Osteology of a forelimb of an aetosaur Stagonolepis olenkae (Archosauria: Pseudosuchia: Aetosauria) from the Krasiejów locality in Poland and its probable adaptations for a scratch-digging behavior

Proportions and general description of the forelimb bones

Forelimb elements of S. olenkae (humerus, Fig. 1; ulna and radius, Figs. 2–4; the largest carpal bone—fused radiale and intermedium, Fig. 5; manus, Fig. 6) are smaller than the corresponding elements of the hind limbs, therefore the entire forelimb must have been shorter than hind limb (D Dróżdż, pers. obs., 2018; Książkiewicz, 2014). The humerus is about two-thirds the length of the femur (Książkiewicz, 2014). The length ratio of the humerus ZPAL AbIII/2369 to the femur ZPAL AbIII/691 is 0.69 (the specimens possibly belong to a single animal, because they have been found close to each other in the same assemblage). Although there are no humeri found in articulation with radius and ulna, it can be assumed that humerus in S. olenkae is longer than each of the forearm bones, based on the comparison of several specimens (D Dróżdż, pers. obs., 2018; Książkiewicz, 2014). Possibly the ulna ZPAL AbIII/1179 belongs to the same animal as the humerus ZPAL AbIII/2369, because of their similar taphonomic condition and relatively close position in the sediment at the moment of recovery. The length ratio of the ulna ZPAL AbIII/1179 to the humerus ZPAL AbIII/2369 is 0.79. The radius and ulna are orientated parallel to each other (Fig. 2; based on specimen ZPAL AbIII/2407). The ulna is longer than the radius and more massive. The ratio of length between the radius and the ulna in the specimen ZPAL AbIII/2407 is 0.85. The proximal ends of the radius and ulna form a single articulation surface for the humerus. The joint between the radius and ulna is elongated and crescent-shaped (Figs. 3A, 4B; based on ZPAL AbIII/3351, 3322), which seemingly makes the relative rotation of these two bones impossible. The olecranon process of the ulna is high (Figs. 2 and 3; based on ZPAL AbIII/2407, 2014, 3351), and in large specimens of humeri the olecranon fossa is present (Fig. 1B; ZPAL AbIII/1175, 257). Based on specimen ZPAL AbIII/2407 the manus together with the carpus is shorter than both the ulna and the radius (Figs. 2A–2D). The carpus consist of at least four bone elements. The largest one of them is probably a fused radiale and intermedium (Fig. 5; ZPAL AbIII/2071, 2407, 3349/1, 3349/2). It is connected with the radius and partially with the ulna at the forearm side and the metacarpals I–III and probably IV at palm side. Its arrangement prevents rotation movements of the carpus and restricts moves of the carpal joint only to a one sagittal plane. Metacarpals and phalanges (Fig. 6E) are stout, relatively short and wide. Medial digits (II, III, IV) are almost of equal length and they are noticeably longer than digits I and V, while digit I is longer than digit V (relative length of digits II∼III∼IV>I>V). Digit I is the most robust in the manus, the medial digits are of similar form and massiveness, while digit V is the tiniest one in the hand. The phalangeal formula is probably 2-3-4-5?-3? and at least digits I to III terminate with claw-like unguals (Fig. 6E, mostly based on ZPAL AbIII/3349/1 and 2071). The ungual of the first digit is the largest one, and it is much bigger than the rest. The ungual of the second digit is about one-third smaller than the first one, and the sizes of following unguals decrease in the same pattern (Fig. 6E; based on ZPAL AbIII/3349/1, 2071). The dorsal part of the hand, forearm and probably also upper arm is entirely covered by osteoderms (Fig. 6F; based on ZPAL AbIII/2407, 3349/1, 3349/2).

