Suminia, IVAKHNENKO, 1994

Fröbisch, Jörg & Reisz, Robert R., 2011, The postcranial anatomy of Suminia getmanovi (Synapsida: Anomodontia), the earliest known arboreal tetrapod, Zoological Journal of the Linnean Society 162 (3), pp. 661-698 : 664-677

SUMINIA IVAKHNENKO, 1994

Type species

Suminia getmanovi Ivakhnenko, 1994 .

Diagnosis

As for species.

Suminia getmanovi Ivakhnenko, 1994

Holotype

PIN 2212 /10, a partial skeleton with skull and lower jaws.

Type locality

Kotel’nich locality in the Kirov region of Russia. The Suminia -bearing horizon is located at the base of the Kotel’nich sequence, and consists of red mudstones, representing a floodplain environment.

Referred specimens

KPM 173, partial skull and anterior part of the skeleton; PIN 2212/62, partial postcranium with skull that formed the basis for the cranial redescription ( Rybczynski, 2000); PIN 2212/102, partial postcranium with posterior dorsal, sacral, and anterior caudal regions of the vertebral column, with parts of the pelvic girdle and hindlimbs; PIN 2212/103a, partial pes (together with PIN 2212/103b, part of specimen 2 on the large block PIN 2212/116); PIN 2212/103b, partial tibia and fibula, as well as astragalus and calcaneum (together with PIN 2212/103a, part of specimen 2 on the large block PIN 2212/116); PIN 2212/104a, partial postcranium with wellpreserved cervical series in dorsal view, anterior dorsal region and partial shoulder girdle, humerus, radius, ulna, and manus; PIN 2212/104b, partial postcranium, including dorsal region with vertebrae, ribs, and partial humerus, radius, and ulna; PIN 2212/116, large block (80 cm by 120 cm) with more than 15 mainly articulated skeletons. The specimens listed above form the basis for the postcranial description of Suminia .

Distribution

Suminia is primarily known from the Kotel’nich locality in the Kirov region of Russia. However, some isolated specimens have been reported from a number of additional localities, all from the eastern European region of Russia. These include Suminia cf. getmanovi from Babentzevo ( Ivakhnenko et al., 1997; Tverdokhlebov et al., 2005), Suminia sp. from Koptyazhevo-1 ( Tverdokhlebov et al., 2005), Suminia cf. getmanovi from Poteryakha-2 ( Ivakhnenko, 2003), Suminia cf. getmanovi from Ust’e Strel’ny ( Ivakhnenko, 2003), and Suminia cf. getmanovi from Navoloki ( Ivakhnenko et al., 1997).

Stratigraphic range

At Kotel’nich, Suminia is known mainly from the base of the sequence, which also represents the base of the Kotel’nich Faunal Subassemblage. The remaining localities are part of the slightly younger Chroniosaurus dongusensis tetrapod subzone, which forms the lower part of the Ilinskoe Subassemblage ( Golubev, 2005). The combined occurrences of Suminia fall within the Upper Sverodvinian Gorizont (horizon) of the Late Permian (Tatarian). The Suminia -bearing strata are roughly correlated with the upper Pristerognathus and Tropidostoma assemblage zones in the South African Karoo Basin, which corresponds to the early Late Permian (Wuchiapingian) on the Standard Global Chronostratigraphic Scale.

Revised diagnosis

Small venyukovioid characterized by the following autapomorphies: large orbit, nearly one-third of skull length; frontal–parietal suture occurring at rim of pineal foramen; postorbital–squamosal contact present ventrally (convergent with Chainosauria ); teeth large relative to skull; wear facets nearly parasagittal; tooth wear striations indicate propalinal motion of mandible that extends each dentary tooth across at least two maxillary teeth; reduced number of 23 presacral and therein 17 dorsal vertebrae; centra of cervical vertebrae longer than wide, indicating an elongate neck; transverse processes of caudal vertebrae fuse to long caudal ribs; procoracoid with indentation at ventromedial margin rather than foramen; iliac blade with robust ridge on anterior edge of medial surface; pubis with puboischiadic fenestra and separate pubic foramen; limbs elongated; manus equals 40% of forelimb length; distal carpal 1 and tarsal 1 enlarged, phalangiform, and diverge laterally; metacarpal I short and robust; crescent-shaped distal tarsal 4; penultimate phalanges elongated; manual phalangeal formula 2-3-4-5-3 and pedal phalangeal formula 2-3-4-5-4, with discshaped phalangeal elements in digits III and IV (manus and pes) and digit V (pes only).

Remarks

No information is currently available on the postcranial anatomy of the other venyukovioids ( Otsheria , Ulemica , and Venyukovia ). Neither are there any postcranial remains known for the most basal anomodonts, Biseridens or Anomocephalus . Therefore, only more complete specimens of these basal anomodonts will allow us to test whether the postcranial features of Suminia listed in the diagnosis represent true autapomorphies, or if they are characteristic for a larger clade.

DESCRIPTION AND COMPARISON

AXIAL SKELETON

General aspects and vertebral count

The vertebral column of Suminia comprises 23 presacral vertebrae with six cervicals and 17 dorsals, as well as three sacral vertebrae, and at least 52 caudal vertebrae, resulting in a minimal number of 78 vertebrae. The vertebrae of Suminia are exclusively amphicoelous.

