Saurichthys undetermined, MAXWELL & ROMANO & WU & FURRER, 2015

Maxwell, Erin E., Romano, Carlo, Wu, Feixiang & Furrer, Heinz, 2015, Two new species of Saurichthys (Actinopterygii: Saurichthyidae) from the Middle Triassic of Monte San Giorgio, Switzerland, with implications for character evolution in the genus, Zoological Journal of the Linnean Society 173 (4), pp. 887-912 : 896-904

SAURICHTHYS BREVIABDOMINALIS SP. NOV.

1992 S. paucitrichus – Rieppel, 1992 (partim): Text-Fig. 4C (PIMUZ T 890)

2013 S. paucitrichus – Wilson et al. 2013a: Figure 5

Differential diagnosis

Skull roof not strongly constricted between orbits (unlike S. costasquamosus ) and the part of the dermal skull roof anterodorsal to the orbital opening is formed to a large extent by the nasaloantorbital bone (unlike S. curionii ); teeth small (unlike S. costasquamosus ); opercle long (opercular depth to length ratio = 1.5; 2.0 in S. costasquamosus , 1.8 in S. paucitrichus , 1.9 in S. yunnanensis Zhang et al., 2010 holotypes); ossified medial shelf on medio-ventral lower jaw posterior to the symphysis absent but strong ridges present on ventral surface of lower jaw; abdominal region and caudal region approximately equal in length in adults (unlike S. costasquamosus ); rib-like mid-lateral scales present (shared with S. costasquamosus , S. paucitrichus , S. yunnanensis ); ventro-lateral scales anterior to pelvic girdle absent (unlike S. costasquamosus , S. yunnanensis ).

Etymology

The name refers to the proportionately short abdominal region of the body.

Holotype

PIMUZ T 67 ( Figs 5 , 6 ), the larger of the two fishes preserved on the block.

Description

The holotype ( Fig. 5 ) is a mid-sized individual (364 mm preserved length; tip of rostrum absent). The skull is crushed such that it is preserved in dorso-lateral view. The axial skeleton is complete and exposed in lateral view. The median fins are preserved as impressions on the sediment.

Stratigraphic provenance

Besano Formation, Upper Member, Bed 154 ( Fig. 1 ), early Ladinian, Middle Triassic ( E. curionii Zone : Brack et al., 2005).

Type locality

Mirigioli (P. 902), Monte San Giorgio , Canton Ticino, Switzerland.

Referred material

PIMUZ T 730 (Mirigioli [P. 902], Bed 144, late Anisian, N. secedensis Zone), PIMUZ T 890 (Mirigioli [P. 902], Bed 148, late Anisian, N. secedensis Zone), PIMUZ T 1685 (Miniera Val Porina [Tetto nuovo], no bed number available, late Anisian, N. secedensis Zone), PIMUZ T 972 (Mirigioli [P. 902], Bed 153, early Ladinian, E. curionii Zone ), PIMUZ T 501 (Mirigioli [P. 902], Bed 156, early Ladinian, E. curionii Zone ), PIMUZ T 658 (Mirigioli [P. 902], Bed 156, early Ladinian, E. curionii Zone ), PIMUZ T 664 (Mirigioli [P. 902], Bed 156, early Ladinian, E. curionii Zone ), PIMUZ T 660 ( Fig. 7C ); Mirigioli [P. 902], Bed 158, early Ladinian, E. curionii Zone ), PIMUZ T 661 ( Fig. 7A, B, D ); Mirigioli [P. 902], Bed 158, early Ladinian, E. curionii Zone ), PIMUZ T 665 ( Fig. 7E ; Mirigioli [P. 902], Bed 158, early Ladinian, E. curionii Zone ).

Remarks

PIMUZ T 730 is the stratigraphically oldest of the referred material. The vertebral count is higher than in the other specimens, and the opercle depth: length ratio is lower.

DESCRIPTION

The body is elongate (fineness ratio = 11.1). In general proportions, the skull is 28%, the abdominal region is 37% and the caudal region is 35% of fork length.