Humerus

The humerus of S. olenkae (Fig. 1), examined here in the specimen ZPAL AbIII/257, 1175, 2627, is a massive, strongly built bone with a straight shaft and well-defined, transversely wide, proximal and distal head. The shaft is twisted through about 30 degrees, so that the distal end faces backward as well as upward. The proximal head expands into the coronal plane, mostly medially, and is very wide, almost half of the length of the entire bone–the ratio of the medio-lateral width of the humeral head to the entire length of the bone is about 0.54 (Figs. 1A–1B; ZPAL AbIII/1175, 2627). Its articulation surface is convex and it is covered by multiple irregularly arranged tubercles of random size, which indicate the presence of a well-developed cartilaginous cap (ZPAL AbIII/2627). The medial process of the proximal head forms an internal tuberosity (it is almost as robust as the articular thickening). It is separated from the main articulation surface by a pronounced indentation (ZPAL AbIII/1175, 2627). Below the articulation surface of the proximal head the shaft gently slopes forming a short neck. Under the medial process the neck forms a thin lamina. The deltopectoral crest is well-developed. It starts below the level of the articulation surface of the humeral head and below the level of the internal tuberosity (Figs. 1A–1B; ZPAL AbIII/1175, 2627). The shaft in the medial/lateral view remains straight throughout its entire length (Figs. 1C–1D). In the dorsal/ventral view, the medial edge of the humerus forms an arch (Figs. 1A–1B). The arch is strongly bent, toward the lateral and distal side, in the proximal portion of the shaft, then fluently transits into almost straight line in the middle portion of the shaft, and bends again, toward the medial and distal side, above the medial epicondyle. The shaft expands laterally, in the regions of the deltopectoral crest and the lateral epicondyle, but the expansion is not as significant as on the medial side (Figs. 1A–1B; ZPAL AbIII/1175, 2627). The wide proximal portion of the shaft is much thinner than the middle and distal section. Its thickness increases gradually from the neck of the proximal head, up to the end of the deltopectoral crest (Fig. 1D). On the dorsal surface of the shaft, next to the deltopectoral crest, there is an elevation, which in the specimen ZPAL AbIII/2627 is ornamented by delicate grooves. On the dorsal side, distal to the elevation, close to the medial edge, an elongated knob is present (Fig. 1B). The ventral side of the shaft in the proximal section is smooth (Fig. 1A; ZPAL AbIII/1175, 2627). The middle section of the shaft, below the deltopectoral crest up to the epicondyles, is close to oval in cross-section and of uniform thickness and width. On the dorso-lateral surface there is a straight furrow that originates in the elevated area next to deltopectoral crest and continues up to the epicondyle (Fig. 1C; ZPAL AbIII/1175, 2627). The distal head of the humerus is much narrower than the proximal head, being about 0.6 the length of the proximal head and 0.25 the length of the entire bone. The entepicondyle is wide. The ectepicondylar groove on the lateral side is fully exposed and deep (Figs. 1A–1C; ZPAL AbIII/257, 1175, 2627). The supinator process is thick and prominent (Figs. 1A–1C; ZPAL AbIII/257). In large specimens the olecranon fossa is present (Fig. 1B). In specimen ZPAL AbIII/257 the olecranon fossa is not fully enclosed as in specimen ZPAL AbIII/1175. In distal view, the distal head is transversely elongated, with pronounced narrowing in its center (Fig. 1F). The articulation surface of the distal head is covered by multiple irregularly arranged tubercles of random size, which indicate the presence of well-developed cartilaginous cap (ZPAL AbIII/257, 2627).