Atlas–axis complex

Elements of the atlas–axis complex of Suminia are preserved in a number of specimens (i.e. specimens 1 and 3 on blocks PIN 2212/116, PIN 2212/62, PIN 2212/104a, and KPM 173), although none of the specimens preserves the complex either in its entirety or in articulation. As in other non-mammalian synapsids ( Kemp, 1969; Reisz, Scott & van Bendegem, 1992), the atlas–axis complex of Suminia was probably composed of a paired proatlas, a paired atlantal neural arch, an atlantal intercentrum and pleurocentrum, an axial intercentrum, and a fused axial pleurocentrum and neural arch. However, the various elements anterior to the axis proper, i.e. the fused axial pleurocentrum and neural arch, are disarticulated and difficult to identify in the specimens available.

The usually paired elements of the proatlas are not preserved or identifiable in any of the studied specimens of Suminia . The paired atlantal neural arches are best preserved in PIN 2212/104a, showing them in dorsal view ( Fig. 3A ), and in KPM 173, displaying the left neural arch in lateral view. Each element consists of a median portion that covered the neural canal, and extends anteroposteriorly. Posteriorly, it flares out and becomes more robust to form the postzygapophyseal process. At its anterior end, the neural arch is smoothly rounded and curves laterally to give rise to a prominent transverse process that projects posterolaterally and ventrally. An atlantal intercentrum could not be identified with certainty. The atlantal pleurocentrum is identifiable in PIN 2212/62, as it is the only specimen that shows this element in articulation ( Fig. 3B ). In fact, the specimen shows this element in lateral aspect just slightly shifted ventrally from its original position, in articulation with the adjacent cervical series. However, it is uncertain whether the preserved element represents the atlantal pleurocentrum only, or whether it is fused with the axial intercentrum. In Galeops , which preserves the only atlas–axis complex of basal anomodonts, the atlantal pleurocentrum is a prominent element with the general morphology of any vertebral centrum, but it does not seem to extend to the ventral edge of the vertebral column ( Brinkman, 1981). Instead, a posteroventral articular surface at the pleurocentrum of Galeops indicates the presence of an isolated axial intercentrum. The latter, however, cannot be identified in Suminia , which suggests that the axial intercentrum might be fused to the atlantal pleurocentrum. The lateral side of this potentially fused element is slightly concave. In contrast, the atlas–axis complex of dicynodonts indicates the presence of an odontoid, consisting of a fused atlantal pleurocentrum and axial intercentrum that are in turn fused to the anterior part of the axial pleurocentrum (e.g. Cox, 1959; Kemp, 1969; Cluver, 1971; King, 1981a).

The morphology of the fused axial pleurocentrum and neural arch is typical for amniotes in displaying an enlarged neural spine. Furthermore, the short and stout transverse process is located rather low, at the middle of the vertebral centrum, and projects laterally as well as ventrally. The prezygapophyses project anteriorly, but they are not as pronounced as in the adjacent cervical vertebrae because they connect to the posterior parts of the atlantal neural arches. The postzygapophyses of the axis are well developed, with their articular surfaces facing ventrolaterally.

Cervical vertebrae

Different criteria have been used in the literature to differentiate between cervical and dorsal vertebrae of non-mammalian synapsids (e.g. Romer, 1956; Cox, 1959). Identification of the first dorsal vertebra has previously been based on either the contact of the associated rib with the sternum, a distinct change in relative rib length, the presence of a distinct capitular facet (parapophysis) on the anterior rim of the centrum, an increased length of the diapophysis, modified angles of pre- and postzygapophyses, or a combination of the above. A combination of these approaches was also applied to distinguish cervicals and dorsals in Suminia , namely rib length as well as pre- and postzygapophyseal morphology (see below). In addition to the atlas and axis there are four adjacent cervical vertebrae, resulting in a total number of six ( Figs 2 , 3 ). The cervicals of Suminia are remarkably wide in dorsal view with distinct neural spines. The pre- and postzygapophyses are as wide as or even wider than the remaining body of the vertebra in dorsal view, giving the cervicals a subrectangular outline in dorsal view. Their orientation deviates only slightly from the horizontal. The postzygapophyses are bulbous and continuous, with slightly convex ventrolateral articulation surfaces, whereas the prezygapophyses reach far anteriorly and slightly laterally, with their articulation surfaces projecting dorsomedially. Thus, the articulation of pre- and postzygapophyses of adjacent cervicals permits movement of the neck in several directions, including dorsoventral and rotational flexibility about its longitudinal axis, but with a distinct emphasis on lateral movements. The transverse processes are relatively short and directed posterolaterally rather than ventrally as in the axis. Thus, the same change in orientation of the transverse process occurs in a posterior direction along the cervical series, as has been described for Galeops ( Brinkman, 1981) . Moreover, the position of the transverse processes shifted dorsally from an initial position at the middle of the vertebral centrum in the axis to a gradually more dorsal placement. In addition, the length of the transverse processes gradually increased in a posterior direction. The sixth vertebra of the vertebral column, which we regard as the last cervical, shows a distinct change in morphology in that it displays narrower postzygapophyses. Nonetheless, the associated ribs are still short and less robust than those of the dorsals. Little information is available on the morphological change of the intercentra and pleurocentra at the cervical–dorsal transition, as none of the specimens expose this region sufficiently. Therefore, it is also unclear whether Suminia had longitudinal midventral ridges on any of its cervicals, as has been reported for Galechirus and Patranomodon ( Brinkman, 1981; Rubidge & Hopson, 1996). However, it is apparent that the proportions of the cervical pleurocentra of Suminia are very distinct, in that they are much longer than wide. Similar proportions of the cervical pleurocentra have been described for the biarmosuchians Biarmosuchus , Hipposaurus , and Lobalopex ( Hopson, 1991; Sidor, Hopson & Keyser, 2004). However, this contrasts with the condition in any other basal or derived anomodont, where the cervical centra are at most as long as they are wide (e.g. King, 1981a; Rubidge & Hopson, 1996). Intercentra are preserved in the cervical region of some specimens. PIN 2212/62 shows a distinct but badly preserved element between the axial pleurocentrum and the centrum of the third cervical. In addition, an isolated, wedge-shaped element is preserved in the same position in specimen 3 on the large block (PIN 2212/116).

