General body form and other observations

Measurements from the new specimen, as well as other specimens (from Jin, Reference Jin1999, table 7), are provided in Table 1. The new specimen (Fig. 3) is the largest for which measurement data are available, although proportionately it fits well with those data presented by Jin (Reference Jin1999). The head of †Yanosteus is 21.0%–27.0% of the standard length, which is comparable to other non-polyodontid Acipenseriformes. The eye is anteriorly positioned on the head, resulting in a short snout, although the precise margin of the orbit is unclear as the skull of most specimens is largely disarticulated (e.g., Fig. 4). The mouth is anteriorly directed (i.e., forward facing, as opposed to ventrally facing in acipenserids). It is noteworthy that the skeleton of a teleostean fish (†Lycoptera sp.) was found preserved in the mouth of our new specimen of †Yanosteus (unlabeled in Fig. 5.1). Although not conclusive evidence, this find is suggestive that this acipenseriform was piscivorous. The caudal peduncle is relatively short and stout. Although the specimens are compressed and the true form of the body is unknown, the profile of the body is reminiscent of that of modern Acipenseriformes, and †Yanosteus was likely rounded in cross section in life.

Table 1.

Morphometric data for a sample of †Yanosteus longidorsalis. SL = standard length, recorded as Jin's (Reference Jin1999) body length, which is from the tip of the snout to the ventral insertion of the caudal fin.

Bones of the skull roof and braincase

The bones of the skull roof in †Yanosteus are ornamented with ridges of bone radiating from the presumed center of ossification (Figs. 4, 5). The frontals (= parietals of some authors on the basis of its homology with the bone termed the ‘parietal’ in sarcopterygians; e.g., see Jollie, Reference Jollie1986; Arratia and Cloutier, Reference Arratia, Cloutier, Schultze and Cloutier1996; Schultze, Reference Schultze, Arratia, Schultze and Wilson2008; we follow the more commonly used term for this bone in actinopterygian studies) are the largest of the bones of the skull roof and are the most anteriorly positioned. They are slightly wider posteriorly than anteriorly and meet each other along the midline of the skull. Anteriorly the frontals are smooth, with a series of about seven bony elements on each side overlying this portion of the bones. These ossicles follow the supraorbital sensory canal anteriorly, after it exits the frontal (i.e., there is no true nasal bone, but rather a series of bony ossicles). Their relation to the rostral canal bones is unclear. Jin (Reference Jin1999, figs. 9, 10B) labeled small elements closely associated with the infraorbital ossicles anterior to the frontal bones as nasals. Posteriorly, the frontals meet the rectangular parietals (= postparietal of some authors on the basis of its homology with the bone termed the ‘postparietal’ in sarcopterygians; see also comments for frontal), which are also robust and ornamented over much of their exposed surfaces. A single large extrascapular is preserved on the left side of our specimen and is somewhat triangular in shape; it is unclear whether this is a lateral or median extrascapular. There is a small extrascapular exposed between the two limbs of the posttemporal of the new specimen, as well as in a similar position in other specimens (i.e., posterior to the skull roofing bones); only a single element is clearly present, although additional elements may be absent due to lack of preservation. Jin (Reference Jin1999, fig. 10) illustrated at most two (i.e., paired) extrascapular bones in his specimens. There is also a series of canal bones that lie lateral to the extrascapular (Figs. 4, 5); it is unclear whether these are lateral extrascapulars (i.e., incorporated into the skull roof) or just bony ossicles. The dermopterotic is badly damaged in our specimen, and little detail can be determined. However, there is a foramen on the lateral edge of the dermopterotic at about the level of the hyomandibula that likely represents the exit of the preoperculomandibular sensory canal. The dermosphenotic lies anterior to the dermopterotic and contacts both the frontal and parietal medially. It is somewhat drawn out posteriorly, so that it is almost teardrop shaped. The infraorbitals are small, relatively uniformly sized bones (the posteriormost two are the largest) that carry the infraorbital sensory canal from its exit from the dermosphenotic ventral and anterior to the orbit. The anterior infraorbital bones (i.e., rostral canal bones) appear to form a commissure at the tip of the snout, although their arrangement is disturbed in this region. About 30 infraorbital/rostral canal bones are preserved, but it is unclear where the distinction between the left and right sides should be made.

