Thursday 19 March 2020

Andrias scheuchzeri: A Giant Salamader from the Miocene of southwestern Hungary.

The Cryptobranchids are Giant Salamanders, which can reach over 1.5 m in their total length. Fossil evidence in Asia and North America suggest that the evolutionary history of Cryptobranchidae extends back to Palaeocene, with their fossils being known later from Europe, Asia and North America. The extant species of this family (Cryptobranchus alleganiensis, Andrias davidianus, and Andrias japonicus) range over the cool temperate zones of China, Japan and North America. Within these distribution zones the animals generally prefer clear, oligotrophic (low nutrient) streams and rivers with rocky bed and steep banks. In contrast, late Oligocene to late Pliocene representatives of Andrias occurred in large ponds or lakes as well. Their fossils have been unearthed at Vertebrate sites in Germany (Upper Oligocene of Rott, Middle Miocene of Öhningen, Middle and Upper Miocene of Wartenberg, Upper Miocene of Reisenburg and Upper Pliocene of Willershausen) the Czech Republik (Lower Miocene of Břeštany) and Austria (Upper Miocene of Götzendorf, Brunn-Vösendorf and Mataschen), with all European Miocene Giant Salamanders currently assigned to a single species, Andrias scheuchzeri

In a paper published in the journal PalZ on 12 December 2019, Zoltán Szentesi of the Department of Paleontology and Geology at the Hungarian Natural History Museum, Krisztina Sebe of the Department of Geology and Meteorology at the University of Pécs, and Márton Szabó, also of the Department of Paleontology and Geology at the Hungarian Natural History Museum, and of the Department of Palaeontology at Eötvös Loránd University, describe a series of specimens assigned to Andrias scheuchzeri, from the Pécs-Danitzpuszta sand pit in the Mecsek Mountains of southwestern Hungary.

The Pécs-Danitzpuszta sand pit lies in southwest Hungary, on the southeastern margin of the Mecsek Mountains, at the eastern boundary of the city of Pécs, and exposes deposits of the late Miocene Lake Pannon. This brackish lake evolved from the Central Paratethys and existed throughout the late Miocene and the early Pliocene, from 11.6 to about 4.5 million years ago.

Geological setting of the Pécs-Danitzpuszta sand pit. (a) Location of the site, with published Miocene fossil sites with Andrias scheuchzeri in the Pannonian Basin. Blue patch indicates the extent of Lake Pannon around the time of sediment accumulation at Danitzpuszta (about 9.5 million years ago). (b) Simplified geological map of the area. Legend: T, Triassic; J, Jurassic; K, Cretaceous; M1-2, Lower–Middle Miocene; M3, Upper Miocene; Q, Quaternary rocks. (c) Panorama of the western wall of the sand pit. Bagger in centre for scale. (d) Steeply dipping limonitic sands in the northern part of (c). Note people for scale. Szentesi et al. (2019).

In the Danitzpuszta sand pit two different types of lacustrine deposits are exposed. The succession is heavily deformed due to intra- and post-Pannonian movements, thus the boundary between the two rock types is vertical, and younging is both towards the south and upwards. The older deposits are greyish white calcareous marls, exposed in (sub)vertical layers in the northern wall of the sand pit. Stratigraphically they belong to the Tótkomlós Calcareous marl Member of the sublittoral, open-water Endrőd Formation. They show a relatively sharp but conformable boundary with the overlying sands.

Based on the Mollusc association belonging to the Lymnocardium schedelianum sublittoral Mollusc biozone of Lake Pannon, these marl beds can be dated to 11–10 million years ago. The younger sediments are limonitic, yellow, coarse-grained, gravelly sands, now exposed in the long western wall of the sand pit, which were worked by the mine during the time it was active. The sands were derived from the denudation of the older Miocene and basement rocks of the mountains and are classified into the Kálla Formation. Within the sands, bedding dip gradually decreases upwards and towards the south, pointing to ongoing tectonic activity—uplift of the mountains—during sedimentation. The Mollusc assemblage found in the sands suggests an age of 9.5–10.0 million years for this deposit.

