Madygenerpeton pustulatus, Schoch & Voigt & Buchwitz, 2010
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publication ID |
https://doi.org/ 10.1111/j.1096-3642.2009.00613.x |
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DOI |
https://doi.org/10.5281/zenodo.10545585 |
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persistent identifier |
https://treatment.plazi.org/id/C95C87DF-E164-D707-7B94-4257FC57FCF0 |
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treatment provided by |
Valdenar |
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scientific name |
Madygenerpeton pustulatus |
| status |
sp. nov. |
MADYGENERPETON PUSTULATUS SP. NOV.
Holotype: FG 596 / V/4 , a near-complete skull (103 mm skull midline length) ( Fig. 2 View Figure 2 ).
Paratypes: FG 596 / V/5 , a series of five articulated fragmentary dorsal osteoderms ( Fig. 4B View Figure 4 ); FG 596 / V/6 , an isolated osteoderm fragment ( Fig. 4A View Figure 4 ).
Type locality: South-west Madygen outcrop area (Urochishsche Madygen) near Madygen village, Batken District (Oblast), south-west Kyrgyzstan ( Fig. 1 View Figure 1 ).
Type horizon: ‘Upper Grey-coloured Member’, Madygen Formation (Ladinian or Carnian, Middle or Upper Triassic).
Diagnosis: Madygenerpeton pustulatus has the following autapomorphies: (1) skull outline broad-parabolic; (2) nares tiny and more widely separated than orbits; (3) otic notch shallow, with squamosal-tabular contact; (4) postparietal posterolaterally emarginated (postparietal embayment), tabular horns large and blunt; (5) maxillary and premaxillary teeth tiny and numerous, and closely set; (6) the absence of a premaxillary fontanelle (otherwise a chroniosuchid character); and (7) dermal ornament of entire skull roof pustular, with spherulitic tubercles apparently fused to a shallow base.
Assignment to Chroniosuchidae : The possession of (1) chroniosuchid-type osteoderms (sensu Golubev, 1998b); (2) the transverse elongation of these osteoderms (occurring only in some chroniosuchids); and (3) a preorbital fenestra between jugal and lacrimal indicate that M. pustulatus is a chroniosuchid. Further, despite its numerous autapomorphies, the cranial morphology is generally consistent with that of chroniosuchids in having a parietal-tabular contact but no intertemporal, a narrow jugal, and a substantial jugal-prefrontal suture. These latter features are distributed across a much larger group of early tetrapods, including embolomeres and stem-amniotes.
Diagnosis of Chroniosuchidae : A monophyletic group of tetrapods sharing the following synapomorphies: (1) preorbital fenestra; (2) sheet-like ventral processes of the dorsal osteoderms whose deep ventral incision results in a P- shape in lateral view; (3) a pair of lateral protrusions from the caudal osteoderm margin bearing posterodorsal articulation facets. In addition, all chroniosuchids lack an intertemporal, a derived condition that must have evolved in parallel in temnospondyls and colosteids. Most chroniosuchids (exception: Madygenerpeton ) further share a fenestra between the premaxillaries and nasals.
Description
General structure: The skull is well preserved and reveals much of the original three-dimensional structure despite crushing of the right margin, which has resulted in deformation of the preorbital region ( Fig. 2 View Figure 2 ). Overall, the cranium is flat and of wideparabolic outline; the most elevated regions are parasagittal ridges running along the nasals, prefrontals, postfrontals, the temporal series, and especially the orbits, whose medial margins form the most elevated parts of the skull. The orbits measure only about one tenth of the skull length and are located in the posterior third of the skull, and widely spaced. The widest point of the skull (level with posterior orbit margins) measures two thirds of its greatest length. Compared with all well-known chroniosuchians ( Chroniosuchus , Chroniosaurus , Jarilinus ), the skull proportions of Madygenerpeton differ markedly by (1) the very small orbits; (2) the broad preorbital region, which is almost as wide as the posterior skull; (3) the tiny nares, which are widely separated from the preorbital fenestra; (4) the absence of a premaxillary fontanelle; and (5) the deep posterior embayments between postparietals and tabulars. Similar to Chroniosuchus paradoxus , the squamosal embayment is shallow, whereas the quadrate condyles are level with the occiput as in the smaller specimens of Chroniosaurus dongusensis . As in Jarilinus , the preorbital region is three times longer than the posterior skull table. The high extent of ossification in all preserved parts of the skull in Madygenerpeton suggests that the specimen was probably mature.
