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 : 891-896

publication ID	https://doi.org/ 10.1111/zoj.12224
DOI	https://doi.org/10.5281/zenodo.7011181
persistent identifier	https://treatment.plazi.org/id/03D887E6-E37F-F314-FEB3-FF26FDEFFACE
treatment provided by	Carolina
scientific name	Saurichthys undetermined
status	sp. nov.

Treatment

SAURICHTHYS RIEPPELI SP. NOV.

Differential diagnosis

Opercle long relative to the length of the rest of the skull (ratio of opercle to lower jaw length = 0.16; ratios of 0.10–0.12 in S. costasquamosus , S. paucitrichus , S. striolatus , S. deperditus , S. curionii ); marginal teeth small; small fringing fulcra present on anterior edge of pelvic, dorsal, anal and caudal fins; axonosts articulating directly with lepidotrichia in anterior parts of dorsal and anal fins (independently ossified baseosts present throughout in S. madagascariensis and S. ornatus ); scales anterior to the pelvic girdle limited to a single mid-dorsal row; abdominal region of vertebral column shorter than caudal region (unlike, e.g. in S. costasquamosus and the Sinosaurichthys species group); rectangular haemal arches in the anterior caudal region spanning two neural arch-like structures (shared with S. madagascariensis and S. ornatus ); foramen between haemal arches absent (as in S. ornatus but unlike S. madagascariensis ); anal loop of mid-ventral scale row located immediately anterior to anal fin (placed more anteriorly in most other species).

Etymology

Named in honour of Olivier Rieppel, to acknowledge his contributions to the taxonomy and anatomy of Saurichthys from Monte San Giorgio, Switzerland.

Holotype

PIMUZ T 61 ( Figs 2–4 View Figure 2 View Figure 3 View Figure 4 )

Description

The holotype and only referred specimen consists of a complete fish of 60.2 cm total length/ 58.7 cm fork length. The skull and pectoral girdle are exposed in ventral view, and the axial skeleton in lateral view ( Fig. 2 View Figure 2 ). Soft tissue is preserved in the epaxial portion of the anterior caudal region.

Stratigraphic provenance

Besano Formation, Upper Member, Bed 162 ( Fig. 1 View Figure 1 ), early Ladinian ( Eoprotrachyceras curionii Zone ).

Type locality

Mirigioli (P. 902), Monte San Giorgio , Canton Ticino, Switzerland ( Fig. 1 View Figure 1 ).

DESCRIPTION

As with most species of Saurichthys , the body is elongate (fineness ratio = 14.55). However, the typical proportions of saurichthyid fishes are modified (skull = 24%, abdominal region = 34%, caudal region = 42%). The pelvic fins are positioned relatively anteriorly on the body, such that the caudal region of the axial skeleton makes up almost half of the total length of the fish ( Fig. 2 View Figure 2 ).

Skull

The skull of specimen PIMUZ T 61 is disarticulated and only partially exposed ( Fig. 2B View Figure 2 ). In many cases, identification of the bones is difficult due to fragmentary preservation. Both mandibular rami are threedimensionally preserved and both are exposed in lateral and ventral view. The left jaw ramus is rather complete and measures about 155 mm in length. The right ramus of the lower jaw is fragmented into several segments. The left and right rami remain articulated rostrally ( Fig. 2B View Figure 2 ). The symphysis measures approximately one quarter of the total length of the lower jaw. The mandibular rami are both gently curved outwards so that they increasingly diverge posteriorly. The outer surface of each mandibular ramus is bent in such a way that the upper portion is oriented laterally while the lowermost part faces ventrally. The ventral surface reaches its greatest mediolateral width (c. 4 mm) at a distance of c. 50 to 60 mm anterior to the jaw joint, but the width of the ventral shelf decreases rapidly both anteriorly and posteriorly.

The lateral surface of the lower jaw is composed of the triangular angular located postero-ventrally, and the very long dentary. The supra-angular is not preserved, but a wide notch in the postero-dorsal part of the right mandible could indicate the presence of this bone. The ornamentation of the lateral surface, which is visible on the right mandibular ramus, consists of numerous vertically oriented striae; the ornamentation changes and becomes tuberculate on the ventral surface of the mandible slightly posterior to the symphyseal region, and the tubercular ornamentation spreads over the lateral surface of the mandible in the region of the mandibular symphysis.

The rostral end of the upper jaw (rostropremaxilla) is visible below the lower jaw in palatal view ( Fig. 2C View Figure 2 ). No tooth crowns are preserved, however approximately eight tooth bases are visible. The diameter of the teeth is small relative to the size of the skull, and no large difference in tooth base size between positions exists, suggesting that large, caniniform teeth are absent.

