Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996

Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996

Type and only species Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 .

Type specimen Complete, mostly articulated skeleton

( Fig. 2 View Fig ), listed as BES SC 999 in the catalogue of the MSNM ( BES SC is an acronym for the Besano Sasso Caldo quarry) and coded as 20.S288-2.2 in the Inventario Patrimoniale dello Stato (Italian State Heritage Database).

Type locality Sasso Caldo site, Besano , Varese Province, NW Lombardy, N Italy. Geographical coordinates: 45°54 ′ 03.7 ʺ N 8°55 ′ 10.6 ʺ E, elev. 650 m. GoogleMaps

Type horizon and distribution Middle portion of the Besano Formation (sensu Bindellini et al., 2019), uppermost Anisian (N. secedensis Zone sensu Brack et al., 2005), Middle Triassic.

Referred material PIMUZ T 4376 (complete, mostly articulated skeleton with the best-preserved skull of the taxon; Fig. 3A View Fig ) , PIMUZ T 1895 (incomplete, mostly articulated skeleton with a well-preserved skull; Fig. 3B View Fig ) , PIMUZ T 4847 (incomplete, mostly disarticulated skeleton with a disarticulated skull; Fig. 3C View Fig ) , GPIT 1793 View Materials /1 (disarticulated skull, the holotype of Mikadocephalus gracilirostris Maisch & Matzke, 1997 ) , BES SC 1016 (incomplete, mostly disarticulated skull; the specimen is coded as 20.S288-2.6 in the Inventario Patrimoniale dello Stato (Italian State Heritage Database).

Emended diagnosis Large ichthyosaur (estimated adult body length ~ 8 m) with one possible autapomorphy—a caudoventral exposure of the postorbital in the temporal region—and the following combination of character states: extremely long, slender, and gracile snout; frontal rostrocaudally elongate and relatively flat; frontal participation in the temporal fossa (= anterior terrace; Motani, 1999) but not in the temporal fenestra; L-shaped jugal; ‘triangular process’ on the medioventral border of the quadrate; prominent coronoid (preglenoid) process of the surangular, distinctly raised above its dorsal margin; tiny, conical teeth with a coarsely-striated crown surface and deeply striated roots; mesial maxillary teeth set in sockets; distal maxillary teeth set in a groove shorter than half of the rostral ramus of the maxilla; 61 presacral vertebrae, at least two sacral vertebrae, and at least 138 caudal vertebrae; tailbend forming an angle of ~ 30°; wedge-shaped caudal centra located between the 56–60th position in the caudal series, possessing small and rounded articular surfaces for possibly unossified ribs; caudal series comprising 55% of the length of the axial skeleton in adults; rounded humerus; rounded manual phalanges; pedal phalanges retaining reduced shafts in adults; obturator foramen with a sub-oval outline, open in juveniles and almost entirely enclosed within the pubis in adults.

Remarks Several additional specimens at the PIMUZ are potentially referable to Besanosaurus and are treated as aff. Besanosaurus herein. Previously, these specimens were preliminarily identified in the field as Mixosaurus (except PIMUZ T 2027, which was labelled as Shastasauridae indet.) and include: PIMUZ T 188, two isolated paddle elements preserved on the surface of one matrix slab; PIMUZ T 190, isolated and badly preserved paddle elements on two matrix slabs; PIMUZ T 1935, jaw fragments on one slab; PIMUZ T 1973, incomplete skeletal remains on nine (potentially ten if one unnumbered slab is counted as well) matrix slabs; PIMUZ T 2010, skull remains on two slabs; PIMUZ T 2013, large skeletal remains including trunk and paddle remains on three slabs (incl. T 2015, which was identified as a counterslab to one of the two slabs of T 2013); PIMUZ T 2027, vertebral elements (several sectioned histologically) on one slab; PIMUZ T 2141, vertebrae and a girdle element on one slab; PIMUZ T 2144, skull and lower jaw remains; PIMUZ T 2259, jaw remains on two slabs; PIMUZ T 2359, articulated vertebrae with chevron bones on one slab; and PIMUZ T 2759, large jaw fragments on one slab.