Ulna

The ulna of S. olenkae (Figs. 2 and 3) represented by the specimen ZPAL AbIII/2407, 3349/1, 3349/2, 1100/1, 1179, 2014, and 3351, is straight and dorso-ventrally flattened. The proximal portion of the shaft is triangular in cross-section and it is wider and thicker than the middle and distal portion. The olecranon process is long (around 0.2 of the total ulna length in the specimen ZPAL AbIII/2407 and 3351; Figs. 3A–3D), but it was probably even longer as on its top there is a tubercular area (Fig. 3B, 3D–3E), which indicates the presence of well-developed apical cartilage (ZPAL AbIII/2407, 2014, 3351). The tubercles are of different size and are irregularly arranged. The articular surface for the humerus falls rapidly behind the tubercular area of olecranon process towards the coronoid process, where it becomes almost flat (Figs. 3A–3B; ZPAL AbIII/2407, 2014, 3351). The coronoid process is elongated medially and prominent. The articular surface for the radius (on the dorsal side, below the coronoid process) is elongated and crescent-shaped (Fig. 3A; ZPAL AbIII/2014, 3351). The area below the olecranon process is covered by an irregularly arranged and strongly marked series of pits and grooves (Figs. 2E, 3A–3B, 3D; ZPAL AbIII/2014, 2407, 3351). In crocodiles, this area is the insertion point for the triceps muscle (Meers, 2003). The presence of strongly marked structures in this region suggests that the triceps muscle must have been well-developed in S. olenkae. There is another small ornamented area (with an ornamentation of small pits and grooves) on the dorsal side of the ulna, in the proximal part of the shaft, distal to the articular surface for the radius (Figs. 2E, 3A; ZPAL AbIII/2014). Its ornamentation consists of small pits and grooves. The shaft in the middle section forms two parallel, almost flat surfaces (on the dorsal and ventral side) and (in dorsal/ventral view) slightly tapers symmetrically towards the distal end of the ulna, where a delicate neck can be distinguished (Figs. 3A–3B; ZPAL AbIII/1100/1, 1179, 2014, 2407, 3351). The lateral edge of the shaft in the middle section is smooth and rounded (Fig. 3D). The medial edge of the shaft, in the middle section, ends with a furrow that initiates below the articular surface for the humerus and the radius, continues throughout the middle portion of the shaft and weakens towards the distal end of the ulna (Fig. 3C). The cross-section of the shaft, in the upper part of the middle section, is similar to an irregular pentagon with two parallel sides (dorsal and ventral surfaces of ulna) and sharp edge oriented medially (where the furrow is). The shape of the cross-section changes towards the distal end of the ulna and becomes more oval. On the dorsal side of the ulna, in the middle section of the shaft, two other well marked longitudinal furrows can be recognized (Figs. 2E, 3A). The first furrow on the dorsal surface, situated next to the lateral edge of the bone, continues almost through the entire length of the shaft and is arched laterally in dorsal view. The other one, situated next to the medial edge of the bone, is straight and continues only through the middle section of the shaft. The ventral surface of the ulna forms a single flat plane with a small longitudinal depression in the middle (Fig. 3B). The plane continues through the proximal and the middle section of the shaft. In the distal part, distal to the delicate neck, the shaft slightly expands and gently twists (Figs. 3C–3D). In distal view, the shape of the surface for the wrist bones is oval (Fig. 3F; ZPAL AbIII/2407, 3349/1). Similar to the proximal end, the distal end of the ulna was also partially cartilaginous. Its bony surface is covered by irregularly distributed tubercles, although much smaller than those in the region of the olecranon process (ZPAL AbIII/2407, 3349/1, 3351). Distal to the neck on the ventro-medial side, there is a prominent oval ornamented area (Figs. 3A, 3C; ZPAL AbIII/3351). Its ornamentation consists of small, irregularly arranged pits and grooves.

Radius

The radius (Fig. 4) represented by the specimen ZPAL AbIII/1628, 2407, 2016/2, 2016/4, 3322, 3349/1, 3349/2 is shorter than the ulna and longer than the manus. The ratio of length between the radius and the ulna in the specimen ZPAL AbIII/2407 is 0.85 (Figs. 2A–2D). The shaft is straight and of almost uniform thickness throughout its entire length, except for the proximal or distal ends, where it expands in a funnel-like manner (Figs. 4A–4B). In dorsal-ventral view the proximal and distal ends are of the same width (Figs. 4A–4B; ZPAL AbIII/3322). The articulation surface for the humerus is orientated perpendicular to the shaft (Figs. 4A–4B) and in proximal view it is semi-rounded (Fig. 4E). Its surface is almost flat, with a tiny depression in the centre and a small process at the lateral edge. The articulation surface for the ulna is crescent-shaped and extends longitudinally (Fig. 4B, E; ZPAL AbIII/3322). The middle portion of the shaft is semi-square in cross-section. Two corresponding, sharp, well-marked furrows are present on the dorsal and the ventral side of the shaft (Figs. 4A–4B; ZPAL AbIII/3322). Both furrows are s-shaped, which gives the impression that the shaft is twisted helically. Two furrows of similar pattern, but much less distinct, are also present on the lateral and the medial side of the shaft (Figs. 4C–4D). The bony surface of the articulation area with the carpus is semicircular in distal view (Fig. 4F; ZPAL AbIII/2106/2, 2407, 3322). Its surface is covered by irregularly arranged tubercles (ZPAL AbIII/2106/2), similar to those present on the olecranon process of the ulna and the humeral heads, but smaller. The presence of the tubercles indicates a well-developed cartilaginous finish.