Skull

The cranium of the holotype is flattened but most bones are preserved in situ, with the dermal skull roof exposed in dorsal view ( Fig. 5B ). The skull measures approximately 100 mm from the tip of the rostrum to the jaw joint.

The rostrum is relatively short and anteriorly gracile. However, the tip of the snout is not preserved. The unpaired rostropremaxillary bone forms the main part of the rostrum. The most anterior preserved portion of the rostropremaxilla is ornamented with tubercles, however more posteriorly its lateral surface is ornamented with obliquely oriented parallel striations and its dorsal surface has a reticular ornamentation. Posterior to the rostropremaxilla lie the paired frontal and nasaloantorbital bones. The part of the dermal skull roof that is antero-dorsal to the orbit is formed entirely by the paired nasaloantorbital bones but the outline of the nasaloantorbitals is not clear in the holotype. Distinct supraorbitals are not observed. The large, paired frontals contact the nasaloantorbital bones antero-laterally. The suture between the left and right frontal bones is approximately straight, and is perpendicular to the posterior margin of the frontals. The frontals possess their greatest medio-lateral width at the level of the dermosphenotics, and their width progressively decreases anterior to the dermosphenotics. Distinct parietal bones are not observed, and these elements could be either absent or fused with neighbouring bones. Postero-lateral to the frontals are the relatively large dermosphenotics. The medial suture of the dermosphenotics is gently curved. At the posterior end of the dermal skull roof are the large, paired dermopterotics, the left one of which is largely complete. The left dermopterotic forms a long, pointed postero-lateral process. The suture between the left and right dermopterotic is not easily visible, but is probably straight, as in other species. Tubercles become superimposed on the reticular ornamentation of the dermatocranium, and increase in density toward the lateral margins of the skull roof.

Some bones of the lateral cranium are also preserved. The lachrymal bone is small and has a rectangular outline. However, its contact with the neighbouring nasalo-antorbital bone is not clear. Other elements of the infraorbital series have not been observed. The preopercle consists of an anterior and a ventral ramus, which meet more or less perpendicularly. The posterior margin of the preopercle is gently convex, whereas its antero-dorsal and antero-ventral margins are concave. The height of the maxilla progressively decreases towards the orbital opening. However, at the level of the orbit, the anterior process of the maxilla is still relatively high. The quadratojugal is preserved in situ, but its outline is not clear. Two elements of the sclerotic ring are also preserved in specimen PIMUZ T 661.

Only the most anterior portion of the left mandible is missing. The ventral margin of the lower jaw is sinusoidal in lateral view, being convex in the part formed by the angular bone and concave further rostrally. The dentary lies anterior to the angular and supra-angular bones and forms the bulk of the lateral mandible. The supra-angular extends from the dorsalmost anterior margin of the angular to a point equal to the midpoint of the orbit. However, the contacts between this bone and the neighbouring dentary and angular bones are not clearly visible. The posteroventral part of the lower jaw is made up of the triangular angular bone. The height of the angular rapidly decreases anteriorly until the level of the posterior end of the dermosphenotic, and more slowly further rostrally. The angular extends anteriorly approximately to the level of the anterior margin of the orbital opening. It is ornamented with tubercles, except on its posterodorsal section, on which reticular ornamentation predominates ( Fig. 5B ). Aside from the angular, the lateral surface of the lower jaw is ornamented with obliquely oriented parallel striations. Tubercles become increasingly common anteriorly. Where preserved, the dentition of the lower jaw consists of densely spaced, proportionally small conical teeth.

Sensory canals can be traced along the ventral margin of the lower jaw (mandibular canal), within the preopercle (preopercular canal), within the dermopterotic and dermosphenotic bones (temporal canal), and possibly also within the frontal bones (supraorbital canal).