Description

Axial skeleton

Vertebrae

We recognise 12 cervical centra, 49 dorsal centra, at least two sacral centra, and at least 138 caudal centra (201 vertebrae in total) in the axial skeleton of the holotype of Besanosaurus leptorhynchus (BES SC 999). Te overall count of presacral vertebrae is at least 61. In PIMUZ T 4376, the caudalmost portion of the tail is not preserved, missing about 30 vertebrae from the tip of the tail. Te preflexural and postflexural counts are 57 and 81, respectively. Apical wedge-shaped caudal centra are located between the 56–60th position in the caudal series. Cervical, dorsal, and proximal caudal centra are generally subcircular in outline in articular (anterior/posterior) view; mid-caudal centra become ovoid in outline and mediolaterally compressed ( Dal Sasso & Pinna, 1996: fig. 13). All centra are amphicoelous and possess the typical ichthyosaurian hourglass-shaped sagittal cross-section.

Cervical centra —In PIMUZ T 4376, the specimen in which the neck region is best preserved, 12 cervical centra are present ( Fig. 4A–C). Tese are exposed in right lateroventral view and show facets for dichocephalous ribs; therefore, we regard them as cervical vertebrae (following Sander, 1989). BES SC 999 also shows 12 cervical centra possessing both a diapophysis and a parapophysis

(See figure on next page.)

Fig. 4 Selected elements of the axial skeleton of Besanosaurus leptorhynchus . A, B portions of the axial skeleton of PIMUZ T 4376,

including the cervical series and the anteriormost portion of the dorsal region; C BES SC 999,posteriormost cervical and anteriormost dorsal vertebrae; D BES SC 999, vertebrae belonging to the posterior half of the presacral dorsal series; E BES SC 999,sacral region and anteriormost portion of the caudal series; F BES SC 999,caudal vertebrae around the inferred position of the tailbend; G BES SC 999,caudalmost region of the tail. at atlas, ax axis, cac caudal centrum, car caudal rib, cec cervical centrum, cer cervical rib, chv chevron, Cl clavicle, Co coracoid, dc dorsal centrum, desr distal end of sacral rib, Fi fibula, ic intercentra, Il ilium, Is ischium, na neural arch, ns neural spine, oc occipital condyle, P pubis, S scapula, sc sacral centrum, sr sacral rib. Numbers indicate the relative position of the centra within each of the vertebral column sections (ce, cervical; d, dorsal;ca, caudal).The apostrophe (‘) indicates a left element. Scale bars represent 1 cm in A and B, 5 cm in C–F

( Fig. 4C). Except for the presence of diapophyses and parapophyses, the cervical centra are generally circular along the transverse plane, but the cranial-most centra show a triangular ventral outline that becomes rounder towards the dorsal region. Te height/length ratio of the cervical vertebrae is ~ 1.5, which is much greater than in Grippia longirostris (~ 1.24; Ekeheien et al., 2018), but slightly lower than in Cymbospondylus petrinus and Shastasaurus osmonti (both ~ 1.6; Merriam, 1902, 1908).

Te atlantal and axial pleurocentra are separate both in PIMUZ T 4376 and BES SC 999, although the atlantal pleurocentrum could only be identified in PIMUZ T 4376

( Fig. 4A). Te atlas in PIMUZ T 4376 shows a concave anterior articular facet, like in mixosaurids, shastasaurids and more derived ichthyosaurs, but unlike in Chaohusaurus and Cymbospondylus , in which it is convex ( Huang et al., 2019; McGowan & Motani, 2003; Merriam, 1902, 1908). Te axial pleurocentrum does not show distinctive differences when compared to the more posterior cervical vertebrae. Tree potential intercentra were also identified in PIMUZ T 4376 ( Fig. 4A). Diapophyses and parapophyses are always present on the cervical centra; the diapophyses project dorsolaterally and the parapophyses project ventrolaterally (more markedly in the large specimens than in the small ones). Te diapophyses increase in size posteriorly towards the dorsal region, gradually transitioning from a round to a reniform outline, whereas the parapophyses decrease in size maintaining a rounded outline. Anteriorly, the diapophyses and parapophyses are separated from each other, the dorsoventral distance between them decreases posteriorly, and the parapophyses are no longer present on the 13th centrum, in which only an enlarged diapophysis is visible

