Slavoia darevskii Sulimski, 1984

Tałanda, Mateusz, 2017, Evolution of postcranial skeleton in worm lizards inferred from its status in the Cretaceous stem-amphisbaenian Slavoia darevskii, Acta Palaeontologica Polonica 62 (1), pp. 9-23 : 10-16

Slavoia darevskii Sulimski, 1984

Figs. 1–12 .

Material.— Isolated skulls: ZPAL MgR-III/69, MgR-III/73, MgR-III/74, MgR-III/75, MgR-III/78, MgR-III/81, from Khermeen Tsav II. ZPAL MgR-I/10, MgR-I/11, MgR-I/93, MgR-I/96, MgR-I/97, MgR-I/98, MgR-I/100, MgR-I/101, MgR-I/102, MgR-I/103, MgR-I/104, MgR-I/105, MgR-I/107, MgR-I/108, MgR-I/109, MgR-I/110, MgR-I/111, MgR-I/113, MgR-I/114, MgR-I/115, MgR-I/116, from Khulsan. ZPAL MgR-I/1, MgR-I/2, MgR-I/3, MgR-I/4, MgR-I/5, MgR-I/6, MgR-I/7, MgR-I/87, from Nemegt. Specimens with postcranial material: PIN 3142 /358, PIN 4487 /14, from Khermeen Tsav. ZPAL MgR-III/76 (skull with partial skeleton), ZPAL MgR-III/77 (skull with cervical vertebrae), ZPAL MgR-III/79 (skeleton), ZPAL MgR-III/80 (skull with incomplete skeleton), ZPAL MgR-III/82 (pelvic girdle), from Khermeen Tsav II. ZPAL MgR-I/8 (skeleton, holotype), ZPAL MgR-I/9 (skeleton), ZPAL MgR-I/78 (partial skeleton), ZPAL MgR-I/94 (skull with cervical vertebrae), ZPAL MgR-I/99 (skull and two vertebrae), ZPAL MgR-I/106 (partial skeleton), ZPAL MgR-I/112 (skeleton), ZPAL MgR-I/117 (bone fragments), from Khulsan. ZPAL MgR-I/85 (skull and partial skeleton), no data about specific locality. All material from Barun Goyot Formation, Upper Cretaceous of Mongolia .

Description.—The skull of S. darevskii was described by Sulimski (1984). Numerous amphisbaenian traits in this species have been identified by Tałanda (2016), including the unique synapomorphies: a strong overlap of vomers and palatines or tight wrapping of pterygoid around the posteromedial surface of the quadrate. I suggested there that an enforcement of the snout preceded the elongation of the body and reduction of the limbs. In this paper, I will compare the S. darevskii skull to other burrowing lizards ( Acontinae , Feylininae , Dibamidae , and the crown Amphisbaenia) only in the context of its fossoriality.

The axial skeleton: Sulimski (1984) described that S. darevskii possesses 26 presacral vertebrae (including seven cervical), two sacral and an unknown number of caudals. According to him all are procoelous. I confirm these observations.

Atlas ( Fig. 1 ) consists of two neural arches fused together on the dorsal side and the intercentrum that closes the ring on its ventral side. The neural arches have a gabled form ( Fig. 1A ). They are wider than the intercentrum (expanded laterally) and slightly expanded laterally. The intercentrum is badly preserved in all specimens studied. Its diameter is similar to the centrum of the second vertebra but its length is shorter. The first hypapophysis is visible in ZPAL MgR-I/108 ( Fig. 1C ). It has a prominent, cranially directed ventral process like in Diplometopon zarudnyi ( Trogonophidae ; Gans 1960). It is fused to the first centrum. Other intercentra are not preserved. Apparently, they were rather small and probably not fused to the centra.

The axis has a high and broad neural spine that is descending cranially (SOM 1: fig. 1A). The spine protrudesposterior to the centrum. Prezygapophyses and postzygapophyses are well developed, the latter being a little larger ( Fig. 1A ). They are flat and nearly horizontal with rounded distal margins. The axis has two hypapophyses visible on ZPAL MgR-III/77. Its synapophyses protrude laterally.

They are large and inclined posteriorly ( Fig 1E ).

The following cervical vertebrae resemble each other in their shapes ( Fig. 1A, E ; SOM 1: fig. 1A). Their neural spines are narrow and directed posteriorly. Synapophyses are high and well developed, as in the crown amphisbaenians ( Folie et al. 2013; Bolet et al. 2014). They are inclined posteriorly. Zygapopsyses are broad and similar to those of the axis ( Fig. 1A ). The cervical vertebrae are generally robust, broad and short.