Both opercles are preserved in situ in the holotype specimen. The right opercle is for the most part covered by the left one and by the elements of the dermal shoul- der girdle. The relatively large left opercle is almost complete, broken only towards the dorsal edge ( Fig. 5B ). It measures 18.5 mm long by 27.5 mm deep (estimat- ed based on the unbroken anterior and posterior dorsal corners). It has a semicircular shape and is characterized by a straight anterior margin, a short, oblique anteroventral border, and a long, rounded posterior margin. In the middle of the anterior margin, the opercle thickens slightly and there is a small protuberance for articulation with the hyomandibula. Ornamentation is reticular near the protuberance, and weakly developed tubercles are present along the dorsal and ventral extremities. Otherwise, the ornamentation consists of weak lines which parallel the posterior edge of the element.

Pectoral girdle

Of the dermal shoulder girdle of PIMUZ T 67, only the paired supracleithra and part of the left cleithrum are exposed. Most of the left cleithrum is covered by the left opercle. The cleithrum is triradiate in shape. With reference to the referred material, the anteroventral process is broad and bilobate at the anterior end, presumably for articulation with the clavicle ( Rieppel, 1985). The postero-ventral process is blunt and rounded. The ventral surface of the cleithrum is ornamented with a coarse ornamentation of tubercles and ridges; ornamentation on the opercular facet is weaker. There is a recessed lamina connecting the dorsal and posterior processes, which is unornamented. The medial edge of the cleithrum directed towards the contra-lateral element is straight anteriorly, and only slightly concave posteriorly. The dorsal process is relatively straight, directed postero-dorsally, and bears a ridge at the posterior extent of the opercular facet.

The right supracleithrum is preserved dorsal to the axial skeleton and its anterior portion is partially covered by the right dermopterotic. The left supracleithrum is located posterodorsal to the rest of the skull. The supracleithrum is a long (c. 28 mm), robust element, slightly curved in lateral view and heavily ornamented with tubercles. ( Fig. 5B ). A short segment of the lateral line canal can be seen along its upper margin. With reference to the referred material, the anterior end of the supracleithrum is broader and more rounded than the posterior end. On the lateral side, there is a rounded flange with reduced ornamentation that appears to have underlain the opercle.

The clavicle is preserved in situ in PIMUZ T 890. It is small and oval in shape, and has a weak concentric ornament on its ventral surface.

Axial skeleton

The holotype has 62 neural arch-like structures in the abdominal region and 56 in the caudal region, for a total of 59 vertebrae (31 + 28). Vertebral counts are variable, ranging from 56 vertebrae (29 + 27) in PIMUZ T 890, 58 vertebrae (28 + 30) in PIMUZ T 658 to 65 vertebrae (35 + 30) in PIMUZ T 730. Morphology of the axial skeleton in the abdominal region is obscured by the squamation, but the neural arches and arch-like elements appear to bear long neural spines in all regions of the column anterior to the caudal peduncle. Paired ventral ossifications are intermittently present. Posteriorly, alternating neural arches bear a strong ridge along their pedicles. Ossified haemal arches occur at a point dorsal to the pelvic girdle and modified anal loop scale. Each haemal arch bears an elongate ventral spine ( Fig. 6A ). The haemal arches have a 1:1 relationship with the neural arches. Posterior to the last dorsal axonost, the morphology of the neural and haemal arches changes. The neural arches become T-shaped, and the haemal spines cease to ossify, however rectangular ventral ossifications persist ( Fig. 6B ).

Paired fins

The paired fins are not well preserved in the holotype. The pectoral fins have minimally 12 lepidotrichia, which appear to be unsegmented. The pectoral fins are best preserved in PIMUZ T 890 , but have been twisted such that the shape cannot be accurately determined.

With reference to the referred material, the pelvic bone is large, weakly ossified, and located medial to the pelvic fins in several large specimens (PIMUZ T 660 , PIMUZ T 661 , PIMUZ T 658 ). The pelvic bone is broad, and extends from near the ventral midline to the base of the fin rays as a single homogenous ossification ( Fig. 7D ). The lepidotrichia of the pelvic fin are weakly ossified and are divided into at least two segments. In the largest referred specimens more than 17 lepidotrichia are present in the pelvic fin, similar to the minimum count of 16 in the holotype. Fourteen lepidotrichia are present in the small specimen PIMUZ T 890 , also consisting of at least two segments in the longer lepidotrichia.