Dorsal vertebrae

There are 17 dorsal vertebrae in the vertebral column of Suminia . The dorsal centra are consistently longer than wide ( Figs 2 , 4 ), comparable with but even more pronounced than in Galepus and Patranomodon ( Brinkman, 1981; Rubidge & Hopson, 1996). The neural spines are not always preserved throughout the vertebral column of the specimens studied, but they seem to remain about equal in height. They arise from the posterior part of the neural arch, curve slightly backwards, and have a greater extension in the dorsal direction than anteroposteriorly. In some specimens there is an apparent lack of fusion between the centra and neural arches in the posterior dorsals. However, this does not appear to be related to size as some large individuals (e.g. specimens 2 and 3 on the block; PIN 2212/116) show this lack of fusion, whereas other smaller individuals display well co-ossified centra and neural arches in this region (e.g. specimen 1 of PIN 2212/116 and KPM 173). The transverse processes of the dorsal vertebrae consistently arise from the base of the neural arches and project mainly in a lateral direction. They are robust, dorsoventrally flattened protrusions that are squaredoff at their distal ends. The transverse processes are longer in the lateral direction than their extension in the anteroposterior direction throughout most of the dorsal series. This relation changes only in the posteriormost dorsals, where the transverse processes become shorter laterally. The pre- and postzygapophyses extend far anteriorly and posteriorly, respectively, and are set closer to the midline than in the cervical region. The prezygapophyses are slender projections that are separated by a U-shaped notch, whereas the postzygapophyses are divided by a V-shaped indentation.

Sacrum

There are three sacral vertebrae ( Figs 2 , 5 ). None of the specimens shows any indication of fusion between the sacral vertebrae, sacral ribs, and the pelvic girdle, but all display a clear disarticulation between the individual elements. Moreover, the same specimens that show the lack of fusion between the centra and neural arches in the posterior region of the dorsal series also display this pattern in the sacral vertebrae. The general morphology of the sacral vertebrae is very similar to that of the posterior dorsals, including prominent neural spines, short and robust transverse processes, and narrow pre- and postzygapophyses. A distinct feature that is restricted to the sacrals of Suminia is an extensive articular surface on each centrum for the associated sacral rib. The position of this articular surface is always at the dorsalmost edge of the lateral side of each centrum, stretching across its entire length, whereas the orientation of the facets changes from being posterodorsally inclined in the first sacral, to horizontal in the second sacral, to anterodorsally inclined in the third sacral.

Caudal vertebrae

The minimum estimate of the number of caudal vertebrae in Suminia is 52. The most informative caudal region of the vertebral column is preserved in specimen 2 on the block (PIN 2212/116), comprising a series of 34 caudals ( Fig. 6 ). This includes a continuous series of the 22 anteriormost caudals, a single caudal, and another series of 11 caudals that are apparently part of one individual, as they are only slightly shifted apart. A second specimen on the same block provides information about the posterior caudal region, consisting of an isolated continuous series of the last 30 caudal vertebrae that possibly belong to specimen 10 ( Fig. 1 ). Both specimens combined provide a near complete picture of the tail of Suminia , as they overlap in the middle part of the tail. The caudal vertebrae are distinct in morphology and proportions. They are significantly shorter than the vertebrae of the presacral and sacral regions, but their centra are still longer than wide, as in the remaining vertebral column. This is consistent with what is known from Galepus and Galechirus ( Brinkman, 1981) , but is in contrast to dicynodonts where all vertebrae, and in particular the caudals, are shorter than wide. In the anterior caudals the neural spine is still well developed, whereas it is diminished posteriorly until it is entirely lost in caudal 22. Similarly, the transverse processes are prominent in the anterior caudal vertebrae, and decrease in size posteriorly. In fact, the transverse processes are apparently fused to the ribs throughout most of the anterior portion of the tail, and cannot be distinguished. Neural arches are present throughout the entire caudal region, and also decrease in relative size. As the anterior caudal region is preserved in dorsal view only, haemal arches could not be identified with certainty, but it is most likely that they were present at least in the anterior caudals, as this is the case in Galepus and Galechirus .

Ribs

Cervical ribs could not be identified with certainty in any of the available specimens. However, a potential atlantal rib is present in KPM 173. It is a short, rod-like element that has a robust proximal end and thins out distally to result in an overall triangular shape. The same specimen also displays a rib that possibly belongs to its sixth (last) cervical vertebra. Ribs that most probably belong to the sixth cervical are also visible in specimen 3 on the block ( Fig. 4 ). These ribs are dichocephalous with a clear distinction between the capitulum and tuberculum, although the two heads are not as clearly separated, as is the case in the cervicals of dicynodonts (e.g. Cox, 1959). The shaft is long, slender, and slightly curved medially, but it does not reach the length of the anterior dorsal ribs.

