Megamphicyon giganteus (Schinz, 1825)

Siliceo, Gema, Morales, Jorge, Antón, Mauricio & Salesa, Manuel J., 2020, New fossils of Amphicyonidae (Carnivora) from the middle Miocene (MN 6) site of Carpetana (Madrid, Spain), Geodiversitas 42 (15), pp. 223-238 : 225-232

Megamphicyon giganteus (Schinz, 1825)

DIAGNOSIS. — Amphicyonid similar to Amphicyon major, but with the following differences: much larger size; the M1 is much more mesiodistally elongated, with very high buccal cusps, and with the mesial and distal margins inflated at the level of the paraconule and metaconule; M2 with lower buccal tubercles, protocone formed of a single ridge, connected to the paraconule; vertical or absent metaconule, generally not connected to the protocone; lower canine shorter and stout; m1 with curved walls and the lingual part of the talonid narrower and elongated; m2 high, with strong protoconid, and metaconid strongly dominating a well-formed talonid, presence of a vestigial paraconid; very long m3 ( Ginsburg & Antunes 1968).

HOLOTYPE. — Left M1 from the site of Avaray (Loir-et-Cher), figured by Cuvier (1824: pl. 193, fig. 20), Mayet (1908: fig. 24, 68, pl. VII) and Kuss (1965: fig. 42), housed at the collections of the Musée d’Orléans.

MATERIAL EXAMINED. — The list of fossils of M. giganteus studied here is as follows (catalogue numbers in parentheses): left hemimandible preserving the dental series p3-m3 (08.17.4215 A 13-373) ; twelfth or thirteenth thoracic vertebra ( T 12/ T 13) (08.17.11003A), first lumbar vertebra (L1) (08.17.11003C), Sacrum (08.17.10203 FC), left ulna (08.17.10199 FC), right radius (08.17.10200FC-1), fragment of proximal half of left radius (08.17.10201 AFC). Left Mc V (08.17.20989 FC), distal fragment of right femur (08.17.10271 DFC), left tibia (08.17.10270-6A-FC), and left Mt IV (08.17.99728-538) .

DESCRIPTION OF HEMIMANDIBLE AND LOWER DENTITION 08.17.4215 A 13-373: left hemimandible preserving the dental series p3-m3, as well as two possible alveoli for p1 and p2 ( Fig. 1 A-C); both the mandibular ramus and symphysis are broken. There is a short diastema between p3 and p4, and

a longer one mesial to p3. The preserved mesial border corresponds to the distal surface of the lower canine alveolus. On the labial face there is a well-developed mandibular foramen located at the level of the diastema separating the lower canine and p1; a second foramen is located just below the p3. The mandibular symphysis shows a very rough lingual surface, with a marked genial tuberosity; it is distally developed until the diastema of p2-p3. The p3 has a relatively small crown, although it has a long talonid showing a small cuspid, somewhat higher and more marked than the rounded mesial accessory cuspid. The p4 is relatively large, mesially thickened, and with a high and large distal accessory cuspid; its basal cingulum is very reduced, and there is almost no trace of a talonid. The m1 is buccolingually flattened, with a short paraconid in comparison to the protoconid, virtually without a lingual valley separating the two cuspids; the metaconid is very close to the distal margin of the protoconid; the talonid is almost completely occupied by a large hypoconid, lingually separated from a very reduced entoconid. The m2 is relatively large, with a high and straight protoconid occupying half of the tooth; there is no paraconid, and the metaconid is distally displaced, as in the m1, although it is relatively larger; the mesial trigonid valley is very reduced, and thus the mesiolingual wall is only composed of the protoconid and the metaconid, unlike most Amphicyonidae , which have a mesiolingual wall composed of the metaconid and a very reduced paraconid; the buccal wall of the protoconid is quite vertical, without buccal expansion; the talonid is large and, similarly to m1, it is mostly occupied by the hypoconid, although the entoconid is more developed than that of the latter tooth. The m3 is also relatively well developed; the trigonid is wide, with the protoconid and metaconid being of similar size and located the same distance from the mesial border of the tooth; the distal cristid of both the protoconid and the metaconid extends distally until contacting the hypoconid and talonid, respectively, delimiting a wide and shallow central valley.