Dorsal to the fragmented right mandible lies a large bone that probably represents the left frontal in ventral view. The conspicuous notch in the lateral margin of the left frontal corresponds to the dorsal and anterodorsal edges of the orbital opening. Posterodorsal to the fragmented cranial bones lies a large fragment of the left opercle, exposed in medial view. The preserved length of the opercle, from the point of articulation with the hyomandibula to the posterior margin, is 22.4 mm. The process bearing facets for articulation with the posterior hyomandibula is angled dorsally, and is not greatly offset from the body of the opercle. Ventral to this process, the anterior edge of the opercle is oriented vertically; about halfway along the ventral depth of the element the anterior margin angles posteriorly. The medial surface of the opercle is gently concave, with a shallow, rounded fossa located postero-ventral to the articulation with the hyomandibula.

Dermal shoulder girdle (excluding pectoral fins)

The disarticulated elements of the pectoral girdle are found posterior to the skull ( Fig. 2B View Figure 2 ). Two fragmentary bones behind the right mandible probably belong to the ventral portions of the left and right cleithra. In addition, a slender element lying between the opercle and the two cleithral fragments may correspond to the dorsal process of the left cleithrum. Both the ventral portion and the dorsal process of the left cleithrum are seen from their external side. The medio-ventral surface of the ventral portion of the left cleithrum is distinctly ornamented, while the dorso-lateral surface, which in vivo was covered by the opercle, is smooth. The right cleithrum is preserved in medial view and displays a smooth surface. Two relatively small, slender elements, one partially covered by the opercle and the other by the dorsal process of the left cleithrum, may correspond to the supracleithra.

Axial skeleton

There are 72 abdominal and 80 caudal neural archlike elements, for a total of 152 [76 vertebrae (36 + 40), sensu Maxwell et al. (2013)]. The neural spines in the anterior region of the column curve dorsally and widen at their dorsal extremities, and the prezygapophyses appear to be dorso-ventrally expanded ( Fig. 3A View Figure 3 ). The pedicle plus neural spine has a combined height of 6.9 mm, and the spine alone is 4.9 mm high. Small, triangular bones ventral to alternating neural archlike elements underlie the anteriormost ∼22 neural arches ( Fig. 3A View Figure 3 ). In the posterior abdominal region, the neural spines become more posteriorly inclined, at an angle to the body of the neural arch. They also increase in length, and become narrower. The pedicle plus neural spine has a combined height of 9.3 mm, and the spine alone a height of 7.5 mm. There is no significant change in neural arch morphology between the posterior abdominal and anterior caudal regions. However, at a point coinciding with the insertion of the seventh dorsal axonost and continuing posteriorly into the caudal peduncle, neural arches abruptly become T-shaped in lateral view, bearing an elongate prezygapophysis and an equally elongate posterior extension, corresponding either to a posteriorly angled neural spine or, more likely, to an elongate postzygapophysis ( Figs 3B, C View Figure 3 , 4A View Figure 4 ).

The boundary between the abdominal and caudal regions is located at approximately the level of the pelvic fins, and is characterized by the first appearance of haemal arches. These resemble spool-shaped blocks, which are right-left paired and positioned ventral to the notochord. Each block spans two neural arch-like structures, and is pierced by a foramen ( Fig. 3B View Figure 3 ). Haemal spines are absent in the anterior caudal region (14 haemal arches, 28 neural spines); however haemal spines are developed in the region congruent with the insertion of the anteriormost anal axonost ( Fig. 3B View Figure 3 ). The haemal spines are fused with the block-like haemal arches on both the left and right sides of the body, such that each block bears a single haemal spine. Each bilateral pair of haemal spines articulates ventrally with a single anal axonost. The haemal spines become shorter posteriorly, and both the haemal arches and haemal spines cease to ossify at a point coinciding with the morphological change in the dorsal axial skeleton from elongate to T-shaped neural arches. Ossified haemal arches and spines reoccur in a 1:1 relationship with the neural arch-like structures at a point immediately anterior to the caudal fin ( Fig. 3C View Figure 3 ). In the epichordal lobe of the caudal fin, lepidotrichia insert directly on the neural arches, which once again bear dorsally directed spines. In the hypochordal lobe, lepidotrichia are supported directly by the haemal arches, two lepidotrichia per arch.

Paired fins

At least seven well-ossified elongate pectoral radials are preserved on the right side. These are associated with some carbonaceous remains that we interpret as the remains of an organ. Based on the left fin, a minimum of 18 lepidotrichia were present. Preservation is too poor to conclusively determine whether these were segmented, or if fringing fulcra were present.