The ribs of the dorsal region have the same overall morphology as the sixth cervical rib, but are distinctly longer and are likely to have contacted the cartilaginous sternum along the midline. The anterior dorsal ribs are dichocephalous, but the capitular and tubercular heads become less distinguishable further posterior in the dorsal region, which is comparable with the condition in other basal anomodonts such as Galepus and Galechirus ( Brinkman, 1981) . Moreover, a gradual decrease in rib length occurs towards the posterior dorsal region ( Fig. 2 ). The rib of the eighteenth (last) dorsal vertebra is much more robust than the anterior ones. It has a broad symmetrical base that could be described as holocephalous. Its shaft is only minimally curved and tapers off distally. Despite its robustness this rib was most probably not connected to the pelvic girdle, as it does not show a distal articulation surface.

Each of the three sacral vertebrae has robust ribs with a broad base and a distal end that clearly attached to the medial side of the ilium ( Figs 2 , 5 ). However, none of the ribs is fused to either the pelvic girdle or the vertebral neural arch or centrum. The first sacral rib has a slight anterior angle, and is constricted at midshaft until it widens again distally. The second and third sacral ribs are less constricted at the midshaft, are slightly curved dorsally, and are broad distally for articulation with the ilium.

Caudal ribs are present in the anterior part of the tail, but they are mostly indistinguishably fused with the transverse processes ( Fig. 6 ). The fused structures in the anterior caudal region are as long as the sacral ribs, and decrease in length only gradually, until they entirely disappear after caudal 22. Therefore, they are prominent features in the anterior two-fifths of the tail, providing an extensive attachment area for caudal muscles.

Gastralia

Gastralia are dermal ossifications in the abdominal region, but are discussed here in the context of the axial skeleton. Several specimens of Suminia preserve gastralia or ventral scales, representing rare evidence for these elements in therapsids. The only other therapsids with ventral scales are the basal anomodont Galechirus and the basal carnivorous dinocephalian Titanophoneus ( Orlov, 1958; Brinkman, 1981). The gastralia in Suminia are best preserved in KPM 173, where they appear to be in articulation along the midline. In addition, several specimens on the block (PIN 2212/116) preserve gastralia in various degrees of completeness. Notable are the gastralia in specimen 5, which displays the individual elements slightly shifted apart in ventral view ( Fig. 7 ). The gastralia of Suminia are extremely slender and delicate elements. They consist of several needle-like elements that seem to be arranged in a single row per trunk segment. Along the midline the rows from the left and right sides come together and possibly overlap. This anatomy is consistent with that in Galechirus and Titanophoneus , but the specimens of Suminia provide more precise information about the natural arrangement of these elements in early therapsids.

APPENDICULAR SKELETON

Pectoral girdle

The pectoral girdle of Suminia consists of the unpaired interclavicle, as well as the paired clavicle, scapula, coracoid, and procoracoid ( Fig. 8A ). A sternum or cleithrum is not present in any of the specimens studied. Several specimens preserve at least parts of the pectoral girdle, most often displaying the scapula and clavicle. The remaining structures are much rarer, and all elements are only present in specimen 3 on the block (PIN 2212/116), which exposes the well-preserved and complete right side of the pectoral girdle ( Fig. 4 ).

The interclavicle is partially covered by the scapula, and only exposes part of its stem and head. Although the entire interclavicle is not visible, it is apparent that this element is more slender than in other basal anomodonts or dicynodonts ( Brinkman, 1981; King, 1988). In Galechirus , which preserves the best and most complete interclavicle of any basal anomodont, it displays a broad paddle-shaped stem that is pointed posteriorly and constricted anteriorly just before the widened head. Brinkman (1981) described that head as diamond-shaped, but it more closely resembles an isosceles triangle with an elongate apex. The interclavicle of dicynodonts is not well known, but it seems to be mostly reduced in size to a triangular element, such as in Dicynodon trigonocephalus ( King, 1981a) . The exposed parts of the interclavicle of Suminia indicate that this element had a rather slender stem, and in this case also a diamond-shaped head, which seems to reflect the plesiomorphic condition of synapsids, as it very much resembles the condition in pelycosaur-grade synapsids ( Reisz, 1986).

The clavicle of Suminia is between one-half and two-thirds the height of the scapula. It is a long, robust, and slightly curved element, with an expanded medial end that contacts the interclavicle. The clavicle of Suminia resembles that of other basal anomodonts, whereas the clavicles of pelycosaurgrade synapsids and dicynodont anomodonts show greater medial expansions, and sometimes a greater curvature.

The scapula has an overall slender morphology. It is about four-fifths the length of the humerus, and displays a narrow dorsal blade and an expanded ventral plate. The blade is slightly convex laterally and concave medially. It is narrowest at a point that is just dorsal to the ventral plate. The blade widens slightly towards its dorsal end, where it is squaredoff. Ventrally, the scapula is expanded mainly in an anterior direction, where it thins out to contact the procoracoid. This is comparable with the condition in other basal anomodonts, but different from dicynodonts, which show a reduction of the anterior extension of the ventral part of the scapula ( King, 1988). In Suminia , the ventral plate is thickest at its posterior edge, which forms a continuation of the dorsal blade until it meets the coracoid posteroventrally to form the glenoid. The glenoid itself is not preserved in its entirety in any of the specimens, but from what is observable it appears that the plesiomorphic screw-shaped morphology of basal synapsid was replaced by a more rounded, posterolaterally facing structure. The thickened posterior edge of the ventral plate bears a prominent rugosity for the probable origin of the musculus triceps ( Brinkman, 1981; King, 1988). However, Suminia does not show the presence of a distinct triceps tubercle, as it is autapomorphic for Galeops . Moreover, as in the basal anomodonts Galeops and Galechirus there is no indication of an acromion process in Suminia , although this feature has been reported even in the most basal dicynodont, Eodicynodon , by Rubidge, King & Hancox (1994).