The first trunk vertebrae are morphologically similar to cervicals (SOM 1: fig. 1A). They become more and more elongated and narrow posteriorward. Their neural spines are reduced to very small posterior projections. Their synapophyses are small. Only zygapophyses remain well developed. The last trunk vertebrae are less slender than others resembling sacral ones ( Fig. 1D ).

The two sacral vertebrae are robust ( Fig. 1F 3 ; SOM 1: fig. 2C). Both possess higher neural arches than surrounding vertebrae and large zygapophyses. They lack neural spines. The transverse processes are flat and broad antero-posteriorly, but do not touch each other. Processes of the first pair are of a more cylindrical appearance ( Fig. 1F 3 ; SOM 1: fig. 2C).

The tail is best preserved in specimen ZPAL MgR-I/8, in which thirteen proximal vertebrae are represented (SOM 1: fig. 2C). The caudals are long and slender and decrease in size posteriorly rather fast. Their neurapophyses are small and short. The transverse processes are well developed, long, flat and oriented cranially in the most proximal caudal vertebrae ( Fig. 1F 1 , F 2 ). They become shorter and shorter posteriorly. The length of hemapophyses remains unknown. The zygapophyses are small. The preserved caudal vertebrae lack the autotomy septa, but their presence on more distal vertebrae cannot be ruled out because the tail is incomplete.

The cervical ribs are preserved in ZPAL MgR-I/78 and MgR-I/112 ( Fig. 1E ; SOM 1: fig. 1A). The first cervical ribs begin from the third vertebra, as seen in ZPAL MgR-I/78 on the left side (cervical ribs from the opposite side are displaced) and left side of ZPAL MgR-I/112. They are robust, short and broad. They widen distally like in lacertids, unlike narrow ribs of the amphisbaenians ( Gauthier et al. 2012). The number of cervicals is often difficult to determine in fossil taxa. They are usually defined by the rib shape or attachment to the sternum. The ribs on the sixth vertebra are the first one, which is long and slender, resembling the dorsal ribs ( Fig. 1E ). However, both they and the next pair were probably not attached to the sternum, like in most lizards (including the closely related Teiidae and Lacertidae ; Hoffstetter and Gasc 1969). Thus, S. darevskii had seven cervicals. This is lower than the count primitive for lizards, which is eight ( Hoffstetter and Gasc 1969; Gauthier et al. 2012). Also Sulimski (1984) described S. darevskii as having had seven cervicals.

The trunk ribs are poorly preserved in all specimens studied and the number of inscriptional, dorsal and lumbar ribs is hard to determine. The first two types were long, slender and medially curved ( Fig. 1D, E ).

The appendicular skeleton: Several specimens have elements of pectoral girdle preserved (ZPAL MgR-III/76, MgR-III/79, MgR-III/82, MgR-I/8, MgR-I/9, MgR-I/85, MgR-I/101, MgR-I/106, MgR-I/112, PIN 4487/14, 3142/358; Figs. 2–4 ). Sulimski (1984) described and illustrated main bones (scapula-coracoids, interclavicle, and clavicles). He also mentioned the possible remains of a sternum in ZPAL MgR-I/108 and argued that this structure was not ossified in S. darevskii . During my investigation, I have also found ossified epicoracoids, prerequisites for the presence of suprascapulas and an ossified sternum (contra Sulimski 1984).

Sulimski (1984) did not find any remains of the sternum. He proposed that it was not ossified. However, it is preserved in specimen PIN 4487/14, where it is fully ossified ( Fig. 1B ). There are also visible remains of the sternum in ZPAL MgR-I/106. It was probably destroyed during preparation. The sternum has a shield-like form. It is ventrally convex. The sternum has at least four rib attachments and contacts the interclavicle antero-medially.

The interclavicle is cruciform ( Figs. 2A, B , 4B, C ), like in other lacertoids. The length and width of the interclavicle are similar. While the bone is flat in Timon lepidus , the interclavicle in S. darevskii is dorso-ventrally deep ( Fig. 4B ). It has very wide lateral processes. These processes form a distinct transverse crest. The crest delimits a transversal concavity faced antero-ventrally. The anterior and posterior processes are narrow and long. They are convex ventrally except for the concavity at the lateral processes level on the anterior process ( Fig. 2A 2 ). The process is pointed anteriorly. The bone is preserved in several specimens which enables us to observe its variation in proportions. It mainly concerns the relative length of the anterior and posterior processes ( Fig. 2B ).