Neurocranium

No bones of the neurocranium, including the parasphenoid, are clearly preserved in our new specimen, but the shape of the parasphenoid can be seen in the other two specimens examined (Fig. 4). Jin (Reference Jin1999, figs. 9, 10, 11) also illustrated the parasphenoid of †Yanosteus. The parasphenoid of †Yanosteus extends posteriorly by two arms that form a deep aortic notch, as in other Acipenseriformes. There are large, anteriorly directed ascending rami of the parasphenoid at about the midpoint of the bone. The parasphenoid bears a distinct median anterior process (Fig. 4) that appears to be broader than that found in Acipenseridae (Hilton et al., Reference Hilton, Grande and Bemis2011, fig. 43).

Jaws and suspensorium

For consistency and ease of comparison with the skeletons of other Acipenseriformes, the terminology we use for the elements of the oral jaws and ventral hyoid arch of †Yanosteus follows that of our previous work on the anatomy of the order (e.g., Grande and Bemis, Reference Grande, Bemis, Stiassny, Parenti and Johnson1996; Grande et al., Reference Grande, Jin, Yabumoto and Bemis2002; Hilton and Forey, Reference Hilton and Forey2009; Hilton et al., Reference Hilton, Grande and Bemis2011). We acknowledge, however, the recent reevaluation of certain homology statements of Datovo and Rizzato (Reference Datovo and Rizzato2018), who suggested that the dermopalatine is homologous to the maxilla of non-acipenseriform actinopterygians. The elements of the upper jaw that are clearly preserved in our specimen include the dermopalatine, quadratojugal, palatopterygoid, and autopalatine (Fig. 5); we could not identify the entire outline of the ectopterygoid in our specimens (e.g., Fig. 5), but its anterior tip is a rounded point. The dermopalatine is elongate and forms the lateral margin of the anterior portion of the jaw. It is slightly broader anteriorly and bears a slight notch in its anterior margin. The quadratojugal is approximately the same width and length of the dermopalatine but is positioned more posteriorly and appears uniform in its width across its length. The precise shape and extent of the palatopterygoid (= endopterygoid of other authors; e.g., Arratia and Schultze, Reference Arratia and Schultze1991; Findeis, Reference Findeis1997; although this bone is likely homologous to the endopterygoid of other actinopterygians, we follow the terminology of Grande and Bemis, Reference Grande and Bemis1991, Reference Grande, Bemis, Stiassny, Parenti and Johnson1996 and Grande et al., Reference Grande, Jin, Yabumoto and Bemis2002 for consistency with our other publications on acipenseriform osteology) could not be determined, but it is the largest of the upper jaw bones, forming much of the roof of the mouth. Posteriorly, it has a rounded margin and appears to have a straight margin medially. Anteriorly, it is overlain by the autopalatine. Only the slightly rounded posterior margin of the autopalatine is exposed on our new specimen, but the entire outline of both the left and right autopalatine is seen in the specimens illustrated in Figure 4.

The dentary is the largest element of the lower jaw preserved in our specimen of †Yanosteus. It is a robust bone that is roughly rectangular in shape with a slight curve at its midpoint. The anterior end is forked, while the posterior end is slightly rounded. The medial surface of the dentary bears a groove, possibly housing Meckel's cartilage. The prearticular is broad and equal in length to that of the dentary (Fig. 5). The mentomeckelian is a small, rectangular bone (Fig. 4.3) that is visible in lateral view at the anterior end of the dentary (Fig. 5.2). Jin (Reference Jin1999, figs. 9, 10, 11) labeled an articular in †Yanosteus, but we could not confidently identify this bone in our specimens.