The procoracoid of Suminia is a large, elongate plate that forms the anterior and ventral extension of the ventral plate of the scapula, and curves slightly in the medial direction. Its dorsolateral margin is straight where it contacts the anterior portion of the ventral plate of the scapula. The posterior part of the procoracoid contacts the coracoid, and probably lacks a contribution to the glenoid. The ventromedial edge of the procoracoid is distinctly convex, and exhibits a pronounced indentation along its margin, rather than having an enclosed foramen as is typical in synapsids. The indentation is elongate and extends in a perpendicular angle into the procoracoid just before the posterior border of the element. When compared with other Palaeozoic synapsids, the procoracoid of Suminia retains its plesiomorphic large size, as seen in pelycosaur-grade synapsids and some other early therapsids such as Biarmosuchus and Moschops ( Gregory, 1926; Sigogneau & Tchudinov, 1972). In contrast, the procoracoid and coracoid are subequal in size in all other anomodonts and more derived therapsids (e.g. Jenkins, 1971; King, 1981a).

The coracoid of Suminia is best preserved in specimen 3 on the block, but it is shifted from its natural position and dips into the sediment with its posterior portion. Although its dimensions are not fully exposed, it is possible to determine that the coracoid is smaller than the procoracoid, reaching only about two-thirds the size of the latter. Based on the exposed part, the coracoid seems to be an essentially triangular, plate-like element. It is thickest and rounded at its anterior end, where it contacts the scapula and procoracoid, and forms part of the glenoid. The coracoid thins out in posterior and ventral directions, and displays concave medial and convex lateral surfaces. There is a distinct ridge at the posterodorsal edge towards the medial surface of the bone.

Humerus

The humerus of Suminia is one of the most commonly preserved elements ( Figs 1 , 2 , 9 , 10 ). It is a slender element that is about 75% of the length of the femur, and shows only moderately expanded proximal and distal ends (see Appendix 1 for a list of measurements). The shaft is twisted about 90° between its proximal and distal ends. This anatomy is most clearly observed in specimen 3 on the block. Most of the other specimens display a smaller angle, which is probably the result of slight compression during preservation. The large angle offset between the proximal and distal ends is typical for basal synapsids ( Reisz, 1986), whereas this torsion is distinctly reduced to at least 45° in non-dicynodont anomodonts and other basal therapsids ( Brinkman, 1981; Sigogneau- Russell, 1989). Nonetheless, the humerus of Suminia very much resembles that of the basal anomodont Galeops , rather than pelycosaur-grade synapsids, which display much more expanded epiphyses, and in which the distal end is always significantly wider than the proximal one ( Reisz, 1986). In Suminia , the proximal end displays the head of the humerus that is restricted to the proximal edge and not extended onto the dorsal side of the bone, as this is known from derived dicynodonts and some other therapsids ( King, 1988; Sigogneau-Russell, 1989). A distinct but modest deltopectoral crest extends from the head of the humerus in an anteroventral direction, and reaches a length of about one-third of the length of the shaft. A posterodorsal expansion of the proximal head is minimal in Suminia , but results in a slight curvature dorsally, which is less pronounced than in Galeops and more comparable with the morphology of Galepus and Galechirus . The supinator process of the humerus is absent in Suminia , as is the case in all therapsids ( Sidor & Hopson, 1998). Both entepicondyle and ectepicondyle are moderately well developed considering the overall slenderness of the element. As in Galechirus , entepicondylar and ectepicondylar foramina are present, with the latter being visible in most specimens, as the majority of the humeri are preserved in dorsal view. Romer (1956) noted that as a result of the general shape of the therapsid humerus, the entepicondylar foramen can only be seen in ventral aspect, which for Suminia is best displayed in PIN 2212/62 and specimen 15 on the block, PIN 2212/116 ( Figs 9 , 10 ). The radial and ulnar condyles are only weakly developed, continuous, and are positioned at the ventral side as well as the distal end of the humerus.

Radius

The radius of Suminia is a slender, cylindrical bone with slightly expanded proximal and distal ends ( Figs 2 , 9 ). With a relative length of about 85% of the length of the humerus, the radius is distinctly longer than that of Galeops (60%) and other anomodonts. The proximal and distal articulation surfaces are distinctly concave. The width of the proximal end of the radius is less than that of the ulna, whereas the distal end of the radius is at least equal in width to that of the ulna.

Ulna

The ulna of Suminia is slightly longer and more robust than the radius ( Figs 2 , 9 ). The distal twothirds of the shaft are straight, whereas the proximal portion shows a distinct curvature on its medial side. A weakly developed olecranon process is present, comparable with the condition seen in Galechirus but not as pronounced as in Galeops and Galepus . Wellossified and greatly elongated olecranon processes are present in most pelycosaur-grade synapsids, Eodicynodon , some large Triassic dicynodonts, and the burrowing dicynodonts Cistecephalus and Kawingasaurus ( Cox, 1972; Cluver, 1978; Walter, 1985; Reisz, 1986; Rubidge et al., 1994).