The clavicles are curved, L- or J-shaped ( Fig. 2C, D ). They have variable morphology and do not depend on size. Holotype possesses slender, J-shaped clavicles that are rounded in cross-section. They become flat and wider medially and bifurcate at the medial end of the bone. They widen dorsally to form funnel-shaped endings, which suggest a clavicle-suprascapula contact. Other specimens of similar size, like ZPAL MgR-I/106 or MgR-I/101, have robust, L-shaped clavicles. They are wide and flat over the entire length. It is not clear whether they bifurcate medially. Both endings are badly preserved in all specimens with robust clavicles .

The scapula and coracoid are fused (in synostosis), like in lacertids ( Müller 2001) and the suture is retained as a scar between the glenoid cavity and coracoid foramen ( Figs. 3 , 4 ). Their morphology is unique among Squamata because scapula is not fenestrated and forms a very thin subtriangular medial blade ( Fig. 3A ). It is often broken. Sulimski (1984: fig. 2) misinterpreted the damage of the blade as a scapular fenestra. The shaft of the scapula is narrow but robust. It is wider at the dorsal end (funnel-shaped), where it was in contact with the suprascapula. This extension reminds one of Bipes biporus Cope, 1894 (see Zangerl 1945). The variability of scapulae is difficult to determine due to the incompleteness of almost all the specimens. Some scapulae do not have the funnel-shaped dorsal ending but it might be a result of bad preservation. The scapulocoracoid fenestra is present. The coracoid of S. darevskii is flat and fan-shaped ( Figs. 3 , 4 ). There are no openings and the procoracoid is absent. The coracoid contributes to the ventral part of the glenoid cavity. The coracoid foramen is positioned medially to the glenoid. On specimen PIN 4487/14 a calcified epicoracoid is partially preserved ( Figs. 3E , 4C ).

The humerus of this lizard is well developed and elongated ( Figs. 4–6 ), about 10 mm long in adult specimens. The proximal and distal ends are subtriangular in outline and rotated in respect to each other. The proximal end is concave ventrally. The humerus has a distinct deltopectoral crest oriented ventrally ( Figs. 5B , 6 ). It extends for one fourth of the humerus length. The humerus epiphyses are weakly ossified or even not preserved in smaller individuals. The medial tuber is weakly developed. However, it is clear that the bone becomes wider towards the distal end and has a distinct attachments area with radial and ulnar condyles. Sulimski (1984) observed the presence of the ectepicondylar foramen. It is visible in ZPAL MgR-I/78.

The ulna is longer and more massive than the radius but is shorter than the humerus ( Figs. 6B , 7 ). The olecranon is prominent. The shaft is compressed laterally. Both ends of the bone are expanded. The proximal end has an indentation for the humerus. The distal end is probably convex and contacts the ulnare.

The radius is slender and delicate ( Fig. 7B ). It is shorter than the ulna because of olecranon on the latter bone. The proximal end is not well visible because of the surrounding bones but the distal end is distinctly expanded into two articulation surfaces ( Fig. 7B 2 ). In B. biporus the bone is also broader in this part but lacks a distinct lateral facet ( Zangerl 1945).

The carpus is usually not well preserved but seems to be not reduced ( Fig. 7 ). It is complete in ZPAL MgR- III/79 but its particular elements are indistinguishable ( Fig. 7A 1 ). In ZPAL MgR-I/8 and MgR-I/9, it is partly articulated but incomplete. The first specimen bears four elements around the distal head of the ulna ( Fig. 7C ). The large posterior element is probably the pisiform. The bone articulating with the ulna is the ulnare. The two bones next to the metacarpal are the distal carpals. The smaller and laterally-placed one is probably the distal carpal V. The larger, medially-positioned and contacting the ulnare one is probably the distal carpal IV. Four carpals are also visible in ZPAL MgR-I/9 ( Fig. 7B ). The first one is a small element between the ulna and radius. It is probably the carpal intermedium, the presence of which is also corroborated by a distinct

C

articulation surface on the radius. More distally, there are two carpals articulated to each other and to the metacarpal II. They represent the lateral centrale and second distal carpal. The fourth element, placed next to IV metacarpal is probably the distal carpal IV. There is also another bone placed distally to the metacarpals. It is rectangular and has a relatively large distinct concave surface. Perhaps it is the disarticulated ulnare. To sum up, probably all ten carpals including pisiform) were present in S. darevskii just like in the Lacertidae ( Rieppel 1992; Maisano 2001). The B. biporus has nine elements, lacking an intermedium ( Zangerl 1945; Kearney 2002).