At first sight, the skull outline, broad interorbital region, alignment of ridges, and the occipital margin of the skull table in Madygenerpeton parallel the situation in edopoid temnospondyls, notably Cochleosaurus bohemicus ( Sequeira, 2004) . However, these similarities are rather superficial when studied in detail: the ridge system is different in several key features (e.g. anterolateral ridge formed by the lacrimal in Madygenerpeton , but the prefrontal in edopoids), and the wide interorbital region results from broadening of the postfrontal in Madygenerpeton as opposed to the parietal and frontal in edopoids. Finally, the occipital margin of the skull bears posterior projections of the postparietal in Cochleosaurus , contrasted with deep emarginations of the postparietals in Madygenerpeton . Although the phylogenetic analysis reported below includes Cochleosaurus , the position of Madygenerpeton within the chroniosuchians is strongly confirmed.
Ornament and ridges: The entire dorsal surface of the skull roof is covered by numerous pustules, which are located on gentle elevations, rising from the otherwise smooth dermal bones. The pustules are generally very small and numerous, mostly round or oval in outline. Many of them appear to be spherules attached to a narrow base, rather than forming outgrowths of bone. Such a kind of composite ornament is known from some extant anurans ( Ruibal & Shoemaker, 1984). The largest and most densely set pustules are located on top of the ridges and along the medial margin of the orbits. In places, neighbouring tubercles are fused to form short ridges. On top of the various ridges, the pustules are larger and aligned in more orderly rows than in adjacent regions.
Similar to Chroniosaurus , the dorsal face of the skull roof bears by the following paired ridges ( Fig. 3A View Figure 3 ): (1) a parasagittal ridge connecting the premaxilla with the prefrontal, continuing around the medial and posterior margin of the orbit, and finally connecting the postfrontal with the supratemporal and tabular; (2) an oblique ridge emerging from the parasagittal ridge in the prefrontal region to run anterolaterally towards the lacrimal where it forms the medial margin of the preorbital fenestra and there continues up to the posterior margin of the naris; (3) a marginal ridge running along the maxilla and anterior jugal, culminating in a larger boss just lateral to the orbit. Smaller and less raised crests are found along the squamosal-jugal border and on the posterior skull table lateral to the pineal foramen, and in the postparietal region. As in other chroniosuchians, lateral line sulci are absent.
Dentition: A few maxillary teeth are exposed on the left side. These are very small, of similar size, and closely set ( Fig. 3D View Figure 3 ). They are conical, pointed, not labyrinthodont, and slightly curved medially. Tusks are not present; instead the palatine and ectopterygoid are covered with small teeth. These are not closely set and do not form a shagreen like in many other basal tetrapods. In addition, the palatine bears a single series of teeth that are similar to the maxillary dentition; two of these palatine teeth are exposed on the right side of the skull, and on the left side there are traces of similar teeth.
Dorsal side of skull roof: In general, the sutures are difficult to trace because of the heavy ornamentation and the generally high level of ossification ( Figs 2 View Figure 2 , 3 View Figure 3 ). The interpretation of sutures given in Figure 2 View Figure 2 is derived from examination of the original using a binocular microscope. The elongate premaxillae together form the broad parabolic anterior end of the snout, and on each side form the anterior half of the narial margin. They form most of the lateral and medial margins of the nares. Whereas the suture with the maxilla is narrow and slightly concave, that with the nasal is serrated and posteriorly convex. In the raised central region of the premaxillae, the pustules are larger and more prominent. There is no trace of an inter-premaxillary fontanelle whatsoever. The nares are much longer than wide, but their outline is unclear, as it differs on both sides, which may have resulted from crushing; they are bordered by the premaxillae, nasals, lacrimals, and maxillae. The rectangular nasals are 3.5 times as long as wide. The parasagittal ridges at their lateral margins and a lower, transverse ridge in their posteriormost part delimit a rectangular depression that is subdivided by a shallow and poorly defined sagittal elevation along the midline suture. The lacrimals are nearly as long as the nasals and posteriorly frame the preorbital fenestrae. The frontals are relatively short and asymmetrical, the right one being longer and reaching further anteriorly than the left. Posteriorly, they are laterally constricted by the postfrontals. The prefrontals border the posteromedial part of the preorbital fenestra and form a broad connection with the jugal anterior to the orbit. Because of this contact and the extension of the preorbital fenestra, the lacrimal is widely separated from the orbit and meets the jugal only in a narrow suture at the anterior end of the preorbital fenestra. The prefrontal has a pronounced relief, ranging from the deepest region of the skull roof (the posterior end of the preorbital fenestra) to one of the most elevated regions anterior to the orbit. The postfrontals are sutured to the prefrontals and relatively large. The postfrontal constricts the posterior end of the frontal and the anterior region of the parietal. A posteromedial projection of the postfrontal holds a place occupied by the intertemporal in many lower tetrapods such as anthracosaurs and seymouriamorphs. As in all chroniosuchids, the intertemporal is absent in Madygenerpeton , and the postfrontal contacts a short supratemporal. The tabular and parietal are both longer than the supratemporal, and there is a narrow suture between the tabular and parietal. The postorbital is very narrow with a triangular highly raised portion bordering the orbit and a deep posterior portion wedging in between squamosal and supratemporal. The posterior margin of the skull table is further apomorphic in bearing large posterior projections of both postparietal and tabular (blunt tabular horns), which are separated by a deep notch or postparietal embayment. The squamosal borders a shallow, semilunar squamosal embayment and just about reaches the posterior margin of the postparietal, indicating that the quadrate and occipital condyles were at the same level.
