5 mm with six teeth in the lateral row what is at least three times as much as in Podolian material. Another ischnacanthiform jaw bone fragment, SMNH P596 from Scania ( Sweden), also has a medial additional denticle on at least one tooth ( Ørvig 1967: pl. 2: 3). This specimen has three teeth preserved and is 7.3 mm long. Its other features cannot be compared because of poor preservation.

Burrow (1995) described as “Ischnacanthid fam., gen. et sp. indet.” a jaw bone fragment ( MMMC 02279; Burrow 1995: fig. 3C) from the Trundle beds (now Connemarra Formation; Lochkovian–Pragian) of central New South Wales ( Australia). Subsequently she conditionally referred the specimen to Trundlelepis cervicostulata ( Burrow 2002: fig. 29E, F). This specimen is, in some morphological features, similar to the material described here. The jaw bone fragment is 4 mm long with five teeth of the lateral tooth row preserved. The anterior teeth (“main cusps” in Burrow's terminology) of this fragment have “one medial cusp”. Judging from an enlarged photograph (Burrow 1995: fig. 3D) the position of small additional medial denticle is the same as in Podoliacanthus zychi gen. et sp. nov. However, in contrast to the Podolian material the lingual tooth row bears “irregularly clustered tubercles varying from less than 0.1 mm wide to 0.5 mm wide at the base, and with from five to 13 or more radiating ribs”. The lingual tooth row of this specimen has its largest tubercles concentrated near the lateral tooth row, while the smallest ones are situated more medially ( Burrow 2002: fig. 29E, F).

Microfossils assigned to Gomphonchus ? turnerae Burrow and Simpson, 1995 from the Late Silurian of Australia include a 1 mm long fragment of dentigerous jaw bone. The specimen is figured in non−occlusal (basal) view showing concavity of its base ( Burrow and Simpson 1995: fig. 6D). “Mesial ridge and a higher lateral ridge, separated by a shallow groove” as well as one preserved “cusp” are mentioned in the description of its occlusal surface. However, its morphological details are not known.

A dentigerous jaw bone from the Early Devonian of the Northwest Territories of Canada, (specimen NMC 22728) was described by Bernacsek and Dineley (1977: text−fig. 11) as an “atypical” Ischnacanthus sp. The specimen, less than 10 mm long, has nine teeth in the lateral tooth row (the foremost one is completely broken). Usually two or three additional denticles are situated near the tooth lingual side. These denticles are of irregular size and shape. There are also single denticles on inter−tooth pits between 6th and 7th teeth, and 7th and 8th ones. The lingual ridge bears “two rows of blunt denticles” which are densely set. Referring to the specimen as “atypical” Ischnacanthus sp. Bernacsek and Dineley (1977) noted its resemblance to Nostolepis (sensu Ørvig 1973) . Now there is consensus that Ischnacanthus lacks of lingual tooth row ( Denison 1979; Burrow 2004a), and near consensus concerning the lack of dentigerous jaw bones in Nostolepis ( Gagnier and Wilson 1995; Gagnier and Goujet 1997; Hanke et al. 2001b). Thus the specimen should not be recognized as belonging to these taxa (see also Burrow 2004a, 2007). It differs from Podoliacanthus gen. nov. in morphological details of the medial side denticles and lingual tooth row. Another jaw bone fragment, specimen NMC 22708 ( Bernacsek and Dineley 1977: text−fig. 16C) from the same locality is about 4.5 mm long and has six teeth in the lateral tooth row. Each of the teeth has a small denticle at its base, as in Podoliacanthus gen. nov., but it differs in the morphology of the lingual row which has long−based “laterally compressed cusps” surrounded by numerous small denticles. Preservation and/or illustration of the specimen does not allow determination of its affinity.

Only one jaw bone fragment from outside Podolia almost certainly belonging to the new genus is MGUH VP 3617 from the Late Silurian or Early Devonian of North Greenland ( Blom 1999: fig. 41K). Its features indicate that it should be assigned to the type species of the new genus (see Remarks in the description of Podoliacanthus zychi gen. et sp. nov. below) .

The new genus is provisionally assigned to the Ischnacanthidae , but there is not enough data to be sure in this opinion. Diversified scales and fin spines, which were found in examined samples, could belong to any other acanthodian taxon. Moreover there are insufficient family diagnostic criteria concerning jaw bones within the ischnacanthiforms.

