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The Humerus of Cryptotis colombiana and its Bearing on the Species' Phylogenetic Relationships (Soricomorpha: Soricidae)

Neal Woodman, Carlos A. Cuartas-Calle, Carlos A. Delgado-V
DOI: http://dx.doi.org/10.1644/BME-007 832-839 First published online: 29 August 2003

Abstract

The Colombian small-eared shrew, Cryptotis colombiana Woodman and Timm, was described from the Colombian Andes in 1993. Its original allocation to the C. nigrescens group recently was questioned based on several cranial characters the species appeared to share with some members of the C. thomasi group. We review characteristics of the C. nigrescens and C. thomasi groups, and we describe the humerus of C. colombiana and the humerus and manus of C. medellinia. The morphology of the humerus joins the suite of characters that supports the hypotheses that C. colombiana is not a member of the C. thomasi group and that all remaining South American species form a cohesive, definable set that is probably monophyletic.

Key words
  • anatomy
  • Cryptotis
  • Insectivora
  • morphology
  • osteology

Small-eared shrews, genus Cryptotis, are endemic to the New World, extending from southeasternmost Canada and the eastern United States to the Andean highlands of northwestern South America. Recent systematic treatments of the genus have organized most species into 4 definable “species groups” (Choate 1970; Woodman 1996; Woodman and Timm 1993, 1999, 2000): C. mexicana group, with 9 species distributed from northeastern Mexico to central Honduras; C. nigrescens group, comprising 6 species distributed from southern Mexico to Colombia; C. parva group, consisting of at least 2 diagnosable species (C. orophila and C. parva) that occur from northeastern North America to the Valle Central of Costa Rica; and C. thomasi group, with at least 8 species in the northern Andes of South America.

Most South American shrews traditionally have been placed in the C. thomasi group (Choate 1970; Woodman 1996, 2002). However, Woodman and Timm (1993) described the Colombian small-eared shrew, C. colombiana, from the Central Cordillera of the Colombian Andes, as a member of the mostly Central American C. nigrescens group. This designation was based entirely on traditional external and craniomandibular characters because C. colombiana was known then from only a single skin and skull, and no postcranial skeleton was available. More recently, the relationship of C. colombiana to the C. nigrescens group has been questioned and a closer relationship between C. colombiana and some members of the C. thomasi group inferred (Vivar et al. 1997).

Recent collections on the Central Cordillera of Colombia have yielded additional specimens of C. colombiana as well as a poorly documented member of the C. thomasi group, C. medellinia. These new specimens include portions of the forelimb skeleton that previously were unknown for either species. The external forelimb yields suites of characters that aid in understanding the relationships within the genus Cryptotis (Choate 1970), and the skeleton of the forelimb, particularly the humerus, has proven especially valuable (Woodman and Timm 1999, 2000). In this study, we review the morphological differences between the C. nigrescens and C. thomasi groups. We also describe the structure of the humerus of C. colombiana and the humerus and manus of C. medellinia and show how the morphologies of their respective forelimbs aid us in understanding the relationships of these 2 species.

Materials and Methods

Specimens of shrews were collected during a series of biotic inventories in the municipalities of Argelia, Caldas, and Sonsón along the Central Cordillera, Departamento Antioquia, Colombia (C. A. Cuartas-Calle, in litt.; C. A. Cuartas-Calle and M. Peña, in litt.; C. A. Cuartas-Calle et al., in litt.) and as part of a study of food habits of the crab-eating fox, Cerdocyon thous, in Antioquia (Delgado-V. 2002). Two specimens of C. colombiana were collected using traditional trapping methods. Postcranial material of C. medellinia is from an individual found dead along the side of a road and remains (including diagnostic craniomandibular elements) recovered from scat of Cerdocyon thous.

Taxonomy of Cryptotis, characters, and character polarities follow Woodman (1996), (2002) and Woodman and Timm (1993), 1999, 2000). Terminology of dentition and dental characteristics follows Choate (1970). Anatomical terminology of the humerus (Fig. 1) follows Reed (1951). Specimen localities and abbreviations for systematic collections are explained in Appendix I.

Fig. 1

Left humerus of Cryptotis goldmani, indicating anatomical features mentioned in the text. Abbreviations: CA = capitulum; DP = deltoid process; GT = greater tuberosity; HD = head; LE = lateral epicondyle; ME = medial epicondyle; PP = pectoral process; TR = trochlea; TT = teres tubercle.

