Andrias sp.
Creators: name: Syromyatnikova, Elena V.
name: Titov, Vadim V.
name: Tesakov, Alexey S.
name: Skutschas, Pavel P.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Amphibia
Caudata
Cryptobranchidae
Andrias

description: Andrias sp.
(Figs 2-6; 7A, D)
MATERIAL. — Two frontals, PIN 5882 /1 and 5882/2; two dentaries, PIN 5882/3 and 5882/4; one prearticular, PIN 5882/5; five trunk vertebrae, PIN 5882/6-10; one femur, PIN 5882/11; one rib, PIN 5882/12; all from the Belorechensk locality, Northern Caucasus, Late Pliocene, and interpreted as belonging to a single individual.
DESCRIPTION
Frontal
Both left and right frontals are almost completely preserved (Fig. 2). They match each other along the medial suture, which indicates they came from the same individual. The length of the complete specimen is 54.5 mm. The medial suture is markedly thickened dorsoventrally and interdigitated. The frontal is widest anteriorly and becomes narrower posteriorly, without a lateral constriction in the olfactory region. The dorsal surface is shallowly convex and smooth. The anteromedial portion of the bone is broad and forms a triangular facet for a nasal; the anterolateral portion is extended anteriorly and forms a narrow facet for a maxilla; and the lateral portion forms a facet for a prefrontal. Judging by the size of the facet on the frontals, the nasals overlapped the frontals for the anterior 30% of the length of the frontals. The ventral surface of the frontal is shallowly concave. The ventral surface bears the anterior portion of the roof of the cranial cavity and, posteriorly, an elongate facet for a parietal (Fig. 2E, H). The anterior portion of the cranial cavity is elongate and subtriangular in outline, narrowing posteriorly. Its anterior margin is straight and indistinctly bordered, whereas the lateral and posterior margins have sharp edges; the anterolateral margin has a bump. A shallow olfactory tract is visible in the olfactory region of the left frontal (Fig. 2E).
Dentary
The dentaries are represented by a complete right ramus and the anterior part of the left ramus (Fig. 3). They are long, slender, and slightly compressed labiolingually. The length of the complete specimen is 109.5 mm in straight line distance between its posterior and anterior tips and 120.2 mm along the curvature of its labial surface. The height of the dentary is 9.2 mm at the symphysis and 14.4 mm at the middle part of the tooth row. The symphysis is wide, convex, and roughly rhomboid in outline. The symphyseal surface is slightly rugose, mostly in its medial part, which suggests high mobility of the mandibular symphysis (Cundall et al. 1987). In lingual view, the dental lamina is of the same height throughout its length. The subdental lamina is half as high as the dental lamina or lower. The sulcus dentalis (subdental shelf) is wide and slightly concave along its anterior portion, but flat and narrow along its middle and posterior portions. It bears several small foramina. The margin of the sulcus dentalis is clear and rounded in PIN 5882/3, but acute along its anterior part in PIN 5882/4. The Meckelian groove is deep and extends anteriorly to a point below the 26-28 tooth positions (as counted from the anterior end). The corpus dentalis is smooth and flattened anterior to the Meckelian groove. Ventrally, it becomes thinner and ends with a ventral keel. In labial view, the surface of the dentary is only slightly rugose. The eminentia longitudinalis is clearly developed throughout the bone. A few small neurovascular foramina extend below the eminentia longitudinalis. The tooth row contains 80 small and closely spaced pleurodont teeth. The tooth crowns are completely missing, but the pedicellar portions show that the teeth were narrow, as in other cryptobranchids. Internally, the dentary is compact and encloses a few small and medium-sized neurovascular cavities and canals (Fig. 3D). In the region containing the anterior part of the tooth row, the cavities are mostly located below the subdental shelf. In the regions containing the middle and the posterior parts of the tooth row, the cavities are located above and below the Meckelian groove. There are no cavities in the area of the Meckelian groove and the ventral keel.