( Fig. 4C). Te diapophyses always contact the facets for the neural arch ( Fig. 4A–C).

In BES SC 999, the 12th centrum possesses a parapophysis that is extremely reduced in size ( Fig. 4C). A similar condition was described for Guizhouichthyosaurus tangae ( Shang & Li, 2009) and Shastasaurus pacificus ( Merriam, 1902: pl. 8, fig. 3). In Shonisaurus sikanniensis there is a parapophysis on the first 10 and in the 12th presacral centra, whereas the 11th centrum only shows a single rib facet ( Nicholls & Manabe, 2004). Around 10 cervical centra were identified in Guanlingsaurus liangae ( Ji et al., 2013) , whereas a parapophysis is visible up until the 9th centrum in the cervical series of Shonisaurus popularis ( Camp, 1980: fig. 28). Te number of cervical vertebrae seems to be variable in Cymbospondylus . Sander (1989) identified six cervical vertebrae in Cymbospondylus buchseri , whereas Merriam (1908: fig. 58) reported the presence of a parapophysis on the first 12 centra of Cymbospondylus petrinus . Furthermore, Fröbisch et al. (2006) noted the presence of a tiny parapophysis on the 8th presacral centrum of Cymbospondylus nichollsi and suggested the 9th to have only a rudimentary parapophysis.

Dorsal centra —Dorsal centra (i.e., non-cervical presacral centra) are generally circular in articular view, showing a regular and constant morphology along the vertebral column. Te height/length ratio of the dorsal centra slightly increases posteriorly along the trunk region (~ 1.5 for the cranialmost dorsal centra; ~ 1.75 for the caudalmost dorsal centra), and the overall size of the centra increases from the neck towards the pelvis (in the holotype, the height of the 10th cervical centrum equals 31 mm and the height of the 46th dorsal centrum equals 49 mm). For comparison, the height/length ratio of the dorsal centra in Cymbospondylus buchseri ( Sander, 1989) and Shastasaurus osmonti ( Merriam, 1902, 1908) is much greater than in Besanosaurus leptorhynchus with a mean value of ~ 2.