The pelvic fins are positioned midway along the length of the fish. The pelvic girdle and radials are not preserved. The pelvic fins consist of more than 20 lepidotrichia, with a minimum of three segments present in the longest (fifth) lepidotrichium ( Fig. 4D View Figure 4 ). At least three small basal fulcra are preserved anterior to the first segmented fin ray. Small fringing fulcra ( Arratia (2009) morphology B) are present along the anterior margin of the pelvic fins at a frequency of one per lepidotrichium. The anterior five lepidotrichia are flatter and more robust than more posterior rays, and are made up of 2–3 segments. There is no evidence for bifurcation of these lepidotrichia. In addition, the pattern of segmentation of these anterior lepidotrichia is offset from that of the more posterior rays.

Median fins

The dorsal fin is supported by 12 axonosts, which decrease in length posteriorly ( Fig. 4A View Figure 4 ). With the exception of a small protuberance on its antero-dorsal edge, the anteriormost axonost is not differentiated from more posterior axonosts. Unlike the anterior axonosts, which contact the lepidotrichia, the posteriormost three axonosts are separated from the lepidotrichia by an unossified gap and by ossified hourglass-shaped baseosts. In addition, there are at least three baseosts posteri- or to the last axonost. The baseosts show a pattern of alternating dark and light banding along the dorsal ends, interpreted as annuli ( Fig. 4C View Figure 4 ). At least six sets of bands are present.

The dorsal fin is made up of more than 40 lepidotrichia, with more than five segments present in the longest rays. Tiny basal fulcra (3–4) followed by a single robust basal fulcrum precede the first fin ray, and appear to be supported by the first dorsal axonost. Small fringing fulcra ( Arratia (2009) morphology B are present along the anterior margin of the dorsal fin, at a frequency of one per lepidotrichium. The fringing fulcra intercalate with the lepidotrichia.

The morphology of the anal fin is similar to that of the dorsal fin, with more than 35 lepidotrichia made up of at least eight segments, and several small basal fulcra supported by the first anal axonost ( Fig. 4B View Figure 4 ). The fringing fulcra can be distinguished from the lepidotrichial segments by the angled articular surface and by the absence of swelling at the lepidotrichial joint (e.g. Fig. 4E View Figure 4 ).

The caudal fin is symmetrical and weakly forked ( Fig. 4E View Figure 4 ). It is 87.6 mm high and 41.9 mm long, containing about 37 lepidotrichia divided into maximally seven segments in the epichordal lobe and 37 lepidotrichia with seven segments in the hypochordal lobe. The terminal axial skeleton and associated lepidotrichia have been taphonomically displaced dorsally relative to the rest of the fin. Small fringing fulcra are present along the anterior margins of the caudal fin at a frequency of one per lepidotrichium ( Fig. 4E View Figure 4 ). The lepidotrichia making up the leading edge of the caudal fin, including the longest lepidotrichia, do not bifurcate distally; those more posterior bifurcate once near their distal tip, and the most posteriorly originating lepidotrichia bifurcate more proximally and at least twice, creating a fringe along the posterior edge of the caudal fin.

Squamation

The mid-dorsal scale row is the only ossified scale row present in the abdominal region. Scales occur in a 1:1 relationship with the neural arches in the anteriormost abdominal region ( Fig. 3A View Figure 3 ), but become longer more posteriorly. Mid-ventral scales are first preserved in the posterior portion of the anal loop ( Fig. 3B View Figure 3 ), immediately anterior to the anal fin but posterior to the osteological abdominal-caudal transition. The anal loop is made up of a minimum of 12 small needle-like scales on either side of the anal opening.

There are paired mid-dorsal and mid-ventral scales on either side of the posterior edge of the dorsal and anal fin, respectively ( Figs 3C View Figure 3 , 4A View Figure 4 ). These scales are long and narrow, and are covered by multiple scales of more typical morphology. The mid-dorsal scales progressively increase in size (length as well as breadth) and degree of overlap posterior to the dorsal fin; the posterior half of each scale overlaps the proceeding scale. The five scales immediately anterior to the caudal fin overlap such that the external surfaces form chevrons with the points directed posteriorly ( Fig. 3C View Figure 3 ). The mid-dorsal scales have a roughened external surface, but distinct ornamentation is lacking. As the cleithrum and mandible are clearly ornamented, it seems likely that this lack of ornamentation is not preservational.

The mid-ventral scale row is exposed in dorsal view in the caudal peduncle ( Fig. 3C View Figure 3 ). Posterior to the anal fin, the scales are elongate ovoids, increasing in length and breadth posteriorly and also becoming more square, with a small, anteriorly directed projection. As in the mid-dorsal scales, the anterior portion of each scale supports the posterior portion of the preceding scale over a long interlocking surface.

In addition to the mid-dorsal and mid-ventral scale rows, a mid-lateral and ventro-lateral row made up of comparatively small scales are present posterior to the dorsal and anal fins ( Fig. 3C View Figure 3 ). The scales of the mid-lateral row are diamond-shaped and bear a pit, presumably related to the lateral line. The ventrolateral scales are more elongate, and resemble commas, with the ‘tail’ directed posteriorly.