The manus is fragmentary in ZPAL MgR-I/8 and MgR-I/9 but almost completely visible on the scan of ZPAL MgR-III/79 ( Fig. 7A ) and is well developed. All metacarpals are slender and have expanded ends. Contrary to typical lizards, their lengths are subequal. The metacarpal I and V are slightly shorter. The metacarpal III is probably the longest one. In B. biporus , the first three metacarpals are also subequal but the last two are shorter ( Zangerl 1945). The phalanges are broad, especially the claws, which are also flat and large. This strongly resembles B. biporus ( Zangerl 1945) . The phalangeal formula is not easy to determine and proportions of most fingers are not typical for lizards (Shapiro third of the acetabulum. The pubic tuber is oriented craniolaterally and appears near the acetabulum.

The ischium is visible only in PIN 3142/358 from its ventral side and on the scan of the holotype pelvis. It generally resembles that of Ameiva ( Teiidae ). It is flat and broad. The ventral part has a parallelogram shape and horizontal symphysial border. It has a distinct ischiadic tuberosity and the notch for hypoischium. The dorsal part contacts the ilium and pubis and contributes to the postero-ventral part of the acetabulum.

et al. 2007). An examination of a CT-scan shows that the first and second fingers have their distal parts “cut off” in the same line ( Fig. 7A 1 ). The preserved part of digit I has the most proximal phalanx complete. The second phalanx is incomplete but the preserved part shows the condition of a regular phalanx—not like a claw. This corroborates the interpretation of the small remains of the third phalanx, which was probably the claw. The second digit has two proximal phalanges complete, and part of the third one. The third digit has three phalanges complete but lacks the claw. The next digit is complete and has four phalanges. The last of them forms a large, broad, flat claw. The fifth digit has an incomplete middle part but boundaries between the phalanges are preserved showing three distinct phalanges. The fifth claw is even larger than the previous one. Assuming that the second and third digits did not develop hyperphalangy, the most parsimonious phalangeal formula would be 3-3-4-4-3.

The pelvic and hindlimb anatomy were only briefly mentioned by Sulimski (1984). In the pelvis of S. darevskii ( Fig. 8 ), the ilium is long and slightly curved caudally. It contacts the ischium and pubis ventrally to form the acetabulum. The dorsal part is slender but it expands antero-posteriorly in the mid part. The uppermost part tapers distally. Unlike lacertids, the preacetabular process is absent in S. darevskii .

The pubis is only partially preserved in the holotype and absent in other specimens from the ZPAL collection. It is long and curved medially. It forms the antero-ventral one

The femur is preserved in ZPAL MgR-I/8, MgR-I/9, and PIN 3142 /358 ( Figs. 9–11 ; SOM 1: fig. 2). It is long and slender. It is approximately 8 mm long in the holotype specimen, not 10 as measured by Sulimski (1984) and is longer than the tibia but shorter than the humerus. The shaft is rather smooth and rounded in cross-section. Both ends are expanded and rotated relative to each other ( Fig. 10A ). There is a well-developed crest next to the proximal head. This is probably the trochanter internalis (or fibular trochanter?). The distal end has a distinct tibial condyle .

The tibia is about 6 mm long as seen in ZPAL MgR-I/9 and PIN 3142/358 ( Figs. 9 , 10 ). It is more massive than the fibula. It has distinct heads on both its ends. The proximal end is broad. It has a flat proximal surface where the proximal epiphysis attached. A distinct crest is projecting dorsally to the proximal end of the tibia. It is a cnemial crest, which is also well developed in the Lacertidae ( Parker 1900) . The distal end is broad as well but compressed antero-posteriorly. It bears the remains of the distal epiphysis but they are too incomplete to determine the type of tibioastragalar articulation ( Russell and Bauer 2008).

The fibula is only partially preserved. It is long and slender ( Figs. 9 , 11A ). The proximal end was not expanded, contrary to the distal end.

The foot of S. darevskii is preserved only on PIN 3142/358 ( Figs. 9B , 11 ). The tarsus is not well preserved. Particular bones are displaced. However, a distinct astragalocalcaneum is articulated to the fibula. Other elements are also present but difficult to distinguish. The rest of the foot is well preserved.

All the metatarsals are long and slender. They have expanded ends. Metatarsal V is the shortest and the IV is the longest one presenting the proportions typical for lizards. However, metatarsal V seems to be simplified and not hooked.

The phalangeal formula is uncertain but it was probably 0-1-1-2?-2?. The first digit is absent. The second and third are composed only by one phalanx each. Both are poorly ossified but resemble unguals in their shape. One complete, wide and short phalanx articulates the metatarsal IV and has a distal articulation surface for at least another phalanx. Similarly, digit V has only one phalanx preserved but it probably had more. The phalanx is long and slender unlike the one from digit IV.