Results

Differentiation of C. nigrescens and C. thomasi groups.—As currently understood, the 4 species groups of Cryptotis defined by Choate (1970) and Woodman and Timm (1993), (1999), (2000) are informal groupings of species sharing suites of characters rather than being clades supported by polarized synapomorphies, yet, they have proven useful in working out the taxonomy of the genus. The C. parva and C. nigrescens groups, although superficially appearing closely related, are defined primarily (but not entirely) by plesiomorphic and unpolarized characters. In contrast, the C. thomasi and C. mexicana groups both possess clearly apomorphic characters, including modifications of the forelimb, that link their respective members phylogenetically (Woodman 1996; Woodman and Timm 1999, 2000). The latter 2 groups also possess characters (e.g., shape of unicuspids and enlargement of processes of the humerus) that indicate a possible close relationship between the 2 groups, and it is likely that the C. thomasi group represents a basal branch of the C. mexicana group. In addition, their respective memberships are likely to change as additional Central and South American taxa are analyzed and we attain a more comprehensive grasp of the distributions of characters among these species. Despite the shortcomings of our present level of knowledge of the phylogenetic relationships within the genus, the C. nigrescens and C. thomasi groups represent 2 very different and clearly definable sets of species.

The complete suite of characteristics defining the C. nigrescens group is based on C. nigrescens and other Central American and Mexican species for which the postcranial skeleton and fluid-preserved specimens are available. However, the majority of characters have been confirmed for most species. Members of the C. nigrescens group are typically small to medium in size for the genus, with mean head-and-body length < 70 mm (the largest specimen, C. mayensis, has head-and-body length <= 77 mm). Fur is typically shorter (4–5 mm) than in the C. mexicana and C. thomasi groups, and there is an obvious bare patch marking the positions of the lateral glands in males. Their forefeet are not enlarged, and the foreclaws are neither elongated nor broadened. Metapodials and phalanges of the manus and pes are long and narrow. First through 3rd unicuspid teeth (U1–U3) typically are cone shaped, with a straight or convex posteroventral margin. The anterior border of the coronoid process of the mandible joins the horizontal ramus at a relatively high angle. The coronoid process is moderately high to high. The articular process is short, broad, and robust. The lower sigmoid notch is very shallow. The humerus in the C. nigrescens group is similar to that of the C. parva group: it is long, straight, and narrow (particularly distal to the teres tubercle), with the teres tubercle more proximally located than in either the C. mexicana or C. thomasi group (but more distally positioned than in Sorex); the narrowest portion of the shaft tends to be broader in lateral aspect than in anterior aspect; and the head is more or less rounded. Although most of the processes, particularly the medial epicondyle and teres tubercle, are prominent relative to Sorex, they are small for Cryptotis. The lateral epicondyle is unexpanded. The posterior edge of the falciform process of the tibia is not deeply pocketed. The body and the trochlear process of the calcaneum are relatively small. Many of these characters are synapomorphic for Cryptotis, but most are plesiomorphic or of uncertain polarity within the genus. In fact, the only clear synapomorphy for the C. nigrescens group is that the anterior element of the ectoloph of 1st upper molar (M1) is of approximately the same size as the posterior element. It is possible that the C. nigrescens group may prove to be paraphyletic.

Members of the C. thomasi group typically are medium to large, with mean head-and-body length > 72 mm; all the named Colombian species have mean head-and-body length > 83 mm. Members of the C. thomasi group have longer (about 6–7 mm), more luxuriant fur than members of the C. nigrescens group. Unlike most soricines, males of the C. thomasi group lack obvious bare patches marking the location of the lateral glands. The forefeet of these shrews are somewhat enlarged, and the foreclaws are elongated (but not broadened). Previously, the skeleton of the feet had not been studied. U1–U3 typically are relatively narrow and concave to very concave on the posteroventral margin. The anterior element of the ectoloph of M1 is reduced relative to the posterior element. The anterior border of the coronoid process of the mandible joins the horizontal ramus at a relatively low angle. The coronoid process is low to moderately high. The articular process is high, broad, and less robust than in the C. nigrescens group. The lower sigmoid notch is shallow to very shallow. The humerus in the C. thomasi group is moderately long, quite robust, and slightly curved; the narrowest portion of the shaft tends to be broader in anterior aspect than in lateral aspect; and the head tends to be dorsoventrally elongate. The processes, particularly the medial epicondyle and teres tubercle, are more prominent than in the C. nigrescens group, and the teres tubercle is more centrally located along the shaft. The posterior edge of the falciform process of the tibia is deeply pocketed. The calcaneum has not been studied. Most of these characters are apomorphic relative to those of the C. nigrescens and C. parva groups.