Comparisons

In overall view, the teeth of Amphicyon major from Sansan are smaller than those from Carpetana, both samples showing a set of morphological differences ( Table 1 ). The p4 of A. major is relatively smaller and lower than that from Carpetana, with a narrower mesial part. The m1 of A. major is more primitive, with a well-developed lingual valley separating the paraconid from the protoconid, a relatively large metaconid, and a large and high hypoconid that only occupies the buccal half of the talonid. Concerning the m2, the differences can be related to a greater degree of specialization of the form from Carpetana : A. major retains, as primitive features, a lingual wall composed of the metaconid and paraconid, a mesial valley in the trigonid, a marked buccal expansion of the protoconid wall, and a relatively wider talonid, with a relatively smaller hypoconid.

Although there is no upper dentition in the Carpetana sample, the locality of Arroyo del Val (Zaragoza), of similar age, has yielded a nice sample of P4, M1 and M2 of Megamphicyon giganteus ( Peigné et al. 2006) that can be used for comparison with the sample of A. major from Sansan. The P4 from Arroyo del Val shows a more developed parastyle than those from Sansan and a wider, lower and more distally located protocone; the M1 has a more developed buccal wall, larger and lower paracone and metacone, and a more centrally located protocone; and the M2 is buccolingually wider, and the protocone is more centrally located.

DESCRIPTION OF POSTCRANIAL ELEMENTS

Vertebrae

Although several vertebral fragments were found in Carpetana, only two vertebrae can be certainly assigned to an amphicyonid. These two vertebrae have most of the processes broken, although the preserved morphology indicates that one of them (08.17.11003A) is probably one of the last thoracic vertebrae (T12 or T13) ( Fig. 2 F-I), whereas the other one (08.17.11003C) can be identified as the first lumbar vertebra (L1) ( Fig. 2 D-E). The size and morphology of these vertebrae fit well with both the relative size and morphology of other known amphicyonid vertebrae, such as A. major or Magericyon anceps Peigné, Salesa, Antón & Morales, 2008 ( Ginsburg 1961a; Argot 2010; Siliceo et al. 2019). The thoracic vertebra of the amphicyonid from Carpetana shows some of the distinctive features of the last thoracic vertebrae (T12 and T13), which typically exhibit a similar morphology to that of the lumbar series. Thus, the right cranial articular process is medially oriented, with a slightly concave surface, and is located at the base of a dorsally developed mammillary process. The two caudal articular processes are also preserved: they are laterally oriented and show slightly convex surfaces. This thoracic vertebra (08.17.11003A) shows a large fovea costalis cranialis on its right side (the left one is barely visible because of the poor preservation) for the articulation with the head of the rib, but lacks both the transverse processes and the fovea costalis transversi. The other vertebra from Carpetana (08.17.11003C) shows a poorer preservation than the thoracic one, but it can be identified as the L1 based on the presence of the basal portion of the costal processes; one of these processes preserves its caudolateral border, suggesting a small size for this costal process, and thus indicating that this vertebra is probably L1, or, less probably, L2. One of the caudal articular processes is also preserved, showing a lateroventral orientation of its articular surface. The base of a strong left accessory process is also visible, showing the typical morphology of the L1.

Sacrum

A complete amphicyonid sacrum from Carpetana is preserved (08.17.10203FC) ( Fig. 2 A-C). It is composed of three fused sacral vertebrae, like that of most extant carnivorans, except ursids, which have a sacrum that is usually composed of five vertebrae. The sacrum from Carpetana is sub-rectangular, with its maximum mediolateral width at the level of the cranial margin. The three sacral vertebral bodies are of similar size, but the first sacral vertebra is markedly wider. Only the last spinous process is present. It is well-developed and shows a thickened dorsal tip. The cranial articular processes are laterocranially projected and their articular surfaces are dorsomedially oriented. The caudal articular processes are laterally oriented, their articular surfaces being markedly convex. The sacral wing, formed by the transverse processes of the first sacral vertebra, is ventrally projected, has thickened borders, and shows a rough lateral surface for the articulation with the os coxae. The lateral sacral crest (the fused transverse processes of the second and third sacral vertebrae) is not laterally expanded; it has a thick and rough border, but at least one of its borders seems to be broken, so the actual expansion of this crest is not easy to assess. The caudal extremes of the lateral sacral crest (transverse processes of the last sacral vertebra) are broken. The intermediate sacral crest is rough, with the tuberosities of the second sacral vertebrae being the most marked ones. Along these crests, several fascicles of the

m. sacrocaudalis dorsalis lateralis and mm. intertransversarii dorsales caudae are attached, as well as the strong lig. sacrotuberalis, which ends on the ischiatic tuberosity. On the ventral side of the sacrum (pelvian facies), the lines between the sacral vertebrae (= lineae transversae) are present. Along this ventral surface, the caudal muscles, m. sacrocaudalis ventralis lateralis and m. sacrocaudalis ventralis medialis are attached.

