Diagnoza

Small stem-frog (less than 50 mm snout-vent length) that resembles Triadobatrachus and all other salientians in having an anteriorly extended iliac shaft. It resembles Triadobatrachus and differs from crown-group anurans in retaining relatively long neural arches, unfused epipodials, a series of unfused caudal vertebrae instead of an urostyle, a scapulocoracoid that is a single ossification, and an ilium with very strong dorsal prominence and a slender elongate shaft, rounded in cross-section. It differs from Triadobatrachus in having a long slender scapular blade (short and broad in Triadobatrachus), in having a single atlas ossification with no trace of rib facets, in having a higher level of ossification, long fused posterior transverse processes and sacral ribs, fully ossified components in elbow joint, ischium fused to ilium (the last feature unique within the Salientia; Roček, personal communication 2007) despite its much smaller size, and in having longer, more slender limbs. As reconstructed, the ilio-sacral joint of Czatkobatrachus has a morphology quite distinct from that of Triadobatrachus, in that the sacral rib is short and fused to the vertebral centrum (rather than free and posteriorly elongate), and has an expanded distal end that is both dorsoventrally and anteroposteriorly bifurcate, with a posterior notch and groove. Czatkobatrachus also differs from Triadobatrachus and resembles many crown-group frogs in having a fully ossified, capitate eminence (eminentia capitata) that equals or exceeds 60% of the width of the distal end of humerus, and asymmetrical epicondyles (ulnar epicondyle larger). It differs from crown-group frogs in having the anterior margin of atlas pedicel notched for the exit of the first spinal nerve and spinal nerve notches or foramina in some posterior vertebrae, and in retaining an ossified remnant of the pubis, fused to the body of the pelvis and perforated by an obturator canal.

Porównanie

Czatkobatrachus shows a combination of primitive and derived character states that together place it between purported stem-lissamphibians (e.g., Gerobatrachus) and crown-group anurans. However, its position with respect to Triadobatrachus remains unclear. Many of its characters (humeral morphology, ilio-sacral morphology, scapula shape, the fusion of the transverse processes) appear to position Czatkobatrachus above Triadobatrachus on the anuran stem, but these differences could relate to function and lifestyle, if Triadobatrachus were less terrestrial.

The most obvious derived characters shared by Czatkobatrachus, Triadobatrachus and the Anura relate to the pelvis (i.e., the anteriorly elongate ilium and, consequently, posterior position of the acetabulum with respect to the ilio-sacral joint, and the reduced pubis). The ilium is less elongate than it is in the crown-group frogs but more so than in Triadobatrachus.

A shortened presacral column and the reduction in length of the tail, would be further synapomorphies of the Salientia, if reconstruction, based on vertebra size and structure in Czatkobatrachus, is confirmed by the recovery of further articulated material. Similarly, long slender propodials and the loss of the interglenoid tubercle are also putative salientian synapomorphies. The posterior shift of the first spinal nerve, to a position that notches the anterior margin of the neural pedicel of the atlas, is an expression of a lissamphibian evolutionary trend.

Although the material is still limited, there is data on four key regions in the evolution of salientian locomotion: the axial skeleton, the pectoral girdle, the pelvis, and the limb proportions. These suggest Czatkobatrachus had taken the following steps:

1. A shift in locomotor pattern away from lateral undulation toward greater dependence on the limbs (fusion of the ribs to the vertebrae, strong transverse processes, the tail probably reduced in length).

2. The development of a slender scapula, and a large circular glenoid facing laterad within a basically arciferal pectoral girdle. These characters might be Czatkobatrachus autapomorphies related to the importance of the forelimbs in propping up the body, in a stage transitional between caudate-like locomotion and that of anurans. The strongly and fully ossified elbow joint suggests that this was an important part of the same functional complex for this small amphibian.