The sand pit is a famous Vertebrate locality in the region because of the large number of fossils found in the limonitic (iron-rich) sands. Remains are dominated by Fish (Bony Fish, Sharks, and Rays), Reptiles (mostly Turtles) and various aquatic and terrestrial Mammal remains. A considerable portion of them, e.g. those of euhaline (saltwater) taxa, must have been reworked from slightly older sediments of Badenian (16.3-12.8 million years old) and Sarmatian (12.6-11.6 million years old) age. However, partially articulated skeletons have also been found in the sand pit, which indicate that at least some of the fossils belonged to animals which lived during the accumulation of the exposed sediments. Cryptobranchidae fossils were collected in the described Vertebrate-bearing limonitic sands. by multiple collectors between 2006 and 2018.

The material examined comprises two left and two right fragmentary maxillae (upper jaw bones), eight left and eight right dentaries (lower jaw bones), four fragmentary atlases (first vertebrae), three fragmentary trunk vertebrae, one left ilium (part of the pelvis), and one left femur (thighbone). 

Maxillae of Andrias scheuchzeri from Pécs-Danitzpuszta. Left maxilla (a), (b) PTE-5152, right maxilla (c), (d) MBFSZ V.2019.97.1. (a), (c) ventromedial view; (b), (d) dorsal view. Szentesi et al. (2019).

The specimens share several characteristics with all crown Cryptobranchids, including large body size; massive bones; bilateral asymmetric kinetics of the lower jaw; and a trochanter fused with the proximal head of the femur. The rough inner surface of pars facialis (with ridges and depressions), the strong curvature of dentaries, and the arrangement and features of the teeth, characters typical for Andrias. The strongly curved maxillae with narrow and long teeth pedicels; the well-ossified, strongly curved dentaries with thick cortex, the mediumsized medullary cavities surrounded with smaller cavities; the amphicoelous atlas with large processus odontoideus and a larger central foramen among several small foramina ventrally; the dumbbell-shaped, amphicoelous trunk vertebrae with short neural spine suggest these fossils belong to the species Andrias scheuchzeri. The anatomical features of the ilium are the same as Urodeles, while the size of this bone suggests that this bone also belongs to the species Andrias scheuchzeri.

Left dentary of Andrias scheuchzeri (a)-(f) PTE-5148, from Pécs-Danitzpuszta. (a) lingual view; (b) labial view; (c) medial view; (d) dorsal view; (e) ventral view; (f: medial view. Szentesi et al. (2019).

The robust, strongly curved, low maxilla differs from that of Andrias matthewi (a species known from the Miocene of North America). The densely situated, high tooth pedicels of the maxillae and the strongly curved dentaries with deep sulcus dentalis also relate these specimens to Andrias scheuchzerii. The dentary differs from Ukrainasurus hypsognatus (a species known from the Miocene of Ukraine) and Andrias davidianus (the living Chinese Giant Salamander) in its more robust structure of the corpus dentalis. It is less robust labiolingually in cross-section than  Zaissaurus beliajevae (Palaeogene of the Zaisan Basin in Central Asia) and Aviturus exsecratus (from the Palaeocene of Mongolia). In cross-section view, the bone tissue of the dentary differs from that of Andrias davidianus and Ukrainasurus hypsognatus in the thick cortical bone and the significantly smaller cavities, from that of the Zaissaurus beliajevae in more numerous and denser medullary cavities, and also from that of Aviturus execratus in significantly less numerous and larger medullary cavities. The prominent and wide processus odontoideus of the atlas, the dumbbel-lshaped centrum of vertebrae, the vertebrae less massive than in Andrias matthewi, the foramen central significantly smaller than in vertebrae of Ukrainasurus hypsognatus. and the massive, short neural spine, ending in cartilage also refer to a close affinity with Andrias scheuchzeri. When comparing the fossil skeletal elements from Pécs-Danitzpuszta with those of recent Giant Salamanders, no significant differences can be observed.

Dentaries of Andrias scheuchzeri from Pécs-Danitzpuszta. Right dentaries: (a), (b) JAM T/2019.4.1., (c) MBFSZ V.2019.100.1., (d) MBFSZ V.2019.101.1., (h) MBFSZ V.2019.110.1.; left dentaries: (e) MBFSZ V.2019.103.1., (f) MBFSZ V.2019.107.1., (g) MBFSZ V.2019.108.1.. (a), (c)–(h) lingual view; (b) posterior view. Szentesi et al. (2019).