The overall morphology of the sacrum of the amphicyonid from Carpetana shows some similarities with those of large felids such as Panthera leo, as it is composed of three vertebrae and lacks the marked distal narrowing shown by canids. Nevertheless, it differs from that of felids in some aspects: the spinous processes are stronger in the amphicyonid, they have thickened dorsal tips, and their overall shape is more rectangular. The morphology of 08.17.10203FC suggests that the amphicyonid from Carpetana probably had a long and muscular tail, a typical feature of the family Amphicyonidae ( Argot 2010; Siliceo et al. 2019). For example, the lack of reduction in the size of the sacral vertebrae indicates a relatively high number of caudal vertebrae, whereas the presence of strong spinous processes in the sacrum is indicative of strong and well-developed caudal muscles.

Radius

There are two radii of M. giganteus from Carpetana; one of them is almost complete (08.17.10200FC-1) ( Fig. 3A, B ), whereas the other one is a fragment of the proximal half that only preserves a small part of the proximal articular surface (08.17.10201AFC). Although the proximal and distal epiphyses are damaged in both specimens, they still preserve some interesting features. In 08.17.10200FC-1, the most complete

of the two specimens, the proximal epiphysis only preserves a small portion of the craniomedial border, and the cranial half of the proximal articular surface (= fovea capitis). The craniomedial border shows a small eminence, proximally projected, also observed in other amphicyonids, such as A. major or Ma. anceps ( Argot 2010; Siliceo 2015). The proximal surface is mainly concave, but there is a small, more or less flat medial portion, which is medially inclined. Along the diaphysis of the radius there are several muscle attachment surfaces; the most marked is the bulky and proximodistally extended radial tuberosity on the proximal portion of the caudal face of the radius, which is the attachment area for the m. biceps brachii. Close to this tuberosity, and laterocranially located, there is a very rough surface for the attachment of the lateral collateral

ligament running from the lateral epicondyle of the humerus. Distally to the radial tuberosity, along the caudal face of the diaphysis, the marked rugosities of the interosseous border can be observed, which is the attachment area for the strong interosseous ligaments described above. Both the radial tuberosity and the rugosities of the interosseous border are very well developed in amphicyonines, more than in most extant carnivorans (felids, canids, ursids or mustelids). Distally to the interosseous border, and caudomedially located there is a well-defined triangular area, delimited proximally by the most distal of the rugosities of the interosseous border, and distally by the distal epiphysis. This is the attachment area for the m. pronator quadratus.

In the distal epiphysis, the articular surface for the scapholunar is elliptical and concave, and the styloid process is moderately distally projected; proximally to the styloid process there is a broad and rough tuberosity for the attachment of the m. brachioradialis. The cranial and lateral surface of the distal epiphysis is damaged, and thus, the articular surface for the ulna and the muscular grooves of this area are not visible.

Ulna

The ulna of M. giganteus from Carpetana (08.17.10199FC) is long, stoutly built, and gently curved caudally ( Fig. 3C, D ). The proximal and distal epiphyses are partially broken. The olecranon is partially damaged, and thus the morphology of the tuber olecrani and the proximal tubercles cannot be described. However, the cranial profile of the olecranon is partially preserved and it seems slightly caudally inclined; also, although it is not possible to know its actual length, the preserved portion of the olecranon is similarly developed as in other amphicyonids such as A. major, so the length would probably be comparable. Among amphicyonine amphicyonids, there are differences in the length and morphology of the olecranon: A. major, M. giganteus, Amphicyon galushai Hunt, 2003 and Ysengrinia americana (Wortman, 1901) show a more developed and caudally inclined olecranon than that of Pseudocyon sansaniensis Lartet, 1851, Amphicyon ingens Matthew, 1924 or Magericyon anceps, with the latter showing a shorter and not caudally inclined olecranon with its short cranial border almost straight in lateral view ( Ginsburg 1961a; Ginsburg & Antunes 1968; Hunt 2002, 2003; Argot 2010; Siliceo 2015).

The lateral side of the proximal epiphysis of the ulna is markedly concave and is mostly occupied by the attachment of the m. anconeus. The crested caudal border of this side is the attachment area for the lateral branch of the m. triceps brachii. The incisura trochlearis and incisura radialis are partially damaged and only the caudolateral part of the incisura radialis is preserved and laterally projected. The incisura trochlearis only preserves its proximal portion, which constitutes the anconeus process.