Median fins

The bases of the dorsal and anal fins are long relative to their height. Twelve dorsal axonosts are preserved (range based on referred material: 10–12), the most anterior of which is broader and flatter than successive elements in the series ( Fig. 6A ). The axonosts have an inconsistent relationship with the axial skeleton, in some places showing a 1:2 pattern with the neural arches and in others a 1:1 pattern. All axonosts appear to support the lepidotrichia directly – there is no evidence for independently ossified baseosts. Each axonost supports up to four lepidotrichia. The dorsal fin is not well enough preserved in the holotype to provide an accurate count of the number of lepidotrichia, however in PIMUZ T 890, 37 lepidotrichia are present, divided into a maximum of three segments.

The anal fin is supported by 11 axonosts (range: 10– 13), the anteriormost of which is expanded and flattened as in the dorsal fin. The posterior six baseosts are independently ossified, however the most anteri- or ones appear to be co-ossified with the axonosts. The anal fin consists of more than 35 lepidotrichia. The longer fin rays consist of a minimum of four segments, and bifurcate at least twice. In the smallest specimen PIMUZ T 890, a maximum of only three segments are present.

The caudal fin is not deeply forked, and has a low aspect ratio ( Fig. 6B ). The epichordal and hypochordal lobes each consist of a minimum of 31 lepidotrichia divided into at least four segments (PIMUZ T 890: 30 and 29 lepidotrichia). Moreover, in both lobes, the second lepidotrichial segment is much shorter than the most proximal; the third segment is more variable in length, possibly correlated with ontogeny. Lepidotrichia bifurcate at least twice distally. PIMUZ T 658 has five segments in one of the lobes of the caudal fin, and minimally three bifurcations of the longest distal lepidotrichia ( Fig. 7E ).

Squamation

In the abdominal region, two scale rows, the middorsal and mid-lateral rows, are preserved. The middorsal scales are long and narrow, with the external surface covered by strongly developed tuberculate ornamentation. Immediately anterior to the dorsal fin, the breadth of the scales increases and their length decreases, such that they appear increasingly chevronshaped in dorsal view. The scale immediately anteri- or to the dorsal fin resembles an ornamented basal fulcrum. Posterior to the dorsal fin, the mid-dorsal scales resemble those from the part of the caudal region anterior to the dorsal fin, but are generally smaller in size. Immediately anterior to the caudal fin, the scales become longer, broader, and increasingly overlapping, forming chevron-shaped scutes overlying the caudal peduncle. The posteriormost scales have an antero-posteriorly short external surface.

The mid-lateral scales are rib-like, and bear a foramen for the lateral line. The area around this foramen is expanded, and is covered with a pronounced tuberculate ornamentation. The ‘ribs’ of the mid-lateral scales become dorso-ventrally reduced posteriorly.

Between the pelvic fin and the dorsal fin, the ventral rib-like processes of the mid-lateral scale row disappear ( Fig. 6A ). The ventral component of the body of the scale, however, remains until a point posterior to the dorsal fin where it becomes reduced. Scales posterior to the dorsal fin are ornamented with a single postero-ventrally directed spine bearing a translucent tip resembling a tooth that overlaps the following scale. The mid-lateral scale row continues posteriorly to the anterior edge of the caudal fin.

In the holotype, there is no evidence for ventral squamation (i.e. mid-ventral and ventro-lateral scale rows) anterior to the pelvic girdle, and the first evidence of mid-ventral squamation is in the region of the anal loop. However, in all referred material, mid-ventral scales extend anterior to the pelvic girdle. In the two smallest specimens, PIMUZ T 890 and PIMUZ T 730, the mid-ventral scale row begins at approximately the midpoint of the abdominal region, immediately ventral to the intestinal cast. Scales are small, narrow and ornamented with small spines. The principal anal loop scale is extremely enlarged ( Fig. 6A ), with a concave medial surface and the antero-dorsal end fanning outward. Posteriorly, this scale overlaps a long, narrow scale that also shows a concave medial surface and a convex lateral surface. This is followed by several more long narrow scales which overlap extensively and are oriented almost at a right angle to the body axis. The posteriormost anal loop scale is slightly bifurcated anteriorly, but otherwise unpaired ( Fig. 6A ). Posterior to the anal loop, the mid-ventral scales become broader and posteriorly rounded, with decreased overlap between successive scales and tuberculate ornamentation on the external surface. The scale immediately anterior to the anal fin is similar to the scale preceding the dorsal fin. Mid-ventral scales posterior the anal fin mirror the morphology of the mid-dorsal row.