Manus

The manus of Suminia forms about 40% of the length of its entire forelimb ( Figs 9 , 11 , 12A ). The carpus consists of ten separate elements, including an ulnare, radiale, pisiform, intermedium, lateral centrale, medial centrale, distal carpals 1, 2, and 3, and fused distal carpals 4 and 5. This arrangement is identical to that of dicynodonts ( Boonstra, 1966; King, 1988), but differs from that of other basal anomodonts such as Galechirus and Patranomodon , for which separate distal carpals 4 and 5 have been described ( Brinkman, 1981; Rubidge & Hopson, 1996). The best carpus of Suminia is preserved in PIN 2212/62, which exposes an almost complete right forelimb in ventral view ( Fig. 9 ). The elements of the carpus are only slightly shifted from their natural position. The ulnare is a plate-like element that represents the largest bone in the carpus of Suminia . An oval pisiform of moderate size is present at the lateral and proximal side of the ulnare. A small mediolaterally compressed intermedium contacts the medial side of the ulnare. Slightly shifted from its natural position is the lateral centrale, which is a subrectangular bone that is slightly larger than the intermedium and would contact the latter distally. Further medially both of these elements would connect to the large and essentially quadrangular radiale. The radiale is pierced by a foramen just proximal to its centre in ventral view. At its distal end the radiale contacts a small, triangular-shaped medial centrale. Distally, the medial centrale connects to the base of a very large distal carpal 1, which is almost equal in size to the ulnare. The distal carpal 1 is unusual in its dimension and shape, in that it assumes the shape of a phalangeal or metapodial element. It displays a large articular facet for metacarpal I on its mediodistal side and a concave facet for distal carpal 2. The morphology of the distal carpal 1 with its distinct articular facets is strongly indicative of a divergent (c. 30–40 °) and somewhat opposable first digit ( Fröbisch & Reisz, 2009). The carpus is continued laterally by the distal carpals 2 and 3, and the fused carpals 4 and 5, with the former two articulating with metacarpals II and III, respectively, whereas the enlarged, fused element contacts both metacarpals IV and V. The metacarpals themselves increase in size from I to IV, with metacarpal V being slightly shorter than metacarpal III. The metacarpals are robust elements with slightly expanded proximal and distal ends. Metacarpal I is particularly massive and short, articulating with distal carpal 1, with a wide base. It is notable that the manual phalangeal formula is 2-3-4-5-3, which makes Suminia the only known anomodont with the plesiomorphic phalangeal count of amniotes ( Fig. 12A ). All other anomodonts in which the manus is preserved display the derived mammalian phalangeal formula of 2-3-3-3-3 ( Hopson, 1995). The condition in Suminia is achieved by the presence of short, disc-like phalangeal elements between the proximal and penultimate phalanges of digits III and IV. Except for the disc-like elements, the phalanges of Suminia are extremely long. In particular, the penultimate phalanges are extremely elongate, clearly exceeding the length of the proximal phalanges, which results in the distinct phalangeal proportions of this taxon, indicative of an arboreal lifestyle ( Fröbisch & Reisz, 2009). The morphology of the proximal phalanges is characterized by a broad articulation with the metacarpals, and a slender shaft that widens again further distally to form a slightly convex articular facet. In digits III and IV this facet is embraced by the short, slightly concave disc-shaped elements. Thereby, the disc-like phalanx displays a distinct distal lip on its dorsal side, which has already been reported in comparably shortened elements of dinocephalians, gorgonopsians, and cynodonts, and that probably prevented hyperextension of the joint ( Orlov, 1958; Hopson, 1995). Moreover, the discshaped phalangeal elements fuse to the penultimate phalanges in some large specimens (e.g. PIN 2212/ 104a), further contributing to the already pronounced elongation of this element in Suminia . The shape of the penultimate phalanges shows a robust proximal end, which becomes progressively more slender towards the distal end of the element; however, a convex distal articular surface forms a typical interphalangeal hinge joint that permits extensive mobility of the terminal phalanx. The terminal phalanges of Suminia are very long, strongly curved, and laterally compressed. The proximal articular surface is deeply concave and the phalanx possesses a very prominent and bulbous flexor tubercle at the proximal end of the ventral side.

Pelvic girdle

In the available specimens of Suminia , the three elements of the pelvic girdle are always isolated and show no indication of co-ossification ( Fig. 5 ). The ilium is the largest element, closely followed by the pubis and ischium ( Fig. 8B ). The ilium is only poorly known in non-dicynodont anomodonts. Partial ilia have been illustrated and described for Galepus, Galchirus , and Patranomodon , with that of Galepus being the most complete ( Brinkman, 1981; Rubidge & Hopson, 1996). The ilium of Suminia is well preserved in several specimens, providing details about its lateral and medial morphology ( Figs 2 , 5 , 8B ). It is a tall element that has well-developed anterior and posterior extensions, but with a slightly larger posterior part. The thin iliac blade is tallest at its anterior extension, as the anterior margin of the bone curves only gently forwards, and projects mainly dorsally. In contrast, the posterior part of the iliac blade curves strongly backwards up to a pointed end that meets the distinctly convex dorsal margin in an angle of about 80°. The iliac part of the acetabulum is a deeply concave structure that together with the contributions of the ischium and pubis results in a circular shape for the articulation of the femoral head. A pronounced supraacetabular buttress is developed above the acetabulum. The lateral surface of the iliac blade is concave, and the medial surface is correspondingly convex. However, a feature that is unique to the ilium of Suminia is the presence of a prominent and broad ridge at the medial side of its anterior margin ( Fig. 5 ). Moreover, the medial surface of the ilium shows three distinct articular facets for the sacral ribs, approximately at the level of the supraacetabular buttress of the lateral side.

