Research Article |
Corresponding author: M. Julieta Pérez ( mariju_perez@hotmail.com ) Academic editor: Raquel López-Antoñanzas
© 2017 M. Julieta Pérez, Rubén M. Barquez, M. Mónica Díaz.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Perez J, Barquez R, Diaz M (2017) Morphology of the limbs in the semi-fossorial desert rodent species of Tympanoctomys (Octodontidae, Rodentia). ZooKeys 710: 77-96. https://doi.org/10.3897/zookeys.710.14033
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Here, a detailed description of the forelimbs and hindlimbs of all living species of the genus Tympanoctomys are presented. These rodents, highly adapted to desert environments, are semi-fossorial with capacity to move on the surface as well as to build burrows. The shape, structure, and size of the limbs are described. Contrary to what was expected for scratch digging semi-fossorial species, Tympanoctomys have slender humerus, radius and ulna; with narrow epicondyles of the humerus and short olecranon of the ulna with poorly developed processes. Following our descriptions, no intrageneric morphological variation regarding to the configuration of the limbs was detected, probably due to phylogenetic proximity, and not related to specific variations in response to different use of substrates or habits. The obtained results constitute a source of previously unpublished information as well as an important base for future analysis in different studies, such as morphometric, morpho-functional, or phylogenetic researches.
Argentina, Chalchalero Vizcacha Rat, description, morphology
Tympanoctomys is member of a clade of the family Octodontidae, restricted to Argentina (
It was estimated that Tympanoctomys diverged about six million years ago (
Tympanoctomys is a small- sized octodontid, with body mass ranging from 67–104 g, head and body length from 130–170 mm (
Members of the genus Tympanoctomys have the capability to move on the surface as well as to build complex burrows (
However, studies of postcranial anatomy of this genus are insufficient. The objective of our research was to describe in detail the morphology of the forelimbs and hindlimbs of all species of the genus Tympanoctomys, mainly analyzing shape and size of some elements. Expecting to generate new data set that allow infer correlation between postcranial morphology and digging strategy.
Sixty specimens of all the living species of the genus Tympanoctomys were examined: Tympanoctomys aureus (22), Tympanoctomys barrerae (35), Tympanoctomys kirchnerorum (2), and Tympanoctomys loschalchalerosorum (1). All with complete postcranial skeletons deposited in three argentine collections, CML (Colección Mamíferos Lillo; Universidad Nacional de Tucumán, Tucumán), CNP (Colección de Mamíferos “Elio Massoia”, Centro Nacional Patagónico, Puerto Madryn, Chubut), and CMI (Colección de Mamíferos IADIZA, Mendoza). Also, four individuals of Ctenomys opimus were measured for comparisons with the species of Tympanoctomys. The specific localities and collection numbers of specimens are given in Appendix I.
The morphology of elements of the stylopodium and zeugopodium, excluding the autopodial elements, of the limbs was described considering form, size, and orientation. For a more comprehensive description, it was divided into A) forelimb: humerus, radius, and ulna; and B) hindlimb: femur, tibia, and fibula. The nomenclature for the anatomical description follows that of previous studies on different groups of mammals (
Based on the previous researches by Biknevicius (1993),
Measurements of the long bones. apdh, anteroposterior diameter of the humerus; deh, diameter of the epicondyles; dlh, deltoid length of the humerus; fl, functional femur length; hl, functional humerus length; ful, functional ulna length; ol, length of the olecranon process; tdf, transverse diameter of the femur.
Humerus.
The diaphysis is robust with a cross-section angled in T. loschalchalerosorum and more cylindrical in the other species (Fig.
Left humerus, proximal, anterior, posterior, and distal views. A Tympanoctomys aureus B Tympanoctomys barrerae C Tympanoctomys kirchnerorum D Tympanoctomys loschalchalerosorum. bg, bicipital groove; dc, deltoid crest; gt, greater tuberosity; hh, humeral head; le, lateral epicondyle; lec, lateral epicondylar crest; lt, lesser tuberosity; me, medial epicondyle; sf, supratrochlear foramen. Scale bars 5 mm for all views except distal view where the scale bars are 1 mm.
The deltoid crest is located, in all species, in the proximal half portion of the diaphysis, it is well developed and greatly expanded laterally in T. loschalchalerosorum and T. barrerae, and ends as a pointed tip; whereas in T. aureus the distal tip is rounded and slightly extends laterally, it is more cranially oriented, similar to T. kirchnerorum. In the distal epiphysis, the capitulum is flattened, expanded, and separated from the trochlea by a well-marked groove. The trochlea is broader than the capitulum, is pulley-shaped, and its orientation with respect to the longitudinal axis of the humerus is straight in T. barrerae and oblique in the other species. The lateral epicondylar crest is well developed in T. loschalchalerosorum (Fig.
Radius and ulna. In T. loschalchalerosorum, T. barrerae, and T. kirchnerorum the diaphysis of the radius is somewhat cylindrical, with a flat side on the contact surface with the ulna, whereas in T. aureus it is more compressed (Figs
In the ulna (Fig.
Right ulna, proximal portion in cranial and distal views of radio and ulna. A Tympanoctomys aureus B Tympanoctomys barrerae C Tympanoctomys kirchnerorum D Tympanoctomys loschalchalerosorum. lcp, lateral coronoid process; mcp, medial coronoid process; ol, olecranon; r, radio distal surface; rn, radial notch; ul, ulnar distal surface; ulptc, ulnar lateral proximal trochlear crest; umptc, ulnar medial proximal trochlear crest. Scale bars 1 mm.
The distal epiphysis of the radius and ulna are here described for only two species (T. aureus and T. barrerae), because in other species these structures were broken and missing in the specimens examined. The medial styloid process of the radius is poorly developed in both species; the carpal surface is more concave and triangular in T. barrerae, whereas it is crescent-shaped in T. aureus. The medial styloid process of the ulna is well developed in both