Humerus of C. colombiana.—External characteristics of the forelimb of C. colombiana were described previously (Woodman 1996), and they match the plesiomorphic characters of other members of the C. nigrescens group: forefeet are unen-larged, and foreclaws are neither elongated nor broadened. We studied 3 nearly complete humeri from 2 individuals of C. colombiana, and, in general, these display the mostly plesiomorphic (for Cryptotis) characteristics typical of the C. nigrescens group (Figs. 2AC). The shaft is relatively long, narrow, and straight in anterior aspect, especially distal to the teres tubercle. The narrowest portion of the shaft is broader in lateral aspect than in anterior aspect. The head of the humerus is more or less rounded. The proximal face of the greater tuberosity is broadened with a slight pocket. As in C. nigrescens, the pectoral process is high, the teres tubercle is slightly enlarged, the medial epicondyle is slightly enlarged, and the lateral epicondyle is unexpanded; none of these processes is as prominent as in members of the C. thomasi group. The position of the teres tubercle along the shaft is similar to that in C. parva and C. nigrescens, and it is more proximal than in the C. thomasi group; the distal edge of pectoral process extends well distal to the teres tubercle.

Fig. 2

Left humeri of A) Cryptotis nigrescens, KU 142054; B) C. colombiana, MUA 062 and C) MUA 060; and D) C. thomasi, FMNH 71027; and E) fragmentary left and F) right humeri of C. medellinia, CADV 029 and MUA 061, respectively. Dashed lines represent areas of breakage. Compare also humeri figured in Woodman (2002).

Humerus and manus of C. medellinia.— As in other members of the C. thomasi group, forefeet of C. medellinia are slightly enlarged, and foreclaws are slightly elongated but not broadened. The humerus has not been described previously for C. medellinia, and the manus has not been described for any species within the C. thomasi group, making the postcranial materials described below valuable despite their incompleteness. A right and a left humerus from 2 individuals of different sizes were available for study. Both humeri are broken, so that only the distal portion of the shaft and the distal epiphysis are present, and even these are incomplete, so only some of the major characters can be recorded. In all preserved characters, as well as overall shape, the humeri match most closely those of other members of the C. thomasi group (Figs. 2DF): the shaft is broader in anterior aspect than in lateral aspect, the medial epicondyle is elongate, and the lateral epicondyle is expanded.

Of secondary interest, the humeri share a gross structural similarity in cross section, visible along the broken portion of the shaft, to avian long bones. Externally, there is a thin layer of compact bone forming the cortex. Internally, the shafts of the bones are mostly hollow, with a few thin struts supporting the walls. The lack of internal spongy bone may be a result of the small size of shrews and the resulting low level of stress on the skeleton, or possibly the struts are spongy bone but on a shrew-sized scale. Despite the superficial resemblance, the cortex of avian bones is distinct histologically, being more complex than that of soricid bones (Foote 1916).

The metacarpals of C. medellinia are relatively long and broad (Fig. 3) compared with those of C. parva and C. nigrescens, in which these bones are long but quite narrow. The broadening is similar to that observed among less-derived members of the C. mexicana group (Woodman and Timm 1999). However, the proximal phalanges of C. medellinia are relatively long and narrow as in C. parva and C. nigrescens. In the C. mexicana group, the proximal (and middle) phalanges exhibit shortening and broadening along with the metacarpals; this is particularly evident in more derived species, such as C. goldmani. Metacarpal V of C. medellinia is more strongly curved (laterally concave) than in any species of Cryptotis for which the manus has been studied. The distal tip of the proximal phalange of digit V extends distinctly distal to the tip of metacarpal IV, rather than the 2 terminating at approximately the same position (as in C. parva). Carpals remain unknown.

Fig. 3

Metacarpals and phalanges of the left manus of Cryptotis medellinia (CADV 029). Distal elements with stippled borders are claw sheaths.

Discussion

Cryptotis colombiana possesses external and craniomandibular characteristics that define the C. nigrescens group, and based on these characteristics, it was described as a member of this group (Woodman and Timm 1993). Vivar et al. (1997), however, reached a different conclusion, suggesting that C. colombiana be removed from the C. nigrescens group and that the C. thomasi group be redefined to consist only of C. thomasi and possibly C. medellinia. Confusion about the relationships of C. colombiana perhaps resulted from the use of several characters, most of which originally were used to distinguish C. colombiana from other members of the C. nigrescens group (rather than establish it as a member of this group):

  1. The presence of a very large foramen on the posterior edge of the tympanic process of the petromastoid clearly and easily distinguishes C. colombiana within the C. nigrescens group. This character is problematic because a very large foramen also is present on the tympanic processes of 2 members of the C. thomasi group: C. thomasi and C. medellinia. Foramina of various sizes (but never as large as in C. colombiana or C. thomasi) also are present in C. meridensis and several other Andean taxa. Among Central American and Mexican species, the foramen is always minute or absent.

