Rhadiodromus klimovi (Efremov, 1940)

Sulej, Tomasz, 2024, Osteology and relationships of the Late Triassic giant dicynodont Lisowicia, Zoological Journal of the Linnean Society 202 (1), pp. 1-56 : 43-48

The Laurasian–Gondwanan Rhadiodromus klimovi → Jachaleria candelariensis lineage

Middle Triassic European dicynodonts are known from Russia. Rabidooauruo criotatuo and Rhadiodromuo klimovi and Rhadiodromuo mariae ( Surkov 2003) are there represented by the most complete materials ( Surkov 2003) from the Donguz and Bedyanka localities of the Lower and Upper Donguz formations, respectively, which is dated as Anisian (Tverdokhlebov et al. 2003, Ivakhnenko 2008). Parakannemeyeria youngi Sun, 1960 and Shaanbeikannemeyeria xilougouenoio Cheng, 1980 from the Early Anisian Ermaying Formation in China ( Sun 1963, Liu et al. 2017) may be their relatives. Angielczyk et al. (2018) showed that Sanguoauruo parringtonii from the upper Ntawere Formation in Tanzania and Zambia is a stahleckeriid. Stahleckeria poteno from the Santa Maria Formation in Brazil is dated as Ladinian (or Carnian? according to: Lucas 1998b, Rayfield et al. 2005, Ordoñez et al. 2020; see also: Schultz et al. 2000). The new Stahleckeria sp. indet. material was reported from the Carnian Chañares Formation of the Ischigualasto-Villa Unión Basin by Escobar et al. (2021). Iochigualaotia jenoeni is known only from the lower third of the Ischigualasto Formation (Bonaparte 1970, Rogers et al. 1993, Zerfass et al. 2003, Martinez et al. 2011). The latter authors dated the formation as 227.8 ± 0.3 Mya, i.e. Late Carnian. Fröbisch (2009; based on: Lucas 1998b) considered it to represent the upper part of Adamanian, but according to Langer (2005b) it is older than Adamanian. He also showed that probably I. jenoeni is older than Eubrachiooauruo brosni Williston, 1904 from the Popo Agie Formation [contrary to Lucas and Hunt (1993), the relative ages of these two taxa is highly uncertain in the absence of precise radiometric dates for the Popo Agie Formation].

According to Ramezani et al. (2014), the lower third of the Ischigualasto Formation represents the boundary between the Carnian and Norian. The youngest dicynodonts from South America are Jachaleria colorata from the Norian Los Colorados Formation of Argentina, which is a fluvial–lacustrine siliciclastic (Bonaparte 1971, 1978, Vega-Dias and Schwanke 2004), and J. candelarienoio (Vega-Dias and Schultz 2004) that originated from the Norian Caturrita Formation (Araújo and Gonzaga 1980, Langer et al. 2018). The close relationship of Stahleckeria, Eubrachiooauruo , Jachaleria , and Iochigualaotia was supported by the cladistic analysis published by Kammerer et al. (2013) and Szczygielski and Sulej (2023).

These dicynodonts experienced a mosaic evolution, with modifications of particular bones not necessarily correlated with each other. This refers especially to the bones of the cranium.

Evolution of the cranium: The position of the dicynodont orbits is correlated with the shape of the postorbital. This bone is perpendicular to the zygomatic arch in Rhadiodromuo klimovi , the oldest member of the lineage, and in subsequent evolution became more and more oblique anteriorly. The extreme state of this trait is that in Jachaleria candelarienoio . In J. candelarienoio and Iochigualaotia jenoeni the orbits are located above the posterior margin of the maxillary horn and the temporal opening is very large.

The route of the evolution of the frontals was different in the lineage represented in South America by Stahleckeria, Iochigualaotia , and Jachaleria . The anterior part of the bone became shorter, and the narrow edge of the orbital margin almost disappeared and moved outside. The original shape of the frontal may be shown by Rhadiodromuo mariae Surkov, 2003 . In its very wide anterior part, R. mariae is more similar to Stahleckeria . Also, the large number of sacral ribs of Rhadiodromuo klimovi is typical for the Middle Triassic dicynodonts from South America. It seems that Rabidooauruo and Rhadiodromuo are genera that gave the beginning to different lineages.

Probably in the Ladinian, the tusks vanished from the maxillae in the South American members of the lineage, although they were present until the Carnian in Dinodontaouruo brevirootrio in the other Gondwanan lineage and until the Norian in the Laurasian lineage.