On the lateral side, the dorsal centra have only one articular facet for holocephalous ribs. Te diapophysis is dorsoventrally long on all dorsal centra, has a concave cranial margin, and shows a slight constriction in the middle ( Fig. 4D). It is generally confluent with the anterior articular surface of the centrum, although never truncated by it ( Fig. 4D). Tis condition is more similar to Shastasaurus and Californosaurus ( Merriam, 1902, 1908) than to Cymbospondylus ( Sander, 1989, 1992), in which the diapophysis is visibly truncated by the anterior articular facet. Te diapophysis contacts the facet for the neural arch up until at least the 37th or 38th dorsal centrum (the 49th and 50th centrum in general, respectively; Fig. 4D). In comparison, in Cymbospondylus buchseri the connection between the rib facet and the neural arch is lost at the 29th dorsal centrum (35th centrum overall; Sander, 1989), in Cymbospondylus petrinus it is lost between the 24th and 26th dorsal centrum (36th–38th centrum in general; Merriam, 1908), in Californosaurus perrini it is lost in the 27th dorsal centrum ( Merriam, 1902), and in Shonisaurus popularis it is already lost anterior to the 23rd dorsal centrum ( Camp, 1980). Te dorsal centra of Besanosaurus leptorhynchus differ greatly from those of Mixosaurus : in the latter, they are relatively longer (height/length ratio of a mid-dorsal centrum in BES SC 1000 is 1.45) and two rib facets are visible on the lateral side of the centra close to the pelvic region (e.g., Brinkman, 2004; Renesto et al., 2020). In lateral view, the dorsal centra of Besanosaurus leptorhynchus and Cymbospondylus are very similar. However, unlike in Cymbospondylus buchseri , the posterior dorsal vertebrae of Besanosaurus leptorhynchus , except for the last four caudalmost ones, possess a diapophysis located around or above the mid-height of the centrum that never shows the anteroventral extension visible in the dorsal centra of the caudal half of the trunk in Cymbospondylus buchseri ( Sander, 1989) . In addition, the typical triangular shape of the posterior dorsals in craniocaudal view of Cymbospondylus petrinus ( Merriam, 1908) and likely Cymbospondylus buchseri ( Sander, 1989) differs from Besanosaurus leptorhynchus , in which the centra are sub-pentagonal in shape ( Dal Sasso & Pinna, 1996). Te dorsal centra of Besanosaurus leptorhynchus are also similar to those of Shastasaurus osmonti ( Merriam, 1902) in that they are sub-circular in outline in articular view. However, in Besanosaurus leptorhynchus the neural arch facets are raised markedly above the dorsal surface of the centrum in posterior dorsals, in contrast to Shastasaurus osmonti , in which they are proportionally much lower. In addition, the diapophysis almost never contacts the anterior articular surface of the dorsal centra in Shastasaurus osmonti and is dorsoventrally much taller than in Besanosaurus leptorhynchus .

Aberrant dorsal centra —In PIMUZ T 4376, the first eight dorsal centra (the 13–20 centra overall; Fig. 4B) are remarkably narrow in lateral view in comparison with the preceding and following centra. Each of these centra measures about half of a normal centrum in length. We rule out the possibility that taphonomical compression selectively and plastically reduced the craniocaudal length of these eight centra since the associated elements (ribs and neural arches) maintain typical proportions. On the other hand, these centra are preserved packed closely to each other. To explain this particular condition, we tentatively suggest the existence of a developmental abnormality that affected the craniocaudal length of centra 13–20 in PIMUZ T 4376.

Sacral centra —We distinguish at least two sacral vertebrae in BES SC 999, identified by the presence of at least two large, disarticulated rib pairs with clearly expanded distal ends, that we recognise as sacral ribs ( Fig. 4E). Two sacral centra were also reported in Shonisaurus popularis ( Camp, 1980) and Guizhouichthyosaurus tangae ( Shang & Li, 2009) . Sacral centra are also preserved in PIMUZ T 4376 but are not visible in PIMUZ T 1895 and T 4847. Te sacral centra in Besanosaurus leptorhynchus occupy the 62nd and 63rd position along the vertebral column (and possibly two further positions caudally), so that the presacral vertebral count can be determined as 61. Te sacral centra do not show diagnostic characters that can clearly differentiate them from the caudalmost dorsal centra and the cranialmost caudal centra. A gradual, slight increase in size in the caudal direction of the precaudal vertebrae makes the two sacral centra the largest in the entire axial skeleton (63rd centrum measures 52 mm in mediolateral width and 25 mm in craniocaudal length in BES SC 999).

In lateral view, both sacral vertebrae show a dorsoventrally elongated diapophysis that is reniform in outline and facets for neural arches that are enlarged lateromedially in the middle.

Caudal centra — BES SC 999 is the only specimen that possesses a complete, partially articulated tail ( Figs. 2 View Fig , 4, and 5). In this specimen, the caudal series (possessing at least 138 vertebrae) measures ~ 280 cm (measured along the vertebral column, without taking the tailbend into account), which is ~ 1.2 times the length of the rest of the body and accounts for 55.3% of the entire length of the axial skeleton. In the holotype, the tailbend is inferred to be present 155 cm caudally to the sacrum, based on the position of the tailbend preserved in PIMUZ T 1895 (see below) . PIMUZ T 4376 also preserves an almost complete tail measuring ~ 104 cm, albeit it is mostly disarticulated and comprises at least 105 caudal centra, missing only the distalmost ones .