Carpus

The carpus consists of at least four bony elements, probably oriented in two rows (Fig. 6E). The largest carpal bone is the fused radiale and intermedium (Fig. 5), present in the spec. ZPAL AbIII/2407, 3349/1, 3349/2, 2071, which probably forms the proximal row with the narrow, square-shaped ulnare, present in the specimen ZPAL AbIII/2407, 3349/1, 3349/2, and 2071. The second row consist of at least two bony distal carpals (present in the specimen ZPAL AbIII/2071, 2407 (only one), 3349/1, and 3349/2), among which one is larger and crescent-shaped and the second one much smaller and pea-shaped.

The fused radiale and intermedium (Fig. 5) is a thick bone, elongated in the coronal plane. Proximally it articulates with the radius and partially the ulna, and is adjacent distally to metacarpals I, II, III, and partially IV (Fig. 6; based on ZPAL AbIII/2407, 3349/1). Książkiewicz (2014) described a single specimen (UOBS 02609), in which the radiale and the intermedium are separated, but he suggested that the bone is probably broken. The radiale and intermedium are joined together by a suture (Figs. 5M–5N, 5R–5S; ZPAL AbIII/2071, 3349/1). The suture is less pronounced, or totally fades, in the ventral and middle portion of the bone, which suggests a continuous process of fusion of both elements. It seems that the two bones initially ossified separately and fused later in ontogeny. For the purpose of further description, the suture will be used as reference point dividing the bone in two sections: the one corresponding to the radiale and the second corresponding to the intermedium. In proximal-distal view the section corresponding to the radiale is semi-oval or rounded and the section corresponding to the intermedium is rectangular (Figs. 5M–5N). In this views, the section corresponding to the radiale is much larger than the section corresponding to intermedium. In dorsal-ventral view the shape of the fused radiale and intermedium is close to rectangular (Figs. 5Q–5R). On the proximal side of the bone, at the section corresponding to the radiale, the surface forms a prominent ridge surrounding the center of the section dorsally and medially (Fig. 5M; ZPAL AbIII/2071). Farther, on the proximal side at the section corresponding to the intermedium, a modest but distinct elevation is present (Fig. 5M). The elevation originates at the middle of the proximal articulation surface, and continues towards the dorsal edge of the bone forming a process-like structure. On the distal side, at the section corresponding to the radiale, there is a large, prominent tuber (Figs. 5N–5O; ZPAL AbIII/2071). The tuber originates in the middle of the articulation surface, and continues towards the distal edge, covering a little less than half of the section corresponding to the radiale (Fig. 5N). Excluding the tuber region, the articulation surface of the distal side of the fused radiale and intermedium forms an almost uniform plane, with a modest elevation close to the dorso-medial edge and in the section corresponding to the intermedium (Fig. 5N; ZPAL AbIII/2071). On the dorsal surface of the fused radiale and intermedium, two distinct areas are visible, one at the section corresponding to the radiale, and one at the section corresponding to intermedium (Figs. 5Q, 6C; ZPAL AbIII/2071, 3349/1). The area in the section corresponding to the radiale is flat with a small depression in its central part. It continues through the medial and part of the ventral side of the section. The area in the section corresponding to the intermedium forms a deep depression. Opposite to it, on the ventral side of the section corresponding to the intermedium, there is another area that forms depression but it is smaller and shallower (Fig. 5R; ZPAL AbIII/2071). The lateral side of the fused radiale and intermedium forms one square-shaped articulation area with a prominent tuber in the distal-dorsal corner (Fig. 5P).

The ulnare is a cube with square bases and rectangular sides. The width of the sides is half the width of the bases. It was probably arranged in one line with the fused radiale and intermedium, with the bases oriented proximally/distally (based on the specimen ZPAL AbIII/2407, 3349/1). The bases are almost flat, with slight depressions in their centres.

The carpus consists of at least two other (probably distal) carpals. One of them (larger) is elongated and lunar-shaped, and the other one is about five times smaller and pea-shaped. These are probably distal carpals III and IV, and based on the specimens ZPAL AbIII/2407 and 3349/1 they are arranged adjacent to metacarpal III in the second (distal) row of carpals.