3. The development of an anteriorly directed cylindrical iliac shaft, with a large dorsal prominence (associated with gluteus magnus or its stabilising tendon). The orientation of the iliac blade implies that thrust forces from the legs were already being directed anteriorly parallel to the body axis, even if the animal was not yet leaping. The ilio-sacral joint had been modified from an abutting cartilage covered joint into a primarily suspensory one, the two components being fastened by a ligament. The ilium was still positioned lateral to the sacral transverse process at the ilio-sacral joint, but was stabilised both dorsally and ventrally by parts of the sacral diapophysis. However, the posterodorsal convexity might represent a rudiment of the sacral flange that overlaps the ilium dorsally in crown-group frogs. The combination of a pelvis modified to transmit thrust anteriorly, with a weak ilio-sacral joint and a short but probably flexible tail, would have placed considerable reliance on the soft tissues holding these elements together. This may explain the increased size of the peri-sacral transverse processes and the enlargement of the dorsal prominence of the ilium (for a stabilising tendon of gluteus magnus).

4. The development of longer, more gracile limbs without strong muscle attachment surfaces. The retention of fore- and hind limbs of similar length is a primitive character, compared to the disparity found in crown-group anurans. However, the elongation of the humeri, by comparison with both Triadobatrachus and crown-group frogs, may represent another autapomorphy of Czatkobatrachus, reflecting an unusual, transitional, locomotor pattern.

Together Triadobatrachus and Czatkobatrachus demonstrate that the evolution of the pelvis (elongate anteriorly directed blades, reduced pubis) and some aspects of the ilio-sacral joint (loosening of the ilio-sacral contact, development of a suspensory system), preceded the evolution of those features uniquely associated with saltation (the urostyle, fusion and elongation of the antebrachial and tibiofibular bones, a separate coracoid element and more parasagittal position of the fore-limb for jumping). The saltatory functional complex had evolved by the Early Jurassic, but further Triassic material is needed in order to determine the sequence of intermediate steps. These may, in turn, shed light on the peculiar functional complex presented by Czatkobatrachus, as would the recovery of more of its Permian antecedents (Gerobatrachus being an important recent addition).

Autekologia

Czatkobatrachus was a small agile terrestrial animal that walked, and perhaps hopped, with a stiffened back, and a reduced tail. It was substantially smaller than Triadobatrachus, was longer limbed, and would have been more agile. It was probably also more fully terrestrial, judging by the higher degree of ossification throughout the skeleton (even by comparison with basal crown-group taxa).

The reconstructed palaeoenvironment for Czatkobatrachus was a small non-permanent water body (or group of such water bodies), surrounded locally by vegetation (oasis), but in a rather arid environment. Food, in the form of insects, is likely to have been concentrated in the vegetation around the water, providing a focus for small vertebrates.

Czatkobatrachus is a rather routine, although subordinate, member of the local tetrapod community, comparable in number of fossils with the record of accompanying temnospondyl amphibians. (This condition much differs from that displayed by Triadobatrachus, the only other Triassic stem-frog so far known, detected by but a single specimen).

The fact that Czatkobatrachus is not uncommon in the assemblage may provide support to the hypothesis suggesting that the anurans originated in upland habitats. This concept assumes that the structural pattern of primitive anuran larvae along with the mode of their feeding (scraping algae from the rock) arose as an adaptation to living in mountain brooks. It can be admitted that the type of environment exemplified by the Czatkowice biotope did not depart too much from conditions under which the anurans are thought to have originated.

Występowanie geograficzne

Only locus typicus

Zasięg czasowy

Only stratum typicum

Materiały muzealne

Institute of Paleobiology PAN, Warsaw (ZPAL)

Literatura

Borsuk-Białynicka, M. and Evans, S.E. 2002. The scapulocoracoid of an Early Triassic stem-frog from Poland. Acta Palaeontologica Polonica 47, 79–96.

Evans S.E. and Borsuk-Białynicka, M. 1998. A stem-group frog from the Early Triassic of Poland. Acta Palaeontologica Polonica 43, 573–580.

Evans, S.E. and Borsuk-Białynicka, M. 2009. The Early Triassic stem-frog Czatkobatrachus from Poland. Palaeontologica Polonica 65, 79–105.

Shishkin, M.A. and Sulej, T. 2009. The Early Triassic temnospondyls of the Czatkowice 1 tetrapod assemblage. Palaeontologia Polonica 65, 31–77.