Te size of the caudal centra decreases immediately posterior to the sacrum. Moreover, the postsacral centra gradually become more and more mediolaterally compressed. Centra that are clearly dorsoventrally taller than mediolaterally wide are visible posterior to the 30th caudal centrum of BES SC 999 ( Figs. 4F, and 5D View Fig ). From the cranialmost to the caudalmost position, the height/length ratio of caudal preflexural centra gradually changes from ~ 1.8 to ~ 2.8, whereas in the caudalmost postflexural preserved centra the ratio is ~ 1.65.

Wedge-shaped centra are visible on one of the slabs comprising PIMUZ T 1895, where at least five centra with a trapezoidal outline in lateral view can be recognised ( Fig. 5A View Fig ). Te neural spines corresponding to these centra are shorter than the preflexural neural spines and oriented vertically ( Fig. 4F). Neural spines positioned caudally to the tailbend become even shorter and cranially oriented. Te tailbend in PIMUZ T 1895 possesses an angle of ~ 30°. In comparison, the tailbend reported in Guanlingsaurus liangae formed an angle of approx. 15° and the tailbend in Shonisaurus popularis formed an

(See figure on next page.)

Fig. 5 View Fig Selected elements of the axial skeleton of Besanosaurus leptorhynchus . A, B PIMUZ T 1895, tailbend region and anteriormost region of the presacral vertebral series,respectively; C two chevrons belonging to PIMUZ T 4847; D BES SC 999, anteriormost portion of the tail. a astragalus, c centrum, cac caudal centrum, car caudal rib, chv chevron, Cl clavicle, dia diapophysis, Fi fibula, Il ilium, Is ischium, na neural arch, ns

neural spine, S scapula, sc sacral centrum, sr sacral rib. Numbers indicate the position of the centra within each of the vertebral column sections (ce, cervical; d, dorsal; ca,caudal).The apostrophe (‘) always indicates a left element. Scale bars represent 5 cm

angle of only about 4° ( Ji et al., 2011; McGowan & Motani, 1999). A slight tailbend was also reported for Cymbospondylus petrinus , but the angle it formed is not clear (Holger & Kosch, 1993).

In BES SC 999, the cranialmost 4 or 5 caudal centra, unlike the following ones, lack haemal arches ( Fig. 5D View Fig ). Te last ossified caudal ribs likely articulate with the sixth caudal centrum. In BES SC 999, PIMUZ T 4376, and PIMUZ T 1895, caudal centra between the 6th position and the wedge-shaped centra still show small and rounded diapophyses, which are visible up until the tailbend (PIMUZ T 1895), indicating they likely articulated with non-ossified elements ( Fig. 5A, D View Fig ). In the proximalmost caudal centra, the diapophysis contacts the anterior margin of the centrum (at least until the 10th caudal position in BES SC 999), although it does not merge with the cranial articular facet. From the proximal to the caudal-most caudal where a diapophysis is visible, they gradually decrease in size until they are a point-like element in caudal centrum 55 (PIMUZ T 1895). A similar condition is seen in Shonisaurus popularis ( Camp, 1980) and Cymbospondylus petrinus ( Merriam, 1908) .

Te presence of articular surfaces for ribs, as well as the change in the direction of the dorsoventral axis of the neural spines, was used to infer the location of the tailbend in BES SC 999: the positions of the apical centra are inferred as positions 56–60 in the caudal series ( Fig. 4F).

Neural arches and spines —In PIMUZ T 4376, most of the presacral neural arches are exposed in right lateral view and preserved in articulation ( Fig. 4A, B). In PIMUZ T 4376, the element previously described as possible opisthotic ( Bindellini et al., 2021) is here reidentified as the atlantal neural arch ( Fig. 4A). PIMUZ T 1895 shows a well-preserved articulated series of neural arches in the cranial most portion of the trunk ( Fig. 5B View Fig ). In BES SC 999, the neural arches are also well preserved, although they are sometimes partly covered by other bones

( Fig. 4C–G).