Internal side of skull roof: The ventral side of the skull roof is well exposed as a result of the absence of the medial palate elements in the holotype specimen ( Fig. 2D View Figure 2 ). Most of the skull roofing bones (premaxillae, nasals, prefrontals, postfrontals, postorbitals, squamosals) are smooth on their internal side, but the parietals and postparietals are roughened, indicating attachment of cartilaginous parts of the otic capsules and braincase. The premaxillae and nasals are entirely flat, whereas the frontals are convex anteriorly and slightly concave posteriorly. The concave region continues posteriorly onto the parietals, where it houses a transversely oval depression for the pineal foramen. The depression ends in the posterior part of the parietals, where the bone surface is covered in numerous scars aligned posteromedially. This roughened region continues into a medial crest that separates deep, paired pockets of the postparietals. In contrast, the tabulars form ventral projections merging into a thin descending lamella of the squamosal. The prefrontal bears an oblique, anterolaterally aligned crest, bordered by a deep trough on the medial side.
Palate: The palate is not as well preserved as the skull roof and is incomplete: the vomers, parasphenoid, and pterygoids are lacking (probably they had fallen off together prior to burial), whereas the palatines and ectopterygoids are still in place ( Fig. 2D, E View Figure 2 ). The braincase is also absent, but it is unclear whether it was still unossified or became disarticulated and lost. This is unfortunate, because the absent regions were much highlighted by Clack & Klembara (2009) for their resemblance with anthracosaurs. The large flat palatine bears medial shelves for the overlap of the pterygoid, and numerous widely spaced, tiny teeth. At about the midpoint of the preorbital region, the palatine bears a thin and pointed posteromedial process that probably underlapped the anterior end of the pterygoid; the vomerine region is poorly preserved, but there is no trace of the vomer left. The space left for the vomer is very narrow, which is consistent with the slender outline of this element in other chroniosuchids ( Ivakhnenko & Tverdokhlebova, 1980). The palatine and ectopterygoid are much wider than in Ch. paradoxus , and in contrast to the latter they have a broad contact in Madygenerpeton . Although the palate of Chroniosaurus is still partially unknown, Tverdokhlebova (1972) figured rather narrow pterygoids that would leave broad space for the palatines and ectopterygoids; future description of the palate in that genus will add important information. The palatine and ectopterygoid are of about equal length. The ectopterygoid has a pointed posterior end, with the maxillar and pterygoid sutures converging at about the mid-level of the orbit. The choana is still obscured by sediment, which could not be removed without damaging skull roofing bones in this delicate region. Judging from the snout margin, the space for the choana is much wider than in other chroniosuchids ( Fig. 3C View Figure 3 ).
Mandible: The posterior third of the mandible is preserved on the left side of the skull, dislocated and attached to the cheek ( Fig. 2D View Figure 2 ). The only identifiable element is the surangular, exposing the ventral and part of the external (labial) side which bears heavy pustular ornamentation. The ventral side of the surangular is markedly thickened, the internal (lingual) side entirely smooth. The outline of the surangular is very similar to that of Ch. paradoxus ( Ivakhnenko & Tverdokhlebova, 1980) , suggesting that the mandible was substantially deeper in the posterior part than anteriorly.
Osteoderms: The dorsal osteoderms are transversally elongate and of ‘chroniosuchian-type’ sensu Golubev (1998a): They display pairs of peg-like anterior and posterior articulation processes, sequentially overlapping outer wings with posterior dorsal and anterior ventral articulation faces bearing concentric ridges and furrows, and a P- shaped ventral process that enclosed the spine of a respective vertebra ( Fig. 4 View Figure 4 ). The dorsal surface bears parasagittal ridges and pustules ( Fig. 4A View Figure 4 ).
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