The largest bone of the hyoid arch that is preserved in our specimen of †Yanosteus is the hyomandibula, which is similar in its overall form to that of most other Acipenseriformes. There is a relatively narrow proximal head that would have articulated with the neurocranium. This head appears to be somewhat oval in cross section. Ventrally, the hyomandibula flares distinctly to become very broad and flat, with a gently curved posterior margin. The interhyal is large, and is roughly hourglass shaped, with deep concavities anteriorly and posteriorly. Anteriorly, the interhyal approaches the posterior corner of the jaw. More ventrally is an hourglass-shaped anterior ceratohyal and rounded posterior ceratohyal (the posterior ceratohyal is partially preserved and no significant details can be described). The anterior end of the anterior ceratohyal bears a deep, rounded concavity. Both ends of the anterior ceratohyal appear to be flattened in cross section. An element that corresponds to the hypohyal is preserved, but no details can be reported.

Opercular series

The opercle, the subopercle, and a single branchiostegal are clearly preserved (Figs. 4, 5). The opercle is a small, fan-shaped element that is much smaller than the subopercle and has a scalloped posterior margin; the posterior portion of the opercle is strongly ornamented. The subopercle is large and has a rounded posterior margin that is slightly scalloped, although this part of the element is not well preserved in our specimens. The posterior portion of the subopercle is slightly ornamented with fine ridges of bone. Anteriorly, the subopercle bears an unornamented process. This process is elongate, rounded, and distinctly set off from the main body of the subopercle.

There is a flat, broad fragment of dermal bone ventral to the subopercle in our specimens of †Yanosteus. This single branchiostegal is best seen in IVPP V10998.1 (holotype) and IVPP V10998.7 and has a broad horizontal body, positioned ventral to the subopercle, and a narrow anteroventrally directed extension that tapers to a point (Fig. 4).

Gill arches

Most elements of the gill arches are covered by overlying dermal bones of the skull, and therefore few aspects of their morphology can be described from our specimens of †Yanosteus (Fig. 5). In our new specimen, the first ceratobranchial from the left side, which bears short, triangular gill rakers on both its leading and trailing edges, is clearly preserved; the gill rakers are more slender and tightly packed on the trailing edge than on the leading edge. In the holotype, there are more posterior elements of the ceratobranchial series (unlabeled in Fig. 4.3). The preserved dorsal gill-arch elements of our new specimens include at least two epibranchials; other unidentified dorsal gill-arch elements likely include pharyngobranchials. Only one of the epibranchials preserves an association with four gill rakers. The first epibranchial bears a distinctly broad proximal head, which is likely the articulatory surface for the first pharyngobranchial, which has become displaced.

Vertebral column

The vertebral column is composed of ossified supraneurals, neural arches, ribs, and hemal arches (Fig. 3.3). There are 26 supraneurals preserved and 48 neural arches preserved, though there may be additional elements of both series preserved anteriorly that are covered by the dermal skull bones. The supraneurals extend from the anteriormost exposed vertebrae and continue posteriorly to under the anterior quarter of the dorsal-fin pterygiophores. Each supraneural flares proximally where it contacts the distal tip of the neural spines. Distally, the supraneurals taper to a blunt point. The neural arches are paired throughout the length of the series and are broad and rounded proximally but are drawn out to become spine-like elements, particularly more posteriorly in the series. There are approximately nine irregularly shaped ossified basiventral elements in the abdominal region, but this is likely an undercount because additional elements are probably hidden by the ribs anteriorly. The paired hemal arches, which are elongate spine-like elements, begin opposite the anterior insertion of the dorsal fin and continue into the caudal region as hypurals. There are 43 total hemal elements, including the 17 hypurals. The hemal elements mirror the neural elements, with a broadly rounded proximal end that tapers to a blunt or squared-off end (i.e., the hemal spine portion of the element). There are no ossified centra.

There are 11 ribs on both sides of our new specimen of †Yanosteus, although those of the right side are displaced and the count is approximate. As for the neural arches and supraneurals, there may be additional ribs that are covered anteriorly in our specimen.