  2. A high, more robust anterior process of the petromastoid distinguishes C. colombiana within the C. nigrescens group. This character is more variable within many species than the previous character, and the anterior process of the petromastoid often is broken, making it difficult to determine its size. Within and among species of the C. thomasi group, the height and width of the process is extremely variable and therefore does not appear to be particularly useful for either defining the group or distinguishing species within the group.

  3. A shallow lingual notch between the facets of the articular condyle also identifies C. colombiana. As noted above, the articular condyle of the mandible in the C. nigrescens group tends to be short, broad, and robust, whereas that in the C. thomasi group tends to be tall, broad, and less robust. The articular condyle of C. colombiana is particularly robust, and the ascending ramus of the condyle is so broad as to nearly fill the region between the articular facets. Although not delicate, the articular condyle in the C. thomasi group is not nearly so broad, and we have not seen any member of this group comparable with C. colombiana in this regard.

  4. A short articular condyle aids in distinguishing members of the C. nigrescens and C. parva groups from members of the C. mexicana and C. thomasi groups. As partly noted for the last character, the articular condyle in the former 2 groups tends to be short, broad, and robust, and it is tall, broad, and less robust in the latter 2 groups. C. colombiana conforms morphologically with the C. nigrescens group; all other Andean species of shrews that we have inspected match more closely the shape described for the C. thomasi group.

A broad anterior process of the petromastoid and a shallow lingual notch on the articular condyle are autoapomorphies that are uninformative phylogenetically. A short articular condyle is characteristic of the C. nigrescens group but plesiomorphic for the genus and, therefore, not useful for determining evolutionary relationships. In the absence of other data, a large foramen on the tympanic process would serve as a synapomorphy linking all those taxa that possessed it, thus marking C. colombiana, C. thomasi, and C. medellinia as sister taxa. However, even if all 4 characters suggested such a linkage, there exist a greater number of external and craniomandibular characters defining the C. thomasi group (see above) that C. colombiana lacks. Instead, C. colombiana possesses the synapomorphic morphology and the other, generally plesiomorphic characters that mark the C. nigrescens group.

The morphology of the humerus of C. colombiana is nearly identical with that of other members of the C. nigrescens group, and this fact provides additional support for the hypothesis that C. colombiana is not a member of the C. thomasi group. Similarly, the morphology of the humerus of C. medellinia supports its allocation to the C. thomasi group. The distinctiveness of the humeri of the 2 species attests to both the differentiation of this element between the 2 groups and the relative conservatism of its form within each group. That these 2 forelimb morphologies represent distinct lineages differentially adapted for survival in a common habitat makes sense ecologically. In some communities of sympatric soricids, body size reflects foraging mode. Larger species (such as members of the C. thomasi group) are soil foragers that tend to take larger prey, such as earthworms, whereas smaller species (members of the C. nigrescens group) include a greater proportion of smaller, surface-dwelling invertebrates in their diets (Churchfield 1991; Churchfield and Sheftel 1994). Larger size has been hypothesized to reflect the physical requirements of foraging either on larger prey or deeper in the soil horizon (Churchfield and Sheftel 1994). Apomorphic characters of the forelimb in the C. thomasi group may include adaptations that enhance such foraging activities in Andean environments.

Acknowledgments

We thank the following curators and collections managers for loans or for permission to examine specimens under their care: L. R. Heaney, B. D. Patterson, J. Phelps, and W. Stanley (The Field Museum) and R. M. Timm and T. Holmes (University of Kansas Natural History Museum). S. Feinstein, R. M. Timm, and an anonymous reviewer provided valuable, substantiated comments on a version of the manuscript.

Appendix I

Specimens figured.—Abbreviations for systematic collections: CADV = collector Carlos A. Delgado-V., uncataloged material to be deposited in Museo de la Universidad de Antioquia; FMNH = The Field Museum, Chicago, Illinois; KU = University of Kansas Natural History Museum, Lawrence, Kansas; MUA = Museo de la Universidad de Antioquia, Medellín, Colombia; USNM = National Museum of Natural History, Washington, D.C.

Cryptotis colombiana.—COLOMBIA: Antioquia: Corregimiento San Antonio de Prado, 2,100–2,800 m, Municipio de Medellín (MUA 060, 062).

Cryptotis medellinia.—COLOMBIA: Antioquia: Municipio de Santa Rosa de Osos, 3,000 m (MUA 061); Alto San Luis, 2,600–2,800 m, Reserva Ecológica San Sebastían–La Castellana, Municipio de El Retiro (CADV 029).

Cryptotis nigrescens.—COSTA RICA: Puntarenas: Monteverde, Cerro Amigos, 1,760 m (KU 142054).

Cryptotis thomasi.—COLOMBIA: Cundinamarca: San Francisco de Bogotá, 3,000 m (FMNH 71027).

Footnotes

  • Associate Editor was Mark R. Engstrom.

Literature Cited

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