The ischium of Suminia is a roughly triangular bone with a greatly thickened proximal end, bearing the posteroventral portion of the acetabulum ( Figs 5 , 8B ). The latter is defined by the presence of a pronounced buttress. From its proximal end the ischium extends ventrally and posteriorly. The anterior margin of the ischiadic plate is almost straight, the medial margin is slightly convex, and the posterolateral margin is concave, giving the bone a sickleshaped appearance. Thus, the ischium has the typical morphology of basal anomodonts and other basal synapsids, lacking more specialized features like the dorsal flange of derived dicynodonts (e.g. Fröbisch, 2006).

The pubis of Suminia is almost as large as the ilium ( Figs 5 , 8B ). It has a robust and distinctly offset proximal end that contributes to the anteroventral part of the acetabulum. From its proximal end the pubis extends far anteriorly and medially as a large plate with a concave dorsal and a convex ventral surface. Thus, this element appears like a long and slender, anteriorly tapering element in lateral view. The pubis doesn’t seem to have a prominent ridge along its anterolateral edge, as in basal synapsids and several other therapsids ( Gregory, 1926; Romer & Price, 1940). In some derived dicynodonts this ridge is developed into a distinct pubic tubercle ( Surkov, Kalandadze & Benton, 2005). The anterior edge of the pubis in Suminia is slightly rounded, but meets the ventromedial margin of the element in a steep angle. The ventromedial margin itself is long, straight, and extends posteriorly until it meets the anteroventral part of the ischium. Unlike in basal synapsids, the contact between the pubis and ischium is not continuous in Suminia . In contrast, the posterior margin of the pubis bears a pronounced semicircular indentation just medial to the acetabulum, which together with the relatively straight anterior margin of the ischium encloses a large puboischiadic fenestra. In addition, a slightly enlarged pubic foramen is positioned entirely within the pubis of Suminia , and just proximal to the centre of the bone. In contrast, the pubis of Galepus bears an autapomorphically greatly enlarged foramen ( Brinkman, 1981: obturator foramen therein), which does not seem to be related to the puboischiadic fenestra, as it lies entirely within the pubis. In dicynodonts and some other therapsids only the puboischiadic fenestra is present, which is often called the obturator foramen, based on the assumption that the pubic foramen, containing the obturator nerve, is incorporated into this fenestra (e.g. Romer, 1922, 1956).

Femur

The femur is one of the most commonly preserved elements of Suminia . It is the longest bone of the skeleton and has a distinctly slender morphology ( Figs 2 , 13A ). The entire bone is gently sigmoidal in outline, which is comparable with the dorsal tilting of the proximal end and the downturning of the distal end of the femur in the basal anomodont Galepus ( Brinkman, 1981) . A low rugosity on the dorsolateral portion of the femur indicates the probable insertion of musculus iliofemoralis; however, a distinct development of a trochanter major, as has been reported in other basal anomodonts ( Rubidge & Hopson, 1996), is absent. The proximal articular surface of the femur is represented by a weak swelling that is mostly limited to the proximal surface of the bone. The distal end of the femur is best preserved in the holotype (PIN 2212/10), as it is one of the few specimens that preserves the femur in three dimensions. The shaft widens distally to give rise to the paired condyles, which are mainly positioned on the ventral side. The two condyles are separated by a distinct groove that continues onto the dorsal surface of the bone, as has been described for Patranomodon ( Rubidge & Hopson, 1996) .

Tibia

The tibia of non-dicynodont anomodonts is only known from Galechirus , where it is about equal in length to the femur ( Brinkman, 1981). Although similar in general morphology, the tibia of Suminia is only about 80% of the length of the femur ( Figs 2 , 13 ). The tibia is the more robust of the zeugopodial elements, and its shaft displays a distinct lateral curvature. Although the proximal and distal ends of the tibia are expanded, this feature is more pronounced at the proximal end. A well-defined cnemial crest is present on the lateral side of the shaft, and is continuous with the proximal end, resulting in the typical triangular shape of the articular facet in proximal view.

Fibula

The fibula of Suminia has approximately the same length of the tibia, but it shows a stronger curvature of its shaft, bending posteriorly ( Figs 2 , 13 ). In general, the fibula is much more slender than the tibia and displays greatly expanded proximal and distal ends. The fibula of Suminia is similar to that of other anomodonts (e.g. Brinkman, 1981; Rubidge et al., 1994); however, it is relatively longer and more slender, and lacks any anterior processes, as described for Kingoria and Placerias ( Camp & Welles, 1956; King, 1985).

