Paleontological Journal, Vol. 32, No. 3, 1998, pp. 278–287. Translated from Paleontologicheskii Zhurnal, No. 3, 1998, pp. 64–73. Original Russian Text Copyright © 1998 by Golubev. English Translation Copyright © 1998 by åÄàä ç‡Û͇ /Interperiodica Publishing (Russia). Narrow-armored Chroniosuchians (Amphibia, Anthracosauromorpha) from the Late Permian of Eastern Europe V. K. Golubev Paleontological Institute, Russian Academy of Sciences, ul. Profsoyuznaya 123, Moscow, 117647 Russia Received January 21, 1997 Abstract—The Permian and Triassic chroniosuchians are revised and the morphology of the dorsal armor scutes is discussed in detail. The first narrow-armored chroniosuchid Uralerpeton tverdochlebovae from the Late Permian Vyazniki faunistic assemblage of Eastern Europe is described and the age of the Vyazniki fauna discussed. INTRODUCTION armor occurred late in the process of decomposition of the animal: the head, the brachial and pelvic girdles and even The chroniosuchians (Chroniosuchia), an unusual the vertebral column became disarticulated sooner. group of Late Permian and Triassic reptiliomorph amphibians, dominated the aquatic tetrapod assem- In horizontal plane the chroniosuchian scutes are blages of Eastern Europe during the Late Tatarian age. rectangular (Figs. 1a and 1b; 2b and 2c; 4a–4d), except Conventionally this group, consisting of two families: for the anteriormost scute, which is shaped like a semi- the Chroniosuchidae and the Bystrowianidae (Tatar- circle or semiellipsis (Fig. 1d). The scute consists of a inov, 1972; Ivakhnenko and Tverdokhlebova, 1980; massive axial part, or the scute body (corpus scutulumi) Shishkin and Novikov, 1992) is included within the and two lateral horizontal plates, or the scute wings order Anthracosauromorpha as suborder. According to (alae scutulumi) (Fig. 1a). The wings may be wide (the the latest revision of the Permian chroniosuchians (Iva- width usually exceeds the length by no more than khnenko and Tverdokhlebova, 1980), the family Chro- twice) or narrow (length exceeds width), while in some niosuchidae includes the genera Chroniosaurus and forms they may be absent.1 The scute dorsal surface is Chroniosuchus, and the family Bystrowianidae com- sculptured with tubercles, crests or pits-and-ridges. The prises the genera Jugosuchus and Bystrowiana. The ventral surface of the wings of the scute is mainly Lower Triassic genus Axitectum was later referred to smooth and bears numerous small foramina. These the bystrowianids (Shishkin and Novikov, 1992). These foramina give rise to poorly expressed and rapidly flat- authors noted in passing, that according to M. F. Ivakh- tening grooves on the lateral surface, impressions of, nenko and in their view, the genus Jugosuchus should blood vessels. The lock for the articulation of the neigh- be referred to the Chroniosuchidae (Shishkin and boring scutes is a complicated structure located on the Novikov, 1992). Significant new material on the Per- ventral side of the body of the scute. Two main types of mian and Triassic chroniosuchians enables a revision of scute are distinguished by the structure of the lock and this group. articulation: the chroniosuchian and the bystrowianian. The chroniosuchian type. This type includes the DESCRIPTIONS Chroniosuchus, Jugosuchus and Chroniosaurus scutes. The members of these genera stand out in the wide One of specific chroniosuchian features is the armor armor which covered practically the whole dorsal part covering the body and the anterodorsal part of the ani- of the trunk. The trunk scutes bear well expressed wide mal’s tail. The armor consists of separate scutes wings, the caudal scutes narrow greatly posteriorly, and arranged in a row along the vertebral column. Their the posteriormost lack wings. A long strip that is devoid number strictly corresponds to the number of vertebrae of sculpture extends approximately a third of the whole in the region of the armour. In Chroniosaurus don- wing length along the posterodorsal edge and repre- gusensis the armor consists of 30 scutes, including sents the overlap area for the next wing of the scute. those of the cervical, trunk and caudal regions (speci- men PIN, no. 3713/54). The scutes were not fused to Thus, the wings of the neighboring scutes overlap each each other, but instead were effectively connected by 1 Here and further on the length is a measurement taken parallel to ligaments. Completely preserved fossilized armor and the median line, width and depth taken normal to the median line isolated fragments indicate that the disintegration of the in the horizontal and vertical planes correspondingly. 