Along the diaphysis, the most remarkable feature is the large, and markedly rugose interosseous border, a laterocranially developed surface, homologous with a similar surface on the caudal side of the radius diaphysis. This border occupies most of the lateral face of the diaphysis of the ulna, and is the attachment area for the membrana interossea and lig. interosseous, strong ligaments connecting the radius and ulna. Along this interosseous border, the attachment areas for several muscles, such as the m. abductor digiti I longus and the m. extensor digiti I and II, are located.

McV

The left Mc V of the amphicyonid from Carpetana (08.17.20989FC) ( Fig. 4 A-F) is even larger than the Mc V from Pontlevoy assigned to A. giganteus by Ginsburg & Antunes (1968). The total length of the piece from Carpetana is 84.06 mm, whereas that from Pontlevoy is only 79.50 mm. The proximal base of 08.17.20989FC is markedly wide, showing a rectangular, dorsopalmarly elongated and convex proximal articular surface with the unciform. Palmar to this surface there is a small, rounded tuberosity, and on the lateral side of the proximal base, there is a large, bulky protuberance for the attachment of the m. extensor carpi ulnaris. The articular surface for the Mc IV is dorsopalmarly developed on the medial face of the proximal base. The body of the Mc V is slightly dorsopalmarly flattened, with a fairly smooth surface, without irregularities. Distally, the Mc V is mediolaterally wide, with an asymmetric distal articular head. Close to the distal head on the palmar and lateral faces there are two marked tuberosities: the palmar one is smaller and is the attachment area for the m. adductor digiti V, whereas the lateral one is laterally projected and is the probable attachment area for the m. abductor digiti V. The general morphology of this metacarpal is similar to that of other amphicyonines such as A. major ( Argot 2010) and Y. americana ( Hunt 2002).

Femur

Only the distal epiphysis of the right femur 08.17.10271DFC is preserved ( Fig. 5 A-E). Although it is partially damaged, the distal articular surface (femoral trochlea and lateral and medial condyles) is present, and the rough and slightly inflated surface of the lateral and medial epicondyles is observable. The femoral condyles are similar in width, with the medial one showing a more convex surface than the lateral one. It is not possible to know if the condyles had a similar degree of caudal or distal projection due to a fracture affecting the lateral condyle. The two condyles are separated by a deep, broad and symmetric intercondylar fossa. On the caudal side of the epiphysis, just above the condyles, there are two fossae for the sesamoids of the lateral and medial tendons of the m. gastrocnemius ( Barone 2010a, b). The femoral trochlea shows a quadrangular shape, its proximodistal length being slightly greater than its lateromedial width. The trochlear surface is concave but not very deep, with prominent lateral and medial borders. In distal view the distal epiphysis is approximately as wide as high (w = 92.53 mm; h = 91.13 mm), with a greater craniocaudal development of both trochleae and femoral condyles than those of ursids. The femoral condyles also are more proximodistally developed than those of ursids. In overall view, the distal epiphysis of the amphicyonid from Carpetana shows an intermediate morphology between those of felids and ursids, similarly to typical amphicyonines such as A. major ( Argot 2010).

Tibia

An almost complete left tibia (08.17.10270-6A-FC) ( Fig. 6 A-F) from Carpetana is preserved, with only the medial portion of the distal epiphysis and the malleolus missing. This tibia is similar to that of other amphicyonines, such as A. major, in not being particularly robust, in contrast to that of Y. americana ( Hunt 2002), and showing a marked lateral curvature. The proximal epiphysis is triangular in proximal view, and it is mainly occupied by the lateral and medial tibial condyles (for the articulation with the femoral condyles), which are slightly caudally inclined; there is also a marked cranially projected tibial tuberosity. The cranial projection of the tibial tuberosity of the Carpetana tibia is, like that of other amphicyonines, more similar to that of felids than to that of ursids. It is rough, with a marked scar for the patellar ligament (the distal portion of the m. quadriceps femoris joining the patella and tibia) on its distal border. Distally from the tibial tuberosity, the tibial crest for the attachment of m. sartorius, m. gracilis and m. semitendinosus is developed until the middle of the cranial border of the diaphysis. On the caudolateral margin of the lateral condyle, there is an elliptical facet for the articulation with the fibula, and cranially to it there is a rough surface for the attachment of the m. fibularis longus. The lateral face of the proximal epiphysis is occupied by the tibial fossa, which shows a rough and markedly concave surface. On the cranial portion of this fossa, close to the tibial tuberosity, there is the attachment area for the m. tibialis cranialis.