Among the studied specimens, only PIMUZ T 4376 shows a well-preserved axis ( Fig. 4A); its neural arch has a craniocaudally expanded neural spine, around two times broader than the subsequent spines.

In general, the neural arches possess a triradiate outline in cranial/caudal view. Te pedicle is as long anteroposteriorly as the length of the centrum; the medial margin of the articular surface is straight, whereas the lateral margin shows a lateral bulge. Tis expansion is clearly visible up until the 38th/39th centrum, which are the last two centra where the facet for the neural arch contacts the diapophysis (see description of dorsal centra; Fig. 4D). Prezygapophyses and postzygapophyses are of similar size in the cervical and anterior dorsal vertebrae; in the posterior dorsal and anterior caudal vertebrae, the prezygapophyses project cranially more than the postzygapophyses do caudally ( Figs. 4 and 5 View Fig ). Te zygapophyses are paired, similar to the condition in Shastasaurus osmonti and Cymbospondylus ( Merriam, 1908; Sander, 1989), but unlike in post-Triassic ichthyosaurs, in which the anterior neural arches show unpaired zygapophyses (e.g., McGowan & Motani, 2003; Moon & Kirton, 2016). Te articular surfaces of the zygapophyses are almost horizontal ( Figs. 4 and 5 View Fig ).

In lateral view, the dorsal neural spines of Besanosaurus leptorhynchus are craniocaudally expanded dorsally, with a generally convex dorsal margin, more rounded than what is visible in Cymbospondylus buchseri ( Sander, 1989) and Phantomosaurus neubigi ( Sander, 1997) (this feature is more evident in the caudalmost dorsal vertebrae). Te neural spines are inclined at an angle that varies along the vertebral column: cervical and dorsal neural spines are posteriorly inclined at an angle of about 25° from the vertical axis (as also described for Cymbospondylus buchseri ; Sander, 1989) and near the inferred position of the sacrum the neural spines become more vertical. More caudally, the anterior caudal neural spines are posteriorly inclined at an angle of about 15° ( Fig. 5D View Fig ). Te caudal neural spines, in the first half of the tail, are inclined posteriorly almost at an angle of 45°, but posterior to the tailbend, the spines become abruptly shorter and vertically oriented, whereas more caudally their inclination becomes inverted, i.e., the dorsoventral axis of the neural spines changes direction pointing anteriorly ( Dal Sasso & Pinna, 1996: fig. 14). Taller neural spines, visible just before the wedge-shaped flexural centra, likely supported a low dorsal lobe of the tail fluke.

Haemal arches (chevrons) —In BES SC 999, chevrons are visible starting from the 5th or 6th caudal centrum. In cranial/caudal view most of the chevrons are Y-shaped, but the caudalmost chevrons are V-shaped ( Fig. 5A, C, D View Fig ). In lateral view, the chevrons are straight and the cranialmost ones are dorsoventrally taller than the corresponding centrum ( Fig. 5D View Fig ). Approaching the tailbend, the dorsoventral height of the chevrons gradually decreases, whereas their mediolateral width remains constant ( Figs. 4 and 5 View Fig ). Caudally to the tailbend, haemal arches keep decreasing in size, also showing a clear mediolateral width reduction ( Fig. 4F, G) if compared to the more anterior chevrons. In both BES SC 999 and PIMUZ T 4376, tiny haemal arches are visible extending almost until the very tip of the tail ( Fig. 4G). Tis condition contrasts with parvipelvians (excluding Ichthyosaurus communis ) and basal euichthyosaurians (e.g., Californosaurus perrini ), in which the haemal arches are reduced in size and disappear more cranially (e.g., McGowan & Motani, 2003; Merriam, 1902; Moon & Kirton, 2016).