A considerable portion of the vertebrate remains at the Pécs-Danitzpuszta site is reworked from older, Middle Miocene (Badenian and Sarmatian) sediments. The diverse Chondrichthyan fauna is typical for the Badenian of Europe. Partially articulated Bony Fish skeletons (e.g. a yet unidentified species of Latid Perch, Fish which primarily live in freshwater) are evidently coeval with the sediment, i.e. late Miocene. The numerous Reptilian remains are mostly freshwater or terrestrial forms, but have been identified so far very approximately, at a level not sufficient to assign an age to them. The extraordinarily abundant marine Mammal fauna includes predominantly Cetaceans (Whales and Dolphins) and Sirenians, plus, in smaller quantities, Pinnipeds (Seals), identified taxa are of Badenian and Sarmatian age. Diverse terrestrial Mammalian remains include Hipparions, Proboscideans, Rhinocerotids, Tapirs, Suids and Ruminants (Bovids, Tragulids and Cervids). Most of them represent a typical early Pannonian (11.6-9.0 million year old) assemblage, a so-called Hipparion fauna. However, a small sized Deinothere (cf. Prodeinotherium bavaricum) is likely to be indicative of fossils from older (Badenian and/or Sarmatian) sediments, showing that this is also an issue with terrestrial taxa from Pécs-Danitzpuszta. Therefore, although the Andrias fossils were found in Upper Miocene (lower Pannonian/lower Tortonian) sands, their Late Miocene age is not unambiguous. This question can be discussed considering the ecological requirements of the taxon and using analogous finds of similar age.

Atlases of Andrias scheuchzeri from Pécs-Danitzpuszta. (a)–(c) JAM 2006.236.93., (d), (e) MBFSZ V.2019.111.1. (a), (d) ventral view; (b), (e) anterior view; (c) posterior view. Szentesi et al. (2019).

Cryptobranchids occur exclusively in humid areas, with mean annual precipitations exceeding 900 mm. Climatic analysis, based on fossil flora, estimated a mean annual precipitation of around 1000 mm for the deposition time of the Danitzpuszta sediments. Macrofloral remains at Danitzpuszta indicate expressedly thermophilous vegetation, where the prevalence of Lauraceae (Laurels) and other thermophilous elements in the vegetation excludes freezing temperatures. These climatic conditions made the early late Miocene of the southern Pannonian Basin suitable for Giant Ssalamanders.

Trunk vertebrae of Andrias scheuchzeri from Pécs-Danitzpuszta. (a)–(d) JAM T/2019.2.1., e.g. JAM T/2019.3.1. (a) Left lateral view; (b), (f) anterior view; (c), (g) ventral view; (d) posterior view; (e) right lateral view. Szentesi et al. (2019).

A further environmental prerequisite for the presence of Cryptobranchids is considered to be increased relief due to regional uplift, which produces the necessary habitats, mountain valleys for the Giant Salamanders. Uplift of the Mecsek Mountains during the early Late Miocene is unambiguously indicated by the upwards gradually decreasing bedding dip, i.e. syn-tectonic deposition in the sands containing the Andrias bones. Steep topography was proposed based on the event-like input of coarse clastic sediments, sand and gravel, with redeposited Pannonian littoral Mollusks into the open-water calcareous marl succession underlying the sands. Increased relief is shown by vegetation types linked to various elevations from lakeshore to upland. The appearance of the fossil-bearing sands above the carbonate-dominated lacustrine succession itself refers to increased activity of watercourses and to the resulting denudation and clastic input into the lake. Direct evidence for the existence of freshwater habitats, namely swamps is provided by the massive occurrence of Myrica lignitum (a type of Bayberry) leaves among the fossil flora. Shoot fragments of Glyptostrobus europaeus (a type of Cypress), another typical swamp plant, were interpreted as having been transported from a distance into Lake Pannon and thus indicate that freshwater wetlands were present further away on the mainland. 

Left ilium (a)–(c) JAM 2006.185.47. and left femur (d)–(f) JAM T/2019.1.1. of Andrias scheuchzeri from Pécs-Danitzpuszta. (a) Lateral view; (b) medial; (c) ventral view; (d) dorsal view; (e) ventral view; (f) posterior view. Szentesi et al. (2019).