The medial border of the distal epiphysis of the tibia from Carpetana is damaged, and in consequence, the medial malleolus is lacking. The distal surface has its craniocaudal axis slightly medially oriented, and shows two concave grooves separated by a convex surface for the articulation with the talus. On the cranial and caudal borders of the epiphysis, there are two distal projections, coinciding with the central separation of the distal grooves. The concavity of this distal articulation surface is, like in other amphicyonines, more similar to that of large felids than to that of ursids; that of the latter group has a shallower articular surface for the talus. On the lat-

eral margin of the distal articulation surface there is a small semicircular facet for the articulation with the distal fibula. The cranial and lateral borders of the epiphysis are markedly rough and show several small tuberosities and foramina, which are probably scars of the tarsocrural articular capsule. The caudal surface of the diaphysis shows the typical pattern of muscle scars seen in other amphicyonines, composed of two obliquely developed lines that delimit the area of several muscles. These lines define a marked and narrow groove for the m. flexor digitorum medialis and m. tibialis caudalis on the proximal half of the caudal surface; the medial line (the popliteal line) demarcates a triangular area, proximally and medially, for the attachment area for the m. popliteus; and finally, the lateral area constitutes the attachment surface for the m. flexor digitorum lateralis ( Barone 1967, 2010a, b; Evans 1993). Distally, both lines end on the medial border, in the middle of the diaphysis.

Mt IV

The Mt IV of M. giganteus from Carpetana (08.17.99728- 538) is fairly complete ( Fig. 7 A-F). The proximal base is dorsoplantarly lengthened, showing a rectangular and convex facet with a slightly proximally projected medioplantar vertex for the articulation with the cuboid. On the medial face there are two facets, the proximodorsal one being markedly larger than the proximoplantar one. The proximal margin of both facets articulates with the ectocuneiform, whereas the rest of the margin, and most of the surface of the facets, articulate with the Mt III. The two facets are separated by a small rugose fossa where one interosseous ligament attaches. The lateral face of the proximal base is mostly occupied by a wide fossa and by the articulation surface with the Mt V on the proximodorsal portion. On the proximoplantar end there is a marked plantar tuberosity, and distal to that, there is a rough surface for the m. interossei IV.

On the lateroplantar margin of the body there is a rough and proximodistally developed fusiform surface, typical of this metatarsal, which is probably the attachment area for the long plantar ligament ( Barone 2010b). The distal head of the Mt IV is globular as in other metapodials, being just slightly asymmetric.

Figures

FIG. 1. — Fragment of the left hemimandible 08.17.4215 A 13-373 of Megamphicyon giganteus (Schinz, 1825) from Carpetana: A, buccal view; B, occlusal view; C, lingual view. Scale bar: 5 cm.

FIG. 2. — Vertebrae and sacrum of Megamphicyon giganteus (Schinz, 1825) from Carpetana. Sacrum 08.17.10203FC: A, dorsal view, B, ventral view; C, cranial view.First lumbar vertebra (L1) 08.17.11003C:D, caudal view;E, left view. Twelfth or thirteenth thoracic vertebra (T12/T13) 08.17.11003A:F, cranial view;G, right view; H, left view; I, dorsal view. Scale bar: 5 cm.

FIG. 3. — Radius and ulna of Megamphicyon giganteus (Schinz, 1825) from Carpetana. Right radius 08.17.10200FC-1: A, cranial view; B,caudal view. Left ulna 08.17.10199FC: C, lateral view; D, medial view. Scale bar: 5 cm.

FIG. 4. — Left Mc V 08.17.20989FC of Megamphicyon giganteus (Schinz, 1825) from Carpetana: A, proximal view; B, distal view; C, palmar view; D, dorsal view; E, lateral view; F, medial view. Scale bar: 2 cm.

FIG. 5. — Distal fragment of right femur 08.17.10271DFC of Megamphicyon giganteus (Schinz, 1825) from Carpetana: A, distal view; B, cranial view; C, caudal view; D, medial view; E, latera view. Scale bar: 5 cm.

FIG. 6. — Left tibia 08.17.10270-6A-FC of Megamphicyon giganteus (Schinz, 1825) from Carpetana: A, cranial view; B, caudal view; C, medial view; D, lateral view; E, proximal view; F, distal view. Scale bar: 5 cm.

FIG. 7. — Left Mt IV 08.17.99728-538 of Megamphicyon giganteus (Schinz, 1825) from Carpetana: A, dorsal view; B, medial view; C, lateral view; D, plantar view; E, proximal view; F, distal view. Scale bar: 2 cm.