Pes

Like the manus, the pes of Suminia makes up a significant proportion of the length of the limb (38%). The pes of Suminia is preserved in several specimens, providing plentiful information about its anatomy ( Figs 1 , 12B , 13C , 14 ). The tarsus is only complete and well preserved in the holotype, and comprises eight elements, including the astragalus, calcaneum, the lateral centrale, and five distal tarsals ( Figs 12B , 14 ). The astragalus is rounded in outline and not L-shaped, as in more basal synapsids ( Romer & Price, 1940). It is a complex element that displays a deep fossa on its ventral surface. Moreover, a second foramen or canal for the perforating artery is present in between the astragalus and calcaneum ( Fig. 12B ). In dorsal view, the astragalus is approximately semicircular in outline, with a convex medial side and a concave lateral side. The latter embraces the medial margin of the calcaneum, and supports its posteroventral edge with a distinct lip. Distally, the astragalus displays a distinct articular facet for the lateral centrale. The calcaneum is a flat, almost circular plate with slightly concave dorsal and ventral surfaces. At its distal end it articulates with the enlarged fourth and the diminutive fifth distal tarsals. The morphology of the first distal tarsal of Suminia is very distinct. Comparable with the distal carpal 1 in the manus, it almost assumes the morphology of a phalangeal or metapodial element, with a clear shaft and proximal and distal articular facets. In fact, the proximal end consists of two facets with a distinct angle between one another. The smaller of the two surfaces contacts the lateral centrale proximally, whereas the more extensive and elongate surface is located at the proximolateral side of this element, contacting the second distal tarsal. Therefore, the shaft of the first distal tarsal projects slightly medially and results in a distinct angle of the first digit connecting to the distal articular surface of distal tarsal 1, also indicating a prehensile first digit in the pes. The second and third distal tarsals are mediolaterally compressed elements that each articulate with a single digit distally: digits II and III, respectively. Distal tarsal 4 is a distinct and greatly enlarged bone, which is crescent-shaped in outline. Its concave edge contacts the lateral centrale and third distal tarsal medially. The expanded convex side contacts the astragalus and calcaneum proximally, and the very small fifth distal tarsal distolaterally. At their distal margin the fourth and fifth distal tarsals articulate with metatarsals IV and V.

The only other known tarsus of a non-dicynodont anomodont is that of Galechirus , which is generally similar to that of Suminia but does not preserve a distal tarsal 5 ( Brinkman, 1981). The tarsus of dicynodonts is generally reconstructed as consisting of seven or eight elements, depending on the presence or absence of the fifth distal tarsal. However, various combinations have been described, even including a medial centrale and intermedium ( Olson & Byrne, 1938). Romer (1956) stated that in dicynodonts the first distal tarsal also assumes the shape of a metapodial element; however, this has subsequently been questioned ( King, 1988). Therefore, the enlargement and divergence of the first distal tarsal in dicynodonts similar to the condition in Suminia is doubtful, and requires further investigation.

The metatarsals and pedal phalanges of Suminia mirror the patterns apparent in its manus. The metatarsals increase in length from digits I to IV, with metatarsal 5 being as long as metatarsal 3. The pedal phalangeal formula of Suminia is 2-3-4-5-4, again representing the plesiomorphic phalangeal count of amniotes. This number is achieved by the presence of extremely shortened, disc-shaped phalangeal elements between the proximal and penultimate phalanges of digits III, IV, and also V. The penultimate phalanges of the pes show an elongation comparable with those in the manus, resulting in the very distinct phalangeal proportions of this taxon. The terminal phalanges display an elongate, laterally compressed, and distinctly curved overall morphology. As is the case in the manus, they are extremely mobile as a result of the distinct hinge joint formed by the convex, spool-like morphology of articulation of the penultimate phalanges.

Figures

Figure 1. Photograph (A) and colour-coded outline drawing (B) of specimens of Suminia getmanovi on the large block (PIN 2212/116).

Figure 2. Drawing of one of the best-preserved and most complete skeletons of Suminia getmanovi (specimen 1 on block PIN 2212/116) in dorsal view. Refer to the text for the abbreviations list. Scale bar: 2 cm.

Figure 3. Drawing of the cervical region of Suminia getmanovi in (A) PIN 2212/104a (dorsal aspect), (B) PIN 2212/62 (lateral aspect), and (C) specimen 4 on block PIN 2212/116 (dorsal aspect). Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 4. Drawing of the pectoral region of Suminia getmanovi (specimen 3 on block PIN 2212/116). Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 5. Drawing of the pelvic region of Suminia getmanovi (specimen 2 on block PIN 2212/116). Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 6. Drawing of the caudal region of Suminia getmanovi (specimen 2 on block PIN 2212/116). Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 7. Photograph of the gastralia of Suminia getmanovi in ventral view, as preserved in specimen 5 on the block (PIN 2212/116). Scale bar: 1 cm.

Figure 8. Reconstruction of (A) the pectoral girdle and (B) the pelvic girdle of Suminia getmanovi in right lateral views. Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 9. Drawing of the right forelimb of Suminia getmanovi (PIN 2212/62) in ventral view. Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 10. Drawing of right humerus of Suminia getmanovi in ventral view as preserved in specimen 15 on the block (PIN 2212/116). Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 11. Drawing of the left manus of Suminia getmanovi (PIN 2212/104a) in dorsal view. Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 12. Reconstruction of (A) the right manus and (B) the right pes of Suminia getmanovi in dorsal views. Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 13. Drawing of (A) a partial left hindlimb of Suminia getmanovi (PIN 2212/102) in dorsal view, (B) a partial right hindlimb of S. getmanovi (PIN 2212/103b) in ventral view, and (C) a partial right pes of S. getmanovi (PIN 2212/103a) in right lateral view (compressed). Note that PIN 2212/103a and 2212/103b are isolated specimens that belong to the same individual, as specimen 2 on the block (PIN 2212/116). Refer to the text for the abbreviations list. Scale bar: 1 cm.

Figure 14. Drawing of the left pes of Suminia getmanovi (holotype, PIN 2212/10) in ventral view. Refer to the text for the abbreviations list. Scale bar: 1 cm.