Stratigraphic and geographic range.—Barun Goyot and Djadochta formations, Mongolia; late Campanian.

Figures

Fig. 1. The axial skeleton of the stem-amphisbaenid Slavoia darevskii Sulimski, 1984 from late Campanian of Mongolia. A. The reconstruction of the first four vertebrae in dorsal view (based on different specimens). B. The reconstruction of sternum in ventral view (based on PIN 4487/14). C. The reconstruction of first hypapophysis (based on ZPAL MgR-I/108). D. PIN 3142/358, complete presacral vertebrate column in dorsal view; photo courtesy of Vladimir Alifanov. E. ZPAL MgR-I/78, the neck with well preserved, blunt and broad cervical ribs. F. The reconstruction (based on ZPAL MgR-I/8) of the fifth caudal vertebra in lateral (F1) and dorsal (F2) views; sacral vertebrae in dorsal view (F3).

Fig. 2. Reconstructions of clavicles (C, D) and interclavicles (A, B) of Slavoia darevskii. A. Morphotype I based on ZPAL MgR-I/8, in anterior (C1), lateral (C2), and ventral (C3) views. B. Comparison of the morphotypes II and I in the same scale. C. Morphotype I based on ZPAL MgR-I/8. D. Outlines of the morphotype II preserved in ZPAL MgR-III/80; grey lines, damaged edges of the clavicle; dotted line, hypothetical reconstruction of missing part. Both in anterior view.

Fig. 3. Reconstruction of scapulocoracoid of Slavoia darevskii based on different specimens, in anterior (A), dorsal (B), lateral (C), and ventral (D) views. E. Reconstruction of epicoracoid assuming that it did not reach the suprascapula.

Fig. 4. Pectoral girdle of the stem-amphisbaenid Slavoia darevskii Sulimski, 1984 from late Campanian of Mongolia. A. ZPAL MgR-III/76, scapulacoracoid in anterior view; the black line marks missing parts seen in other specimens. B. ZPAL MgR-I/8, pectoral girdle with ventral edge of clavicle, interclavicle (morphotype I), coracoids, and proximal part of humerus. C. PIN 4487/14, sternum with rib attachments, coracoids with epicoracoids, interclavicle (morphotype I), humerus, cervical, and skull in ventral view.

Fig. 5. Humerus of the stem-amphisbaenid Slavoia darevskii Sulimski, 1984 (ZPAL MgR-I/8) from late Campanian of Mongolia. A. Distal part in medial view. B. Humerus in ventral view, displaying also coracoid and interclavicule.

Fig. 6. Reconstruction of right humerus (A) and ulna (B) of Slavoia darevskii. A. ZPAL MgR-I/8, holotype. Proximal head with partially preserved epiphysis in proximal (A1) and ventral (A2) views, the shaft and distal head in medial view (A3), and distal head in distal view (A4). B. ZPAL MgR-I/9, subadult, in lateral (B1), posterior (B2), and medial (B3) views.

Fig. 7. Forelimb (with details of manus) of the stem-amphisbaenid Slavoia darevskii Sulimski, 1984 from late Campanian of Mongolia. A. ZPAL MgR-III/79, CT-scans. The dotted line in A1 marks surface where the digit I and II are eroded; outlines in A2 mark phalanges and metacarpals (preserved and reconstructed). B. ZPAL MgR-I/9; photograph (B1), outlines of the forearm and partially preserved hand (B2). C. ZPAL MgR-I/8, partially preserved hand.

Fig. 8. Reconstruction of pelvis of Slavoia darevskii. A. Based on PIN 3142/358, paired ischia in ventral view. B. Based on ZPAL MgR-I/8, in lateral view; the dashed line marks missing part.

Fig. 9. Hindlimb of Slavoia darevskii. A. Based on ZPAL MgR-I/9, tibia in ventral view. B. Based on PIN 3142/358, outlines of hindlimb in dorsal (B1) and anterior (B2) views.

Fig. 10. The stem-amphisbaenid Slavoia darevskii Sulimski, 1984 (ZPAL MgR-I/9) from late Campanian of Mongolia. A. Left femur in posterior view. B. Tibia in posterior view.

Fig. 11. The stem-amphisbaenid Slavoia darevskii Sulimski, 1984 (PIN 3142/358, posterior half) from late Campanian of Mongolia; displaying nearly complete hindlimb and ischia. Photograph from the rear showing reduced pes (A), ventral side (B). Photos courtesy of Vladimir Alifanov.

Fig. 12. Reconstruction of the whole skeleton of Slavoia darevskii in dorsal view.