In all specimens, a ventro-lateral scale row ossifies posterior to the pelvic fin. These scales are dorsoventrally deep and are ornamented with an elongate spine with translucent tip. This scale row does not persist dorsal to the anal fin, but reoccurs in the caudal peduncle at about the same point as ossified haemal spines reappear.

COMPARISON WITH OTHER SAURICHTHYS SPECIES

Body proportions

Both Saurichthys rieppeli and S. breviabdominalis are mid-sized saurichthyid fishes, smaller than the sympatric S. costasquamosus , and larger than S. paucitrichus ( Table 1 , Fig. 7F ). The pelvic fins and the boundary between the abdominal and caudal regions are shifted anteriorly in both S. rieppeli and S. breviabdominalis relative to S. paucitrichus , S. costasquamosus , and S. yunnanensis ( Table 1 , Fig. 9A , Zhang et al. (2010)), resulting in the anterior displacement of the visually deepest point on the body (the dorsal fin: Webb (1982)) in S. breviabdominalis and S. rieppeli relative to congeners (at 74–77% body length, as in S. deperditus and S. grignae , but unlike S. costasquamosus , S. paucitrichus , S. curionii , and S. striolatus in which the fin inserts at 84–85% body length). The relatively non-elongate and brevirostrine S. macrocephalus and S. brevirostris show dorsal fin placement at 80% body length.

Skull

Although the dorsal portion of the opercle is not preserved in Saurichthys rieppeli , it is clear that the opercle is much longer relative to the length of the jaws than in S. costasquamosus or S. paucitrichus , both of which have relatively long jaws and short, deep opercles (ratios of 0.11 and 0.12, respectively). In many species with proportionately long opercles, such as S. curionii and S. striolatus , this is combined with a more elongate skull such that the opercle length/mandibular length ratio is invariant (0.11 and 0.10, respectively). The length of the opercle relative to the length of jaws in S. rieppeli (0.16) is more consistent with brevirostrine species such as S. macrocephalus , S. brevirostris (both 0.14), and S. breviabdominalis (0.18). Differences in opercle shape in S. breviabdominalis relative to many conspecifics has previously been noted ( Wilson et al., 2013a: note in the cited study, ‘ S. paucitrichus ’ = S. breviabdominalis ).

Axial skeleton

The number of vertebral segments anterior to the caudal fin is slightly lower in S. rieppeli than in the larger S. costasquamosus and S. grignae ( Table 1 ), but is much higher than in S. breviabdominalis , which has the lowest number of vertebrae recorded in any saurichthyid ( Maxwell & Wilson, 2013). The total count is probably also low in S. paucitrichus , although due to pres- ervation, an accurate assessment is not possible. The number of caudal vertebrae in S. rieppeli is proportionately high, as in S. grignae ( Tintori, 2013) . Very high caudal counts relative to abdominal counts characterize some of the geologically youngest saurichthyids ( S. deperditus , S. brevirostris ), and in both of these species, as well as in S. rieppeli , this altered abdominal to caudal vertebral ratio reflects the proportionately shorter abdominal region ( Fig. 9A ).

The rib-like ventral ossifications associated with alternating neural arches in the anteriormost region of the vertebral column of S. rieppeli are also observed in S. curionii and S. macrocephalus ( Rieppel, 1985) . The rectangular anterior caudal haemal arches in a 1:2 relationship with neural arches and pierced by a foramen are reminiscent of the haemal arches of S. ornatus ( Stensiö, 1925) , but differ from those of S. madagascariensis in the absence of a foramen between each haemal element ( Lehman, 1952). Although this haemal arch morphology is generally associated with Early Triassic saurichthyids ( Romano et al., 2012), it should be noted that an unnamed saurichthyid from the Middle Triassic of Spain is described as having haemal arches of similar shape ( Beltan, 1984).