Acritolepis (Early Lochkovian, Severnaya Zemlya) was referred by Valiukevičius (2003) to the Order Climatiiformes fam. indet. based on the morphology and histology of its scales, although its type species, A. ushakovi , has typical ischnacanthiform jaw bones ( Valiukevičius 2003: fig. 3A). Valiukevičius (2003: 134) stated that jaw bones of the type species “bear ankylosed teeth (main cusps with intercusps) which are close to Nostolepis Pander, 1856 ( Gross 1957), but this occur in other genera ( Parexus , Vernicomacanthus , Ptomacanthus , Climatius and Brochoadmones ) supplied with tooth whorls, which have not been observed in Acritolepis ”. However, none of the genera in this list (based on articulated fish) has dentigerous jaw bones. It is worth noting that generic determination of the jaw bones referred by Gross (1957) and Ørvig (1967) to Nostolepis is questionable ( Denison 1976). This genus is characterized (concerning dentition) by having tooth whorls ( Denison 1979). The cartilaginous jaws of Parexus , Vernicomacanthus , Climatius , and Brochoadmones bear tooth whorls, but not ankylosed teeth ( Denison 1979). In Ptomacanthus “the dentition comprises both upper and lower jaw teeth in the form of spirals …; there are no dentigerous jaw bones” ( Miles 1973), and “the upper and lower jaws bear tooth whorls” ( Denison 1979). Presence of dentigerous jaw bones in Acritolepis , whatever the affinities of the genus (see, for example, Burrow 2004a), obliges comparison with Podoliacanthus gen. nov. In contrast with the new genus, Acritolepis dentigerous jaw bones have teeth which “have a triangular longitudinal and stretched rhombic transverse basal form with a rounded outer line” and bear “an emerged inner keel”. The intercusps of Acritolepis are striated. Jaw bones of Acritolepis are 13–32 mm long, and teeth of the lateral tooth row are 2.1–3.5 mm height, far higher than in Podoliacanthus gen. nov.

Acanthospina (Early Lochkovian, Severnaya Zemlya) was originally designated as Acanthodii incertae sedis, with “chondrichthyan−type scales, composed of “ Nostolepis ”− type tissues, … and ischnacanthid−like (particularly Poracanthodes − type) teeth” ( Valiukevičius 2003: 187–188). Burrow (2004a) considered the taxon ischnacanthiform. It differs from Podoliacanthus gen. nov. in having “slightly basally striated main cusps (up to 7 mm high)” with “stretched highpyramidal longitudinal section” ( Valiukevičius 2003: fig. 46A). Position of the “lateral cusps” mentioned in original description was not indicated. Since the teeth (“main cusps’) size is much bigger than the total supposed length of the jaw bones of Podoliacanthus gen. nov. the size difference between the compared genera is significant.

There are a number of supposed ischnacanthiform genera based on isolated scales, tooth whorls or fin spines but their jaw bones are generally not known. These are Acanthopora (Early Devonian [Early Lochkovian], Severnaya Zemlya). Arcticacanthus (Lochkovian, Severnaya Zemlya) , Arenaceacanthus (Late Silurian, Lithuania), Bracteatacanthus (Late Silurian, Lithuania), Ectopacanthus (Early to Middle Devonian, Baltic States, Podolia, Spitsbergen), Garralepis (Early Devonian, Australia), Lietuvacanthus (Early Devonian, Baltic States, Podolia), and Rohonilepis (Late Silurian, Lithuania) (Valiukevičius 1979, 1998, 2000, 2003, 2004; Burrow 2002; Hairapetian et al. 2006).

Gomphonchoporus , Radioporacanthodes (Late Silurian to Devonian, Australia and Northern Europe), and probably, Trundlelepis (Early Devonian [Lochkovian to Pragian], Australia) also have been described after isolated scales and tooth whorls ( Burrow 2003a; Hairapetian et al. 2006). All of these together with Zemlyacanthus (see above) are placed in the family Poracanthodidae (another family of the Ischnacanthiformes ) based on the scale histology ( Vergoossen 1997). Poracanthodes Brotzen, 1934 , the type genus of this family, was also originally established on isolated scales, but jaw bones have since been referred to it as well. Valiukevičius (2003: fig. 28A) figured such a jaw bone fragment from Severnaya Zemlya with tooth cusps of the main row in lateral view. The teeth of this specimen are about 4 mm high, comparable to the total length of jaw bone of the new genus. Burrow (2003b: fig. 3) also figured a jaw bone fragment among dissociated remains of Poracanthodes punctatus Brotzen, 1934 from the Upper Silurian of Nevada. The fragment is about 10 mm long and seems to have three teeth of the lateral tooth row, probably triangular in parabasal section. However, the surface of the specimen is too eroded to discern its morphological features. A small 2.1 mm long fragment of poracanthodid dentigerous jaw bone from the Late Silurian of Scania (southern Sweden) has one tooth of the lateral tooth row with four inter−tooth denticles and strong lingual row dentition which includes densely placed, low, obtuse denticles of differentiated size ( Vergoossen 2004: pl. 8: 86). The largest denticles

http://dx.doi.org/10.4202/app.2011.0079

are comparable in size with the single preserved tooth. Judging from the photograph the tooth and largest denticles have ribs or striation on their sides.

Several genera which had previously been referred to ischnacanthid acanthodians are now referred elsewhere. These are Acanthodopsis (see Denison 1979; Burrow 2004a; Long et al. 2004), Uraniacanthus (see Miles 1973; Denison 1979; Hanke et al. 2001a, b; Burrow 2004a; Hanke and Wilson 2004; Hanke and Davis 2008), Machaeracanthus (see Lehman 1976; Denison 1979; Reed 1986; Burrow and Young 2005; Maisey and Melo 2005; Südkamp and Burrow 2007; Burrow et al. 2010), Apatheacanthus (see Denison 1979; Burrow 2004b), Doliodus (see Denison 1979; Miller et al. 2003), and Helenacanthus (see Denison 1979; Burrow 2004a, b).

Stratigraphic and geographic range.—Early Devonian (Late Lochkovian) of Podolia, Ukraine ( Fig. 1: point 3; Late Silurian or Early Devonian of north Greenland, Denmark).