Philander opossum (Linnaeus, 1758)

Flores, D. A., 2009, Phylogenetic Analyses Of Postcranial Skeletal Morphology In Didelphid Marsupials, Bulletin of the American Museum of Natural History 2009 (320), pp. 1-81 : 34-41

Philander opossum , Didelphis , Lutreolina , Chironectes , Thylamys , Monodelphis adusta , Lestodelphys , Marmosops incanus , M. parvidens , and Gracilinanus agilis .

Character 77: Ulna, longitudinal groove in lateral surface for the insertion of Mm. abductor pollicis longus and anconeus: (0) absent; (1) present and shallow; (2) present and notably deep ( fig. 19). In the lateral surface of the ulna there is an area for the insertion of m. anconeus and origin of the m. abductor pollicis longus, both related to the extension of the elbow joint ( Evans, 1993). According to Argot (2001), those muscles are well developed in arboreal forms. However, some arboreal forms, such as Caluromys , do not have a well-developed groove when it is compared to Caluromysiops . The groove in the lateral surface is absent in Thylamys , Monodelphis brevicaudata , M. adusta , and Marmosa mexicana . It is present though not very deep in Glironia , Metachirus , Caluromys , Philander , Didelphis , Lutreolina , Chironectes , Tlacuatzin , Marmosa rubra , Monodelphis theresa , Micoureus , Lestodelphys , Marmosops (except M. parvidens ), Cryptonanus , and Gracilinanus microtarsus . This groove is very deep only in Caluromysiops and Marmosops parvidens , indicating a very developed m. anconeus. Polymorphism is observed in Marmosa murina , Micoureus paraguayanus , and Gracilinanus agilis (coded {01}). Because I observed an intermediate condition in the development of the longitudinal groove in the lateral surface, this character is treated as ordered (0 « 1 « 2) in all analyses.

Character 78: Ulna, lateral extension of the coronoid process: (0) absent; (1) notably extended, beyond the level of the trochlear notch ( fig. 21). The coronoid process on the lateral side appears notably developed only in Monodelphis , Marmosops parvidens , and M. incanus .

Character 79: Radius, lateral compression: (0) radius little or not laterally compressed; (1) radius notably laterally compressed. A laterally compressed radius has been functionally associated to a major flexibility during pronation-supination movements ( Oxnard, 1963; Walker, 1974). The radius is notoriously compressed in some species such as Glironia , Caluromysiops , Caluromys lanatus , Lutreolina , Monodelphis , Marmosa rubra , Micoureus demerarae , Marmosops parvidens , and Lestodelphys .

Character 80: Radius, shape of articular facet for humerus: (0) circular; (1) anteroposteriorly compressed. This character was defined by Horovitz and Sánchez-Villagra (2003: ch. 61). The radial head has some functional implications in pronation-supination movements ( Argot, 2001). In arboreal forms, the circular radial head allows a wide potential range of pronation-supination movement. A radial head craniocaudally compressed is indicative of a more stable radio-capitullum joint, which is observed in scansorial forms, different from the circular shape observed in arboreal forms ( MacLeod and Rose, 1993). The articular facet is anteroposteriorly compressed only in Philander , Marmosa rubra , and Chironectes .

Character 81: Radius, development of the bicipital tuberosity: (0) scarcely marked; (1) very developed ( fig. 22). The bicipital tuberosity of the radius is the site of insertion of m. biceps brachii. According to Argot (2001), the bicipital tuberosity is more developed in arboreal forms. I found a well-developed bicipital tuberosity in Glironia , Caluromys , Caluromysiops , Didelphis , Lutreolina , Marmosops , Lestodelphys , Cryptonanus , and Gracilinanus agilis . On the other hand, the tuberosity is scarcely marked in Metachirus , Philander , Chironectes , Tlacuatzin , Marmosa , Monodelphis , Thylamys , Micoureus , and Gracilinanus microtarsus .

Character 82: Radius, presence of a thin bony plate extended posterolaterally along the diaphysis: (0) absent; (1) present ( fig. 22). Only in Caluromys , Caluromysiops , and Lestodelphys is there a thin bony plate on the diaphysis of the radius. According to Argot (2001), this structure reinforces the origin for the Mm. flexor digitorum profundus and pollicis longus, both important in the prehensility of the manus. Although this bony plate and a prounced curvature of radius have been associated to arboreal habits ( Argot, 2001; Lanyon, 1980), the morphology of the radius in the terrestrial Lestodelphys suggests an important capacity for manipulating prey.