The presence of haemal spines limited to the region dorsal to the anal fin in a 1:1 arrangement with the anal axonosts and a 1:2 relationship with the neural arches seems to be unique to S. rieppeli . S. ornatus and S. madagascariensis do not have haemal spines ( Stensiö, 1925; Lehman, 1952). The lack of well-ossified haemal arches in the caudal peduncle, posterior to the anal fin, is reported in S. dawaziensis ( Wu et al., 2009) , but more commonly there is a reduction in haemal arches posterior to the dorsal and anal fins ( Rieppel, 1992; Wu et al., 2011). Well-ossified haemal spines appear immediately anterior to the caudal fin in S. rieppeli , and occur in a 1:1 relationship with the neural arches in this region.

Pelvic girdle and fins

The morphology of the pelvic bone in S. breviabdominalis differs from more basally positioned saurichthyids. In S. ornatus and S. dawaziensis , the pelvic bone is mediolaterally narrow, with a medially directed anterior process anterior to the lepidotrichia ( Stensiö, 1925; Wu et al., 2009). Laterally, the pelvic bone is separated from the lepidotrichia by series of ossified proximal and distal radials in S. ornatus , and an independent metapterygial ossification is present posterior to the pelvic bone ( Stensiö, 1925). In addition, S. dawaziensis ossifies an anterior lateral process ( Wu et al., 2009). In S. curionii , a medial anterior process is present, as in the older species. However, the posterior pelvic bone is medially expanded and the right and left plates are not widely separated ( Rieppel, 1985), similar to the state in S. breviabdominalis . In the Sinosaurichthys species group, as in S. breviabdominalis , the lepidotrichia articulate with the edge of the pelvic bone ( Wu et al., 2011), suggesting that the radials have been lost or coossify with the pelvic bone.

Fringing fulcra are present along the leading edge of the pelvic fins in S. rieppeli , as in the Early Triassic species S. ornatus ( Stensiö, 1925) , however the fulcra are much larger in the latter species.

Median fins

In S. rieppeli , the dorsal axonosts are long relative to the height of the fin (about half as long as the fin is high), and are located anterior and ventral to the fin. This differs from S. deperditus and S. calcaratus , where the axonosts are located almost entirely anterior to the fin, and lie almost parallel to the vertebral column. In S. curionii , S. macrocephalus , and the Sinosaurichthys species group, the axonosts are short relative to the fin. In S. rieppeli , the anteriormost axonost is not morphologically distinct from more posterior axonosts in the series, thus differing from S. madagascariensis , S. breviabdominalis , S. deperditus , S. calcaratus , and S. curionii ( Lehman, 1952; Rieppel, 1985, pers. observ. EEM).

Unlike in some Early and Middle Triassic species ( Kogan, 2011; Wu et al., 2011), only the posterior baseosts of the dorsal and anal fins of S. rieppeli ossify separately from the axonosts. Whether this arises due to ontogenetic fusion of the anterior axonosts and baseosts, as argued by Tintori (2013), cannot be determined, although such fusion does not appear to occur in other species for which a range of size classes are known ( Rieppel, 1985).

The fringing fulcra (sensu Arratia, 2009) on the leading edge of the median fins of S. rieppeli differ from those seen in the Sinosaurichthys species group, in that a single fulcrum occurs per lepidotrichium rather than a series of small overlapping elements ( Wu et al., 2011). However, in both taxa the fringing fulcra intercalate with the lepidotrichia ( Arratia (2009) morphology B). This differs from the morphology of the fulcra (both fringing and basal ( Hilton, Grande & Bemis, 2011)) lining the anterior edge of the dorsal lobe of the caudal fin of Acipenser or Birgeria , or the pelvic fins of S. ornatus ( Stensiö, 1925) , but is similar to the fringing fulcra in the pectoral fin and ventral lobe of the caudal fin of the coccolepidid Condorlepis ( López-Arbarello, Sferco & Rauhut, 2013). Fringing fulcra on the unpaired fins have also been reported in S. chaoi , S. madagascariensis , S. grignae , and S. dawaziensis (Piveteau, 1944–1945; Liu & Wei, 1988; Wu et al., 2009; Tintori, 2013).