Prearticular
The prearticular is represented by the right element lacking its anterior and posterior tips (Fig. 4). It is low anteriorly and posteriorly and highest in the area of the coronoid process. The coronoid process is large, but short anteroposteriorly, nearly triangular, and tilted posterolingually. The articular facet is narrow and smooth. The dental facet widens posteriorly and has a sharp dorsal border that forms the lingual border of the coronoid process. It occupies about half of the length of the coronoid process.
Vertebrae
The vertebrae are represented by five nearly complete specimens from the trunk region (Fig. 5). The centrum is deeply amphicoelous, antero-posteriorly short and hourglass-shaped in lateral view, and circular in anterior and posterior views.
Its length varies between 19-21 mm. Both cotyles have a notochordal pit. Lateral and ventral surfaces of the centrum are densely porous. The cotylar rim is robust and ventrally forms a larger anterior and a smaller posterior knob.Ventrally, the centrum is perforated by a large central foramen in all vertebrae. One or two subcentral foramina are situated at the bases of the transverse process in PIN 5882/8 and 5882/9. The transverse processes (rib-bearers) are completely preserved only in two specimens. The transverse processes are unicipital and expanded distally, and articular surface for the rib heads is dumbbell-shaped. On its posterior surface it bears a wide depression that is surrounded by distinct ridges. The base of the transverse process is perforated by a large vertebrarterial canal. The neural arch is inverted V-shaped in anterior view and deeply concave in dorsal view; it is markedly vaulted posteriorly. Its posterior surface bears a pair of depressions spanning between the neural spine and the postzygapophyses. The spinal nerve foramina are absent. The neural canal is depressed and widened, with small internal projections (spinal cord supports; see Skutschas & Baleeva 2012) on its sides. The neural spine is posteriorly high and ends in a facet for a cartilaginous tip. The angle formed by the neural spine with the centrum is about 40 degrees. The pre- and postzygapophyses vary from round to elongate (Fig. 5K, L). Their articular surfaces are slightly inclined medially. The articular surfaces of the zygapophyseal processes show growth ridges, which are most visible across the distal portions (Fig. 5M). Most of the observed articular zygapophyseal surfaces have six or seven growth ridges. The interzygapophyseal ridges are not developed.
Femur
The femur is represented by its distal portion only (Fig. 6 A-G). It is asymmetrically rounded, being convex dorsally and concave ventrally. Cross-sections of the femur (Fig. 6E) show a thick cortex with a few vascular canals and/or erosion cavities in the proximal part of the preserved fragment, i.e., near the diaphyseal region, and a dense vascular network of canals in its distal part. The vascular canals open into grooves on the bone surface.
Rib
The rib is represented by a single proximal fragment (Fig.6 H-L). It is anteroposteriorly flattened and medially slightly concave. The articulation surface is hourglass-shaped with narrow upper and wide lower portions. Ridges extend along the upper and lower margins of the rib.
COMPARISON AND REMARKS
We suggest that all listed bones come from a single individual because they were found close to each other, correspond well to each other in size and type of preservation, and there is no duplication of elements.
The giant salamander from Belorechensk can be assigned to Cryptobranchoidea and Cryptobranchidae (e.g. Gao & Shubin 2012) in having large body size, frontals strongly overlapped onto parietals, anterolateral extension of frontal, absence of the spinal nerve foramina, and unicapitate trunk ribs.
According to the criteria of Gubin (1991), the giant salamander from Belorechensk can be assigned to Cryptobranchinae based on its relatively small size, tooth bases situated directly above sulcus dentalis, and prezygapophyses situated above the base of the neural spine. However, 80 teeth in the dentary are too many for Cryptobranchinae, which are supposed to have 60-75 teeth (Gubin 1991). On the other hand, thecryptobranchine Andrias scheuchzeri has been reported to have as many as 82 teeth (Böttcher 1987). Therefore, it seems that the distinction between Cryptobranchinae and Aviturinae (sensuGubin 1991) has to be revised with respect to this, and possibly to other, characters. The lack of consistent differences in other characters has been mentioned earlier (Vasilyan et al. 2013).