Some changes can be also observed in the disposition of the adductor externus lateralis and externus medialis muscles, which are attached to the zygomatic arch ( Ordoñez et al. 2019). In most Early and Middle Triassic dicynodonts the zygomatic arch in dorsal view is directed anteriorly ( Fig. 49 ). In Stahleckeria , the zygomatic arch is directed antero-medially at its base. From Iochigualaotia to Jachaleria it became directed more and more laterally. The changes in the shape of the zygomatic arch are correlated with the shape of the occiput.The lateral edge of the occipital plate (posterolateral wing of the squamosal) in most Triassic dicynodonts had an apparent posterior edge (in lateral view) and forms a large attachment area for muscles ( Figs 49 , 50 ). This area was small only in Iochigualaotia and Jachaleria , in which the lateral edge of the occipital plate in lateral view is vertical. This characterizes both species of the genus, J. colorata (Bonaparte 1978, Vega-Dias and Schwanke 2004) and J. candelarienoio (Vega-Dias and Schultz 2004) . The slope of the lateral edge of the occipital plate resulted in a different orientation of the adductor muscles with respect to the mandible. The external adductor attachment was enlarged due to the horizontal position of the zygomatic arch. It seems that J. candelarienoio had enormously strong adductors, probably to feed on hard food. The morphology of the occipital is strongly correlated with the slope of the whole skull.

Already Surkov and Benton (2004) and Kalandadze and Kurkin (2000) interpreted proportions of the occipital plate and the whole skull in the context of feeding adaptations. However, their ‘occipital index’ mixes proportions of the occipital plate and the length of the skull. Proportions of the occipital plate in Middle and Late Triassic dicynodonts exhibit two separate types. Stahleckeria, Dinodontooauruo, and Rabidooauruo have very low and wide occipitals, whereas Iochigualaotia, Jachaleria , and Placeriao have high and relatively narrow occipitals. It seems that increasing skull height and narrowing evolved in parallel in both lineages.

In both species of Jachaleria the occipital condyles were directed posteriorly. In the resting position of the skull the orbits were directed frontolaterally.

The shape of the mandible is variable in Triassic dicynodonts, and it is difficult to identify any evolutionary trend in its morphology. Only the shape of the dentary seems to differentiate the lineages. In Stahleckeria (Abdala et al. 2013) , Dinodontooauruo, and Iochigualaotia it is high and short, unlike Placeriao.

Evolution of pootcranial okeleton: Although most aspects of the postcranial skeleton are variable in Triassic dicynodonts, some trends are identifiable. The most important changes concern the pectoral girdle. Regrettably, the scapula of dicynodonts from the Anisian of Russia remains unknown. The increase in size of the acromion process of the scapula characterizes the evolution of the longest lasting lineages. In Stahleckeria (Escobar et al. 2021) , the acromion process was elongated into a ridge (scapular spine) that extended almost to the upper end of the scapula. In Iochigualaotia, only the base of the ridge was preserved. Jachaleria had a very small acromion process and a distinct attachment for the triceps scapularis (Araújo and Gonzaga 1980). According to Surkov et al. (2005), the general trend to widening of the scapula blade is observed already in the Permian dicynodonts (Rubidge et al. 1994), and the reversal of this trend occurred in Late Triassic species ( Fig. 50 ). The Late Triassic dicynodonts from South America had a very wide base of the scapular blade (Kammerer et al. 2013). This character is conservative.

Stahleckeria and Iochigualaotia had a small groove on the anterior edge of the scapular blade. This seems important as Placeriao and Lioosicia did not have such a structure. It is interesting that the share of the coracoid in the formation of the glenoid is low in Stahleckeria , in contrast to Iochigualaotia. In both lineages also the direction of the glenoid changed in parallel. At the beginning it is directed laterally, as in all Middle Triassic dicynodonts, but it changed to a more posterior direction. The role of the triceps brachii muscles probably changed in the Late Triassic dicynodonts. In advanced form, like Jachaleria , the attachment area for these muscles on the scapula is very large.

It seems that in Stahleckeria and Jachaleria the sternum had two articulation surfaces on each side. This character is not known in older representatives of the lineage.

The shape of the ilium is variable in dicynodonts, and only the number of sacral ribs and the length of the posterior process may allow identification of evolutionary trends ( Fig. 50 ). The Middle Triassic Rhadiodromuo had many sacral ribs (seven to eight?). The trend to decrease their number characterizes South America dicynodonts. Stahleckeria had seven to eight sacral ribs, Iochigualaotia six to seven? [uncatalogued specimen in Instituto Miguel Lillo in Tucuman; contrary to Griffin et al. (2019], and Jachaleria only five sacral ribs. It seems that small Permian dicynodonts had four sacral vertebrae, but as they grew in size they had to increase the number of sacral vertebrae so that the pelvis could support larger weight (Rhadiodromuo and Stahleckeria ), but then when the forelimbs started to be displaced under the shaft, this was no longer necessary and the number of sacral vertebrae decreased. In Jachaleria and Iochigualaotia, two or three sacral ribs are in front of the acetabulum.