The number of lepidotrichia and lepidotrichial segments in the unpaired fins was considered to be of particular importance in differentiating S. costasquamosus and S. paucitrichus ( Rieppel, 1992) . S. paucitrichus has three lepidotrichial segments in the dorsal and anal fins, and no more than four in the caudal fin ( Rieppel, 1992), whereas S. costasquamosus was described as having respectively 4–5 and 7–8 lepidotrichial segments per fin. The number of lepidotrichial segments in the unpaired fins of S. rieppeli is similar to S. costasquamosus , as well as to older (Early Triassic and early Middle Triassic) species. S. breviabdominalis has respectively 3–4 and 4–5 segments per fin. This character therefore appears to be of little use in discriminating between these sympatric species, especially with reference to the dorsal and anal fins.

Squamation

The reduction of the abdominal squamation to a single row in S. rieppeli differs from contemporaneous saurichthyids from Monte San Giorgio ( Rieppel, 1985; Rieppel, 1992) and elsewhere in Europe ( Firtion, 1934; Oosterink, 1986; Cartanyà, 1999; Hitij et al., 2010), most of which have six scale rows anterior to the pelvic fin (two medial + right and left paired dorsal and ventral lateral rows). The lateral squamation in S. rieppeli is also drastically reduced in the caudal region, being limited to tiny scales in the caudal peduncle. However, S. grignae , from the early Ladinian of northern Italy, lacks even the mid-lateral scale row in the caudal peduncle, although the mid-ventral row is well developed anterior to the pelvic fins ( Tintori, 2013), unlike in S. rieppeli .

The modified paired scales extending along the base of the dorsal and anal fins in S. rieppeli are similar in overall shape and position to the modified scutes supporting the fins in osteolepiformes ( Carroll, 1988). These have not previously been noted in saurichthyids or other early actinopterygians. However, the elongate elements originally described as the posteriormost dorsal and ventral radials in S. grignae (see Tintori, 2013) may be similar.

The anal loop of S. rieppeli is unusual in structure and position. In most saurichthyids, the anal loop and the boundary between the abdominal and caudal vertebral regions lie immediately posterior to the pelvic girdle. S. brevirostris , in which the vertebral transition is located anterior to the pelvic fins and anus (as indicated by gut contents showing the location of the digestive tract: Urlichs, Wild & Ziegler, 1994), is an exception. The anterior displacement of the pelvic fins and the abdominal–caudal transition relative to the anus in S. rieppeli is unique among saurichthyids. Such dissociations are generally uncommon among fishes with a similar shape ( Maxwell & Wilson, 2013). The scales making up the anal loop in S. rieppeli are small, undifferentiated, and numerous. With over 12 scales on either side, scale counts far surpass those of S. dawaziensis (6), one of the few other saurichthyids in which the anal loop scales are small and undiffer- entiated ( Wu et al., 2009). The scales of the anal loop of S. orientalis also apparently lack a massive, strongly differentiated scale, but are not numerous ( Kogan et al., 2009).

The squamation in S. breviabdominalis is very similar to other members of the Costasaurichthys species group, with some subtle differences. In S. costasquamosus and S. yunnanensis , the ventro-lateral scale rows extend anterior to the pelvic girdle, parallel to the midventral scale row. This extension into the abdominal region is absent in S. breviabdominalis . Likewise, in S. costasquamosus , the ventro-lateral scale row resumes immediately posterior to the anal fin, whereas in S. breviabdominalis there is an unmineralized gap extending approximately one-third of the length of the caudal peduncle. Lastly, in S. costasquamosus , the middorsal and mid-ventral scales become gradually larger over the length of the caudal peduncle, whereas in S. breviabdominalis , there is a relatively abrupt change in scale size ( Fig. 6B ). S. rieppeli is the only other species from the Besano Formation to show this sudden morphological change in dorsal and ventral midline scales in the caudal region. Enlarged mid-dorsal and midventral scales in the caudal peduncle are not uncommon in saurichthyids, however, being reported in S. gigas ( Woodward, 1890) , S. madagascariensis , S. dawaziensis , S. deperditus , S. calcaratus , and S. striolatus (Piveteau, 1944–1945; Griffith, 1959; Wu et al., 2009).