278 NARROW-ARMORED CHRONIOSUCHIANS 279 ala corpus ala scutulumi scutulumi scutulumi paa csvi pv 1 cm (a) (c) csve pap pv 1 cm (b) (d) 1 cm Fig. 1. Chroniosaurus: (a) C. dongusensis trunk scute ventrally, a reconstruction based upon the specimen PIN, no. 3713/1; (b) the trunk scute: dotted zones indicate overlap areas; (c) C. boreus trunk vertebra in left lateral view, specimen SGU, no. 104V/1145; (d) the first cervical scute of C. dongusensis ventrally, specimen PIN, no. 3713/39. Designations: csve—crista scutulumi ventralis externa, csvi—crista scutulumi ventralis interna, paa—processus articularis anterior, pap—processus articularis posterior, pv—pro- cessus ventralis. other posteroanteriorly (specimens PIN, nos. 2005/2579; line is not explicit for a species. All four possible com- 3585/104; 3713/54, 55). binations are known in Chroniosaurus dongusensis (specimens PIN nos. 3713/1 and 3713/37), with differ- The descending ventral process (processus ventra- ent variations of the contact of the ventral processes lis) forms the medial part of the lock; the former partic- even within the same individual armor (specimen PIN, ipates in the articulation of the scute with the neural no. 3713/37). The ventral process in the anteriormost spine (Figs. 1a and 1c). It is a vertical plate strongly scute does not reach its anterior edge and its anterior flattened laterally and its length exceeds its width 8–10 impressio processus ventralis is not expressed (Fig. 1d). times. In the horizontal section it is lens-shaped with pointed anterior and posterior corners. The process The ventral process attains its maximum depth ante- length notably exceeds the length of the body of the riorly and posteriorly, thus becoming arch-shaped in the scute, hence the anterior and posterior parts of the pro- sagittal plane (Fig. 1c). The neural spine enters a deep cess significantly stick out anteriorly and posteriorly in notch formed in the center. The anterior and posterior relation to the corresponding edges of the body of the edges of the neural spine bear deep furrows, which scute. As a result, the ventral processes of the neighbor- housed correspondingly the anterior and posterior parts ing scutes contact each other laterally in articulation. of the ventral process. The scutes do not usually fuse The length of the overlapping surface constitutes from with the vertebrae. As a result, the neural spine is charac- one fourth to one third of the process length. The lateral teristically triangle-shaped when seen laterally. areas of the ventral process anteriorly and posteriorly The anterior and posterior articulation processes bear a well expressed depression (impressio processus (processus articulares anteriores and processus articu- ventralis). The Impressio processus ventralis is a point lares posteriores) are situated lateral to the ventral pro- of contact for the ventral process of the neighboring cess (Fig. 1a). The articulation process is always sepa- scute. The depression position in relation to the median rated anteriorly by a notch from the ventral process PALEONTOLOGICAL JOURNAL Vol. 32 No. 3 1998 280 GOLUBEV from the impressio processus ventralis side. In contrast, ever, similar morphological elements (Fig. 2b and 2c) from the opposite side of the impressio processus ven- may be allocated here. tralis in relation to the median line both processes usu- In Bystrowiana the scute wings overlap each other ally fuse dorsally at a right angle. A clearly expressed anteroposteriorly, hence the overlap area lacking sculp- crest, the crista scutulumi ventralis interna, runs pos- ture is situated along the anterior edge of the dorsal sur- teromedially from the anterior edge at the ventral sur- face of the scute wing (specimen PIN, no. 1100/1, face of the anterior articulation process, which turns 1100/18) (Fig. 2b). The ventral process occupies the caudally into the lateral edge of the posterior articula- posteromedial part of the ventral surface of the body of tion process. A deep furrow gradually becoming more the scute. Its length along the median line is over twice shallow posteriorly, which housed the posterior articu- less than the total length of the scute. In the front sec- lation processes of the preceding scute, is placed tion the process is elliptic, the long medial axis of medial to this crest. Laterally the processus articularis which is 1.5–3 times shorter than the short scute. The anterior is separated from the scute wing by a cleft, anterior and posterior edges of the process are rounded which deepens medially, closes and becomes shallow and never extend behind the corresponding scute edges. posteriorly. This cleft housed the posteromedial edge of The ventral processes of the neighboring scutes never the preceding scute wing. In the anteriormost scute the meet each other in articulation. Accordingly, the lateral anterior articulation processes and related morphologi- depressions on the ventral processes characteristic of cal structures are absent (Fig. 1d). the chroniosuchian type scutes, or the impressio pro- The posteromedial part of the scute wing ventral cessus ventralis, are absent in Bystrowiana and Axitec- surface frequently bears a low crest, the crista scutu- tum. The ventral scute process always fuses to the neu- lumi ventralis externa. It usually starts from the poste- ral spine forming an interdigitating suture and the ven- riormost edge of the wing and runs anteromedially, tral process appears as if mounted on the neural spine describing an arc, slightly convex externally. In front from above (Fig. 2d).