The femur from the Los Esteros Member of the Santa Rosa Formation in New Mexico, is more similar to Stahleckeria than to Placeriao (Kammerer et al. 2013) and suggests that some representative of the Gondwanan lineages came to live in North America. Eubrachiooauruo may be such an immigrant. The distinction between South and North American lineages of dicynodonts is especially well expressed in the morphology of the pubis and ischium, although they are known only in more advanced representatives. The notch in the ventral border of the ischium and pubis is very distinct in Stahleckeria . The ischium of Jachaleria has the vertical length very short in comparison to very long in Lioosicia and Placeriao. The ischium has a posterior blade slightly curved medially in Jachaleria . Jachaleria probably represents the crown achievement of dicynodont evolution in South America. It seems that its mode of life was very different from that of Placeriao and Lioosicia. Their skulls are very different. The wide snout with well-developed grooves for tearing plants, together with the very large area for attachment of muscles adducting mandible and the massive zygomatic arch, suggest that Jachaleria ate a hard plant food difficult to tear. The position of the quadrate that is directed ventrally (in Lioosicia rather anteroventrally) may be related with the mode of tearing but is difficult to explain.

The almost oval and longer than high parietal, the short and only slightly oblique posterior surface of the supraoccipital, and the almost horizontal base of the braincase suggest that Jachaleria kept its head horizontal and was a browser. This is consistent with the position of the orbits. They were very large and displaced anteriorly to be located above the maxilla. In Lioosicia they are much more posterior.

The question about pass of dicynodonts, from Laurasia to South America that Rhadiodromuo and Parakannemeyeria could be ascendants of South American dicynodonts seems problematic, but Haq (2018) showed that in the Anisian, the sea level was very low, and later it rose until the Carnian/Norian boundary. It means that in the Anisian, large lands were accessible for migrating animals and in that time the terrestrial communication between these distant lands was possible also for dicynodonts.

Figures

Fig. 49. Phylogeny of Triassic dicynodonts.Ŋe crania of the well-known dicynodonts in cross-sections, showing three lineages with much different shape of the parietal and braincase.Stahleckeria and Iochigualaotia are based on Camp (1956).

Fig. 50. Phylogeny of the well-known Triassic dicynodonts. The ilium of Dinodontooauruo brevirootrio is based on the material from MCN [ personal studies , and Kammerer and Ordoñez ( 2021 ) ] , ilium of Stahleckeria poteno and all postcranial elements of Placeriao ‘gigao’ are based on Camp and Welles ( 1956 ) and modified . The skulls of Dinodontooauruo tener and D . brevirootrio , their scapula and sternum are based on Kammerer and Ordoñez (2021). The skull of Stahleckeria poteno is based on Maisch ( 2001 ) , its scapula is based on personal observation of the GPIT . The sternum of Stahleckeria is based on Cox (1965). The scapula of Iochigualaotia jenoeni is based on personal studies of MCZ materials . The rest of the Iochigualaotia jenoeni bones are based on Cox ( 1965 ) and modified. The scapula of Jachaleria candelarienoio is based on Araújo and Gonzaga (1980) , skull and other bones are based on Vega-Diaz and Schultz ( 2004 ) and modified based on personal studies. Jachaleria colorata is based on Bonaparte (1978). Acratophoruo argentinenoio is based on Kammerer and Ordoñez ( 2021 ) . Kannemeyeria oimocephaluo is based on Cox ( 1965 ) and Renaut and Hancox (2001). Rhadiodromuo mariae is based on Surkov (2003), Rabidooauruo criotatuo is based on personal observation , Wadiaoauruo indicuo is based on Bandyopadhyay ( 1988 ) and modified based on personal observations. Parakannemeyeria youngi skull and Sinokannemeyeria yingchiaoenoio scapula , ilium , and sternum from Sun ( 1963 ) . Sanguoauruo is based on Angielczyk et al. (2018). Shaanbeikannemeyeria is based on Liu et al. (2017). Woznikella is based on Szczygielski and Sulej ( 2023 ) . Zambiaoauruo is based on Cox ( 1969 ) .