Figures

Figure 1. Stratigraphy of the Besano Formation at the Monte San Giorgio locality, with position of the described Saurichthys specimens.

Figure 5. Saurichthys breviabdominalis sp. nov. holotype specimen PIMUZ T 67. A, Overview. Note the small Saurichthys specimen (PIMUZ T 5981) underlying the caudal region of the holotype is not figured in the line drawing or discussed further. B, Larger image of skull and interpretive illustration. Dotted lines show the location of sensory canals, and dashed lines indicate uncertainty in the location of sutures between bones. Abbreviations: af, anal fin; ang, angular; cl, cleithrum; d, dentary; df, dorsal fin; dpt, dermopterotic; dsp; dermosphenotic; epi, epichordal lobe of the caudal fin; f, frontal; hypo, hypochordal lobe of the caudal fin;?L, putative lachrymal; mand, mandible; md.c, mandibular canal; mx, maxilla; op, opercle;?p, putative parietal; pec, pectoral fin lepidotrichia; pop, preopercle; pop.c, preopercular canal; pvf, pelvic fins; qj, quadratojugal; rpm, rostropremaxilla; san, supra-angular; scl, supracleithrum;?so.c, putative supraorbital canal; t.c, temporal canal.

Figure 6. Saurichthys breviabdominalis sp. nov. caudal region (PIMUZ T 67, holotype). A, Anterior caudal region. B, Posterior caudal region and caudal fin. Abbreviations: af, anal fin; als, anal loop scales; ax, axonosts (pterygiophores) of the dorsal and anal fins; df, dorsal fin; mls, mid-lateral scale row; ha, haemal arch; hs, haemal spine; mds, mid-dorsal scale row; mvs, mid-ventral scale row; na, neural arch; vls, ventro-lateral scale row.

Figure 7. A, Saurichthys breviabdominalis sp. nov. referred specimen PIMUZ T 661. B, S. breviabdominalis referred specimen PIMUZ T 661, close-up image of the skull. C, S. breviabdominalis referred specimen PIMUZ T 660, skull in palatal view. D, S. breviabdominalis referred specimen PIMUZ T 661 showing the structure of the pelvic region. E, S. breviabdominalis referred specimen PIMUZ T 665, caudal fin lepidotrichia showing the pattern of segmentation and bifurcation. As the specimen is disarticulated, it is unknown whether these originate from the hypochordal or epichordal lobe. F, PIMUZ T 322, S. paucitrichus holotype, showing the general proportions of the skull and postcranium. The opercle is preserved as an external mold on the sediment, as indicated in Rieppel (1985: Fig. 38D). G, PIMUZ T 698, specimen referred to S. paucitrichus by Rieppel (1992). Abbreviations: ad.f, adductor fossa; als, anal loop scales; mand (d), mandible in dorsal view; mand (v), mandible in ventral view; mx, maxilla; op, opercle; pel, pelvic bone; pvf, pelvic fin; ps, parasphenoid; scr, sclerotic ring (poorly ossified); vo, plant remains (Voltzia); vls, ventro-lateral scale row; mvs, mid-ventral scales.

Figure 9. A, Ternary diagram showing the range of body proportions in selected Saurichthys species. 1, S. rieppeli sp. nov., 2, S. grignae, 3, S. longimedialis, 4, S. longipectoralis, 5, S. minuta, 6, S. costasquamosus, 7, S. paucitrichus, 8, S. breviabdominalis sp. nov., 9, S. macrocephalus, 10, S. curionii, 11, S. deperditus, 12, S. striolatus, 13, S. brevirostris. B, Phylomorphospace showing the negative relationship between opercular depth and axial elongation in saurichthyid fishes.