The elongated lunar-shaped bone in specimen ZPAL AbIII/2071, alternatively may be interpreted as a pisiform, based on the comparison with modern scratch-digging mammals, such as badger (Hildebrand, 1988). If true, it should be oriented in one line with ulnare. Presence of long pisiform is characteristic for digging mammals (Hildebrand, 1988).

Metacarpals

The metacarpals (Fig. 6E) present in the specimen ZPAL AbIII/2071, 2102, 2407, 3349/1, 3349/2 and are robust, relatively short, wide and dorso-ventrally flattened. Their bases are wider than the heads. Their shafts taper towards the distal ends. They match and partially cover each other in dorsal view. Metacarpals II, III, and IV are of similar shape and length, and are noticeably longer than metacarpals I and V, which are about 0.75 their size (based on ZPAL AbIII/2071, 2407, 3349/1, 3349/2). Metacarpal I is slightly longer than metacarpal V. The relative length between the metacarpals I, II, and III vary among specimens. For example, in specimen ZPAL AbIII/3349/1, metacarpal IV is the longest, but in specimen ZPAL AbIII/2407 the longest is metacarpal III. Metacarpal I is the most robust, and metacarpal V the most gracile. The relationship of robustness can be described as I<II<III<IV<V. The distal articulation surfaces of the metacarpals are slightly asymmetric, each with a larger tuber on the medial side. Metacarpal I is also the widest among the metacarpals (Fig. 6E; ZPAL AbIII/2071, 2407, 3349/1, 3349/2). Its shaft is flat dorsally and rectangular in cross-section. On the ventral surface it has a depression for the subsequent metacarpal. Metacarpals II, III, and IV are of similar shape (Fig. 6E; spec. ZPAL AbIII/2071, 2102, 3349/1, 3349/2). Their shafts are triangular in cross-section. Like in metacarpal I, there is a depression on their dorsal surface for the subsequent metacarpal. The shape of most gracile metacarpal V differs between specimens. In specimen ZPAL AbIII/3349/1 and 3349/2 it is wide and flat. Its width is almost uniform throughout the entire length, and the base and the head are not distinct. In the specimens ZPAL AbIII/2071 and 2407 the base and the head are much wider than the shaft and well-developed. The shaft is slender and oval in cross-section.

Phalanges

The phalanges (Fig. 6E) are present in the specimen ZPAL AbIII/2407 (Figs. 2A–2D)—all phalanges of the first row, ZPAL AbIII/3349/1 (Figs. 6A–6D)—all phalanges of the first and second digit, broken 1st row phalanx of the third digit, the four phalanges of the fourth digit and 1st row phalanx of the fifth digit, ZPAL AbIII/3349/2—all phalanges of the first row, ZPAL AbIII/2071—in the right hand all phalanges of the first, second and third digit, in the left all phalanges of the first and second digit, and ZPAL AbIII/257, 3352, 3353—isolated phalanges. The probable phalangeal formula for S. olenkae is 2-3-4-5?-3? (Fig. 6E), based mostly on the specimens ZPAL AbIII/3349/1 and 2071. The number of the phalanges for the first three digits is certain, because they terminate with claw-like unguals preserved in the first two digits of ZPAL AbIII/3349/1 (Figs. 6A–6D), and first three digits of the right manus and the first two of the left of the spec. ZPAL AbIII/2071. In ZPAL AbIII/3349/1 the phalanx of the fourth row of the digit IV ends with an articulation surface, which indicates the presence of another phalanx or an ungual. However because of small size of the preserved fourth phalanx, it is unlikely that there was more than one element following it (Figs. 6C–6D). As for the fifth digit, the size of the first row phalanx, compared to the size of other phalanges in ZPAL AbIII/3349/1, suggest that at least two phalanges and an ungual were present. In UOBS 02834 described by Książkiewicz (2014), containing hand elements preserved in articulation, two phalanges of the fifth digit are present. The phalanges are dorso-ventrally flattened, short, and wide (Fig. 6E). The phalanx base is always wider than its head. The shaft narrows towards the distal end. It is rounded at the dorsal side and flat at the ventral side. Both the base and the head are slightly asymmetric. In the heads the tuber on the internal side is always larger than that on the external side and a depression is present on the articulation surface. This feature is more pronounced in the phalanges closer to the metacarpals. Interlocking phalangeal articular surfaces are wide and they extend deep into the shaft. The grooves for ligament attachments are well marked in all phalanges despite their size (Fig. 6E).