PELVIS AND EPIPUBIC BONES

Character 83: Acetabulum morphology: not laterally expanded; (1) deep, with the dorsal part extended laterally ( fig. 11). The acetabulum morphology is critical in femur flexion ( Elftman, 1929). A shallow acetabular fossa allows a wide range of movements of the femur ( Jenkins and Camazine, 1977; Argot, 2002), especially in abduction, which is related to arboreal habits ( Elftman, 1929; Muizon and Argot, 2003). However, this morphology implies a reduced stability of the joint, because of as it was established for Caluromys ( Muizon and Argot, 2003) , arboreal didelphids pressumably have slow climbing habits. In this context, Metachirus is autapomorphic for this character since it exhibits a deep acetabular fossa, with the dorsal portion laterally extended, which is a specialized morphology for its cursosaltatorial mode of locomotion.

Character 84: Iliac wing forming a large blade: (0) absent; (1) present ( fig. 11). The iliac wing, which forms a large blade, is an autapomorphy only present in Metachirus , which shows a reduced iliac fossa as well. This extension is occupied by a well-developed m. gluteus medius ( Argot, 2002; Taylor, 1974) and is indicative of high activity of this muscle as well as the development of the epaxial musculature ( Maynard Smith and Savage, 1955; Grand, 1983). The morphology exhibited by Metachirus is consistent with the results of Grand (1983) as well, because the lower back musculature of this taxon represents more than 55% of the total epaxial musculature, different from the 25–35% in other didelphid taxa. Although the iliac fossa is also reduced in Metachirus due to the blade shape of the iliac wing, the m. iliacus is well developed ( Argot, 2002; Elftman, 1929).

Character 85: Angle formed by the two posterior rami of ischium in caudal view: (0) 90 ° or scarcely more; (1) less than 90 °. In caudal view, the angle formed by the two rami of ischium can accentuate the extroversion of the ischiatic spine, which is important in the origin of abductors and gracilis muscles ( Elftman, 1929; Argot, 2002). Additionally, the angle has been related to the range of abduction ( Jenkins and Camazine, 1977). As it was stated by Argot (2002), a sharp angle (or an oblique orientation of the ischium) emphasizes the degree of abduction. In the sample analyzed, the two rami of the ischium form an angle of 90 ° in Metachirus , Chironectes , Philander , Didelphis , Lestodelphys , Hyladelphys , and Gracilinanus . In the remaining taxa, the angle is less than 90 °.

Character 86: Presence of an osseous posteroventral extension on the ischium: (0) absent; (1) present ( fig. 23). The posteroventral extension on the ischium is a synapomorphy observed only in the species of Marmosops . This extension seems to increase the area for origin of the hamstring muscles complex (i.e., Mm. biceps femoris, semimembranosus, and semitendinosus), which inserts on the tibia and fibula and contributes to the knee flexion.

Character 87: Development of iliopubic process: (0) absent; (1) present ( fig. 24). This character was described by Horovitz and Sánchez-Villagra (2003: ch. 73). In this area the m. psoas minor is inserted, which originates in the last thoracic and the first lumbar vertebrae, and whose action is to flex the lumbar part of the vertebral column ( Elftman, 1929; Evans, 1993). Although in some specimens it was not be very conspicuous, the process is present in both terrestrial as well as arboreal forms, such as Caluromysiops , Metachirus , Lutreolina , and Thylamys pusillus . Polymorphism is evidenced in Philander frenatus and Chironectes (coded {01}).

Character 88: Epipubic bones, proximal size: (0) short; (1) long. In the same way as in Horovitz and Sánchez-Villagra (2003: ch. 77), I coded short proximal size of epipubic bones when the contact is equal to or less than half of the distance between the pubic symphysis and the point at the anterior edge of the pelvis, whose level is coincident with the middle of the acetabulum. The proximal size of the epipubic bones is long in Caluromysiops , Caluromys , Philander, Didelphius , Lutreolina , Chironectes , Thylamys , Monodelphis , Micoureus demerarae , Lestodelphys , Marmosa (except M. rubra ), and Marmosops . Polymorphism is exhibited only in Micoureus regina . No information about this character is currently available for Thylamys venustus and Philander mcilhennyi (coded ‘‘?’’).

Character 89: Epipubic bones distal shape: (0) straight; (1) curved ( fig. 24). In general terms I founded evidence of sexual dimorphism in the development of epipubic bones (at least those taxa in which a good series was analyzed, see appendix 1), being larger in females of pouchless taxa (as observed by White, 1989). Its shape shows two apparently defined morphotypes across the taxa analyzed. The condition of straight epipubic bones is the most common morphology exhibited in the sample. On the contrary, the epipubic bones are clearly curved in the distal portion in some taxa, such as Caluromysiops , Caluromys , Marmosops parvidens , Chironectes , and Marmosa rubra . No information is currently available about this character for Philander mcilhennyi (coded ‘‘?’’).