Thus, although their exact age cannot be indisputably identified, based on the listed climatic and topographic arguments the Andrias fossils can be coeval with the host sediments. Moreover, the Giant Salamanders live in streams, i.e. in high-energy sedimentary environments, so the partially eroded surface of the bones does not necessarily indicate their post-depositional transport. The preservation of bones found at the Pécs-Danitzpuszta site is also comparable to that of in situ Andrias bones from molasse deposits. The presence of Andrias in the Mecsek Mountains during the late Miocene would fit into and confirm the view of the early late Miocene palaeogeography as having a relief more rugged and a climate more humid and mild than today.

The nearest sites with Andrias fossils are located along the western margin of the Pannonian Basin. In the Styrian Basin in Austria, Andrias scheuchzeri vertebrae and a maxilla were found in the lower Upper Miocene (uppermost MN7/8) clay pit of Mataschen, at the foot of the Eastern Alps. Two other sites (Brunn-Vösendorf and Götzendorf/Sandberg) are located more to the north, in the Vienna Basin. A vertebra and a humerus were reported from the 'Congeria beds' of Brunn-Vösendorf, again next to the Eastern Alps. This site has a late Miocene age as well and is correlated to the middle MN9 zone. The third location, Götzendorf/Sandberg, lies just west of the Leitha Mountains and provided a rich herpetofauna, including several bones of both adult and juvenile Andrias scheuchzeri recovered from deposits of a side-branch of the Late Miocene paleo-Danube. The small Mammal taxa of the outcrop indicate the upper MN9 zone and chronologically the locality is indirectly dated to about 10 Ma based on the close similarity of the evolutionary stage of the small Mammal fauna to that of radiometrically dated faunas in Turkey and Germany. All three listed Andrias locations are of early late Miocene age, similarly to the Danitzpuszta sands. They lie next to elevated topography, be it the Alps or the lower Leitha Mountains, like Danitzpuszta lies at the foot of the Mecsek Mountains. Therefore, the Pécs-Danitzpuszta site is a further, fourth locality in the Pannonian Basin where Giant Salamanders could find their habitats in the late Miocene. The distribution of late Miocene sediments of various ages shows that the Mecsek Mountains formed an island in Lake Pannon from the beginning of the Late Miocene to about 8-7 million years ago. This island had dimensions of at least 70 × 15 km during the early Late Miocene, an extent large enough to develop a well-developed drainage network and to provide habitat for the Andrias.

Reconstruction of the late Miocene freshwater environment of Pécs-Danitzpuszta and the Andrias scheuchzeri. Márton Szabó in Szentesi et al. (2019).

Cryptobranchids (Megalobatrachidae indet.) have also been mentioned from the Pliocene of southwest Hungary, from the sites Csarnóta 2 and Beremend 26, in the Villány Hills, about 30 km south of the Mecsek Mountains. Szentesi et al. investigated these small amphicoelous vertebrae, deposited in the collection of the Hungarian Natural History Museum, but did not confirm their attribution to the family Cryptobranchydae, as neither their shape, nor their small size (under 3 mm) supports the original identification. Therefore, the remains from Pécs-Danitzpuszta are the only Giant Salamander fossils known from Hungary so far.

Based on cranial and postcranial bones, giant salamanders were present in the Miocene of the Mecsek Mountains in southern Hungary. The studied fossils are attributed to the species Andrias scheuchzeri

The fossils of Andrias from Pécs-Danitzpuszta are isolated, incomplete and mostly eroded, suggesting pre-depositional transport. The most frequent bones are the massive dentaries (especially its anterior part), whilst the more vulnerable maxilla, vertebrae and elements of the appendicular skeleton are significantly rare.

Giant Salamanders lived on an island within the Central Paratethys in the Middle Miocene or within the brackish Lake Pannon in the Late Miocene. Although their exact age cannot be indisputably identified, the Andrias fossils can be coeval with the Upper Miocene host sediments. Arguments supporting suitable conditions for Giant Salamanders in the early late Miocene of the southern Pannonian Basin are the mild and humid climate and the presence of freshwater wetlands on the dryland indicated by the fossil flora, the syn-depositional uplift of the mountains providing for a rugged topography and the intense clastic sediment input into Lake Pannon indicative of a fluvial network on the Mecsek Island and of sufficient precipitation. Even if the fossils are reworked from older sediments, their age cannot be older than Badenian (Langhian).

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