The cryptobranchid from Belorechensk clearly differs from the Paleogene species Aviturus exsecratus (Gubin 1991; Vasilyan & Böhme 2012) in a number of characters, i.e., its smaller overall size, markedly slender bones, different shapes of the frontals and the cranial cavity, a narrower and deeper sulcus dentalis of the dentary, a lower subdental lamina, absence of developed presymphyseal sulcus, fewer teeth, a markedly vaulted neural arch of the vertebrae, the absence of interzygapophyseal ridges, a poorly developed accessory alar process, and the presence of a depression on the posterior surface of the transverse process. It differs from another Paleogene cryptobranchid, Zaissanurus beliajevae (Chkhikvadze 1982), in having lower subdental laminae.
The cryptobranchid from Belorechensk differs from the Mio-Pliocene Andrias scheuchzeri (Westphal 1958, 1970; Estes 1981; Böttcher 1987) in a greater overlap between the nasals and the frontals, a slender and lower dentary, and a shallower sulcus dentalis. Additionally, it differs from individuals of A. scheuchzeri from the Late Miocene of Pécs-Danitzpuszta (Hungary) in its dental symphysis being rhomboid (Fig. 3F) rather than triangular (Szentesi et al. 2020: fig 3c).
The cryptobranchid from Belorechensk differs from Ukrainurus hypsognathus from the Late Miocene of Ukraine in having its dentary slender and lower, the corpus dentalis and the subdental lamina both lower, the subdental shelf anteriorly concave, the lingual crista absent, the neurovascular foramina smaller, the labial side of the dentary nearly smooth, and the dentary facet of the prearticular situated more posteriorly (Vasilyan et al. 2013). It shares with U. hypsognathus the short and broad shape of the coronoid process of the prearticular and may have extremely elongated, elliptical prezygapophyseal articulation surfaces (Fig. 5L).
Among the Recent genera, the cryptobranchid from Belorechensk can be assigned to Andrias based on the frontals excluded from the narial opening (not excluded in Cryptobranchus) and a larger angle (about 40 degrees) between the neural spine and the axis of the vertebral centrum (19-37 degrees in Andrias vs 15-20 degrees in Cryptobranchus) (Estes 1981). Westphal (1958) could not diagnose modern Andrias species based solely on osteology. Some relevant characters were mentioned by Meszoely (1966): the shape of the orbit and the separation between the pterygoid and the maxilla. Unfortunately, neither of these features can be observed in the cryptobranchid from Belorechensk. Finally, Estes (1981) could not find any differences between A. scheuchzeri and A. davidianus, and he placed the latter within A. scheuchzeri. Comparisons between the cryptobranchid from Belorechensk with Andrias japonicus and Andrias davidianus are difficult, because we are not aware of any publications listing osteological characters diagnostic of these modern species. Nevertheless, it seems possible to differentiate the cryptobranchid from Belorechensk from modern species of Andrias by its frontal not being laterally constricted in the olfactory region and having a longer contact with the nasals (Fig. 7 A-C) and by the coronoid process of its prearticular being broad and long (Fig. 4). The cryptobranchid from Belorechensk seems closer to A. davidianus in having the cranial cavity of the frontals subtriangular, widest anteriorly and narrowing posteriorly (Fig. 7D, E), whereas in A. japonicus it is rhomboid (Fig. 7F). However, until reliable data on the comparative osteology of the modern species of Andrias become available and/or new material on the cryptobranchid from Belorechensk appears, we provisionally can assign the latter only to Andrias sp.
description: Published as part of Syromyatnikova, Elena V., Titov, Vadim V., Tesakov, Alexey S. & Skutschas, Pavel P., 2024, A " preglacial " giant salamander from Europe: new record from the Late Pliocene of Caucasus, pp. 45-57 in Comptes Rendus Palevol 23 (3) on pages 48-51, DOI: 10.5852/cr-palevol2024v23a3, http://zenodo.org/record/10579621