Character 90: Ilium shape: (0) straight; (1) with the distal portion barely curved laterally ( fig. 25). In character 84, I considered the general development of the iliac wing. Here, I focused on the direction of the distal portion of the ilium. As mentioned above, the shape of the ilium is important in the movements of the hip, since this is the area of origin of some muscles involved in the extension-flexion of the hip joint such as the Mm. glutei, iliacus, and sartorius. The distal portion slightly curved laterally probably increased the development of the m. glutei, as well as the epaxial musculature (Maynard Smith and Savage, 1956; Grand, 1983). Even though in the study of Argot (2002) the terrestrial Metachirus is the only didelphid species analyzed with this particular morphology of the ilium (see Argot, 2002: fig 7b), I found the same pattern in other didelphid species as well, such as Glironia , Chironectes , and Tlacuatzin , which are not always directly associated with terrestrial habits.

Character 91: Pubic symphysis size in relation to the craniocaudal size of the obturator foramen: (0) shorter; (1) equal or longer ( fig. 25). Although the obturator foramen is well developed in all didelphid species analyzed herein, I detected a remarkable variation in pubic symphysis pubis size. Variation in this character could be linked with the area of origin of m. gracilis, implied in the flexion of the tibia ( Elftman, 1929), and assisting the hamstring muscles in the extension of the femur ( Argot, 2002; Maynard Smith and Savage, 1955). In most didelphid species, the symphysis is equal to or longer than the obturator foramen. However, in Glironia , Tlacuatzin , and Thylamys macrurus , the symphysis is shorther than the obturator foramen.

Character 92: Caudal portion of ischium body curved laterally (other than the ischiatic tuberosity): (0) absent; (1) present ( fig. 26). This is the region of the ischium where important muscles involved in the flexion of the tibia and tail movements originate, such as Mm. biceps femori and semitendinosus. The curved shape of the ischium probably increases the function of those muscles. Both muscles (i.e., Mm. biceps femoris and semitendinosus) are well developed in Metachirus , probably due to the typical cursosaltatorial locomotion (Maynard Smith and Savage, 1956). Besides Metachirus , I found the posterior part of ischium body to be laterally curved also in Philander , Didelphis , Lutreolina , and Chironectes .

FEMUR

Character 93: Development of lesser trochanter: (0) not very developed; (1) well developed, surpassing the half of the mesial extension of the femoral head ( fig. 27). Although the lesser trochanter is present in all didelphids, I detected two character states for its development. In this structure the Mm. iliacus and psoas major insert, which is implied in the flexion of the hip joint, as well as the flexion and fixation of the vertebral column ( Evans, 1993). In arboreal forms, such as Caluromys or Micoureus , the iliacus and psoas major act as flexors and as external rotators and adductor of the leg ( Muizon and Argot, 2003). Argot (2002) reported differences on the development of the lesser trochanter of Caluromys , Metachirus , Monodelphis , and Micoureus . In the sample analyzed, the lesser trochanter is notably developed in Glironia , Caluromys , Caluromysiops , Thylamys pusillus , T. macrurus , Micoureus , Marmosa (except M. rubra ), Marmosops , Cryptonanus , and Gracilinanus . No information is currently available about this character for Thylamys venustus (coded ‘‘?’’).

Character 94: Protuberance between trochanteric fossa and head: (0) absent; (1) present ( fig. 27). This protuberance seems to be an impression of the insertion of obturatores and gemelli muscles. It is present in most species analyzed herein, except for T. pallidior , which is autapomorphic in this character, as this structure is absent. No information is currently available about this character for Thylamys venustus (coded ‘‘?’’).

Character 95: Development of greater trochanter: (0) not surpassing the level of the head; (1) surpassing the level of the head ( fig. 27). According to Argot (2002), didelphids usually exhibit a greater trochanter not higher than the femoral head. However, besides Metachirus , I judged greater trochanter notably developed in Didelphis , Philander , Lutreolina , Chironectes , Tlacuatzin , Marmosa robinsoni , M. mexicana , Marmosops impavidus , M. parvidens , and M. pinheiroi .

Character 96: Distal epiphysis anteroposteriorly compressed: (0) absent; (1) present. The anteroposterior compression of the distal femoral epiphysis is probably related to the intercondyloid fossa being delimited by low crests ( Argot, 2002). Some didelphids with well-developed arboreal habits ( Caluromys , Micoureus ) show this morphology ( Muizon and Argot, 2003), and the same relationship was evidenced in viverrids ( Taylor, 1976) and primates ( Tardieu, 1983). In the sample, the distal epiphysis is anteroposteriorly compressed in Caluromys , Caluromysiops , Philan-