Caudal fin and supports

The caudal fin of †Yanosteus is broad and only weakly forked (Fig. 6). There are 17 ossified hypurals that, together with the posterior hemal arches, support the caudal-fin rays. There are eight unsegmented, unbranched fin rays at the anteroventral end of the caudal fin and 20 segmented unbranched caudal-fin rays, which extend almost the entire ventral margin of the caudal fin; the longest fin ray of the ventral caudal lobe is the second branched caudal-fin ray. There are 61 branched caudal-fin rays on our specimen, all of which are ventral to the tip of the notochord (i.e., there is no trace of epaxial caudal-fin rays). In total, there are 89 caudal-fin rays, which fits within the range provided by Jin (Reference Jin1999, p. 86–98). There are six ossified epurals dorsal to the space the notochord would occupy in life. Farther anterior there are two bones, partially overlying one another, that are of unclear homology but possibly are caudal supraneural elements. There are also two unidentified bones posterior to the caudalmost neural arches; these are oval and flat. Along the dorsal margin of the caudal region are 29 dorsal caudal fulcra (30 reported by Jin, Reference Jin1999) that smoothly grade from short, leaf-shaped elements to more elongate fulcra typical of Acipenseriformes (Hilton et al., Reference Hilton, Grande and Bemis2011). At least anteriorly in the series (i.e., the anterior ~15 dorsal caudal fulcra), these are paired. It is unclear more posteriorly whether they are paired or median structures (as found in other Acipenseriformes; e.g., Hilton et al., Reference Hilton, Grande and Bemis2011). There are a few (~10) rhombic caudal scales located near the posterior tip of the caudal region of our large specimen of †Yanosteus.

Dorsal and anal fins and supports

One of the most remarkable aspects of the morphology of †Yanosteus is its elongate dorsal fin (Fig. 7.1, 7.2), which is unusual among Acipenseriformes and otherwise is found only in †Spherosteus and †Liaosteus (†Peipiaosteidae; Jin et al., Reference Jin, Tian, Yang and Deng1995) and the acipenserid †Priscosturion longipinnis (Grande and Hilton, Reference Grande and Hilton2006). The dorsal fin of †Yanosteus inserts posterior of the midpoint of the body (54.7%–59.3% standard length [SL]) but extends for a large portion of the body (37.0%–46.7% SL). The dorsal fin of our large specimen has 144 segmented branched rays as well as four unsegmented and 30 segmented unbranched rays (178 rays total). This is slightly more dorsal-fin rays than reported by Jin (Reference Jin1999), who found between about 160 and 170 dorsal-fin rays. The dorsal-fin rays are supported by 48 proximal radials and 49 middle radials; no trace of distal radials was found on our specimen. This number of radials agrees with that reported by Jin (Reference Jin1999), who reported 42–51 proximal and 45–52 middle dorsal-fin radials. As seen in other acipenseriforms, some of the middle and proximal dorsal-fin radials are forked (e.g., the proximal end of the twelfth middle radial in Fig. 7.1, 7.2).

The anal fin of †Yanosteus is relatively short and appears to show ontogenetic variation in its proportional size (11.9% SL in the smallest specimen and 19.5% in the largest; Table 1). The anal fin is positioned far posteriorly on the body (78.0%–73.3% SL) and is roughly opposite the midpoint of the dorsal fin. In our large specimen, there are one unsegmented and 18 segmented unbranched anal-fin rays and 55 branched segmented rays (74 anal-fin rays total; Fig. 7.3, 7.4). Jin (Reference Jin1999) found there to be 62 fin rays total in the anal fin for the two specimens that could be confidently counted. These fin rays are supported by 21 proximal and 23 middle radials; Jin (Reference Jin1999) reported the same counts for his specimens, with the exception of one specimen, which had 21 middle radials in the anal fin. As found in the dorsal fin, there is no trace of the distal radial series preserved on our specimens.