As mentioned above, claw-like unguals are present on at least the first three digits (Fig. 6E; based on ZPAL AbIII/2071, 3349/1). The size of the claw-like unguals decreases in more lateral digits. The largest claw-like ungual of the first digit is about one-third longer and more massive than that of the second digit, and the second is longer and more massive than the third one in the same manner (ZPAL AbIII/2071). It can be inferred from the size of the preserved phalanges that other claw-like unguals (if present) kept this tendency. The unguals are laterally compressed, with sharp edges at the top and bottom sides (similar to claws of, for example, armadillos, pangolins, badgers) (Hildebrand, 1988). They are tear-shaped in cross section but asymmetric, with a depression on the medial surface (ZPAL AbIII/2071). Longitudinal grooves for ligaments are well-marked on both lateral and medial surfaces. Almost the entire surface of the unguals is covered by tiny and very densely distributed perforations (Figs. 6A–6B, 6E; ZPAL AbIII/2071, 3349/1). Similar texture can be observed on the bony parts of horns, for example, in modern bovids or in the unguals of armadillos (personal observation, Hildebrand, 1983). It indicates the presence of a well-developed keratin sheath. Considering the general morphology of the whole hand it is probable that very small claw-like unguals were present on the fourth and the fifth digits. In some species of modern digging animals such as armadillos, pangolins, or moles often one or several digits enlarge and take a blade-like shape useful for a scratch-digging, while the others are considerably smaller, reduced or absent (Beddard, 1902; Hildebrand, 1988).

Dermal skeleton

Probably the entire forearm of S. olenkae was covered by numerous appendicular osteoderms (Figs. 6A–6B, 6F). They are preserved in association with the arm, carpus and manus elements in ZPAL AbIII/2071, 2407, 3349/1 and 3349/2. In the specimens ZPAL AbIII/2407, 3349/1, and 3349/2 they are accumulated mostly on the dorsal side of the hand. In the spec. ZPAL AbIII/2407 there is also a large cluster of osteoderms, previously recovered in front of the distal end of radius and ulna, but removed during preparation (not illustrated). The appendicular osteoderms are generally flat, plate-like structures, semi-round to semi-oval in dorsal/ventral view. The edges of the osteoderms can be regular and smooth (mostly in the larger scutes) or irregular and ridged (more often in smaller ones). They are of various sizes, the largest are about 2.5 cm in diameter (ZPAL AbIII/2407, separated cluster of osteoderms), the smallest around 0.5 cm in diameter (several osteoderms in ZPAL AbIII/2407, 3349/1, and 3349/2). They are ornamented on the dorsal surface. The ornamentation consists of delicate grooves and depressions. The ventral surfaces of the appendicular osteoderms are smooth. On both dorsal and ventral side of osteoderms, tiny openings for blood vessels are present.

The appendicular osteoderms in ZPAL AbIII/2407, 3349/1 and 3349/2 are significantly displaced in regard to their in vivo position, likely because of transportation and early diagenesis processes. However, some general observation can be made. The appendicular osteoderms cover the entire dorsal surface of the carpus and the manus (Fig. 6F). The osteoderms that occur in this area are of various sizes and shapes, but generally they are semi-round and small to medium (with diameter about 0.5 to 1.5 cm). In more flexible regions, such as the carpus and joints of the digits, the osteoderms are smaller, but in greater number than in more static regions, such as above the metacarpal shafts, where they are larger but less numerous. As for the region of the arm, it is likely that it was entirely covered by appendicular osteoderms in the manner restored for Aetosaurs ferratus or T. coccinarum (Schoch, 2007; Heckert et al., 2010). The osteoderms of the arm are larger (up to around 2.5 cm in diameter). The lack of articulation structures on the surfaces of the appendicular osteoderms suggests that they did not overlap with each other (unlike the rectangular osteoderms presents on the back of the animals), but rather lay one next to another like the scutes of modern crocodiles and alligators (e.g., Grigg & Gans, 1993).