Pectoral and pelvic girdles and fins

The exposed surface of the posttemporal of †Yanosteus is strongly ornamented with ridges of bone (Fig. 5). The posttemporal articulates with the posterior margin of the skull roof but is not as tightly interlocked with the skull roofing bones as that of other acipenseriforms (e.g., Acipenseridae). The anterior end of the posttemporal is forked, with an elongate dorsal branch and a shorter, more lobate ventral limb. Ventrally, the posttemporal contacts the robust supracleithrum, which is about two times the length of the posttemporal. Much of the surface of the supracleithrum is roughly ornamented; the only smooth external surface of the supracleithrum is the posteroventral region. The cleithrum is large and, as in other acipenseriforms, is curved along its posterior margin where it defines the posterior extent of the pectoral girdle. This thickened curved region is ornamented with ridges. Medially, the so-called cleithral wall is smooth and gently bowl shaped and defines the posterior limits of the branchial chamber. The clavicle is not well preserved, but there is broken dermal bone present between the left and right cleithra that likely corresponds to the clavicles. No detail of shape of these elements can be distinguished. Jin (Reference Jin1999, figs. 9, 10) illustrated the clavicle as bearing a rounded anterior margin and a narrow, dorsally projected process. The lack of an interclavicle was interpreted as a general character of †Peipiaosteidae by Jin (Reference Jin1999). However, we identify a poorly preserved dermal element, found in both the holotype and another specimen (Fig. 4), as likely an interclavicle. There is no trace of a postcleithrum in our new specimen, but this is likely due to preservation as a slender postcleithrum straddling the articulation between the supracleithrum and cleithrum was found in our other specimens (Fig. 4; see also Jin, Reference Jin1999, figs. 9, 10B).

The pectoral fin of our specimen of †Yanosteus is supported by a metapterygium and six ossified radials (Fig. 8). This is a greater number of pectoral radials than reported by Jin (Reference Jin1999; 2–3 radials), but this may due to the specimen described here being larger and possibly more ossified. Our specimen bears 13 unbranched and 29 branched fin rays posterior to the short but well-formed pectoral-fin spine (Fig. 8). The base of the pectoral-fin spine is robust and bears a concavity that would have articulated with the elements of the pectoral girdle. Among Acipenseriformes, a fin spine is otherwise seen in †Chondrosteus acipenseroides Egerton, Reference Egerton1858 (Chondrosteidae; Hilton and Forey, Reference Hilton and Forey2009), †Protopsephurus liui (Polyodontidae; Grande et al., Reference Grande, Jin, Yabumoto and Bemis2002), and all members of Acipenseridae (Bemis et al., Reference Bemis, Findeis and Grande1997; Hilton et al., Reference Hilton, Grande and Bemis2011). The spine of †Chondrosteus appears different from the other acipenseriform taxa in being much shorter and not fused to the fin rays of the leading edge of the pectoral fin. By contrast, in †Protosephurus, †Yanosteus, and Acipenseridae, the pectoral-fin spine forms at least in part by the fusion of adjacent anterior pectoral-fin rays (Findeis, Reference Findeis1994; Dillman and Hilton, Reference Dillman and Hilton2015). The pectoral-fin spine of †Yanosteus appears to include two or three fin rays in its formation.

The pelvic fins of †Yanosteus are positioned just posterior to the midpoint of the body (53.%–58.0% SL) and bear around 17 unbranched and 18 branched fin rays (35 fin rays total; countable only on a single specimen examined here; Fig. 9), which agrees well with the counts reported by Jin (Reference Jin1999; 34 in the one reliably reported individual). We found nine radials that are partially embraced by the proximal ends of the fin rays. In addition, there is a series of ossified basipterygial elements that are moderately narrow bars of bones with flared anterior and posterior ends. The precise orientation and relation to other elements of the pelvic girdle are unclear. There are six of these basipterygial elements preserved in our large specimen, and we suspect that they were arranged as three per side.

Scales

The body of †Yanosteus is largely devoid of scales, with the exception of tubular elements that are associated with the lateral line sensory canal. In addition, there are a few (approximately nine) irregularly shaped rhomboid scales at the very posterior tip of the caudal region. We did not find the round-based scales associated with the pectoral girdle in our new specimen, although this is likely due to lack of preservation as they have been reported in †Yanosteus by Jin (Reference Jin1999, fig. 10A) and can be seen in the holotype (observed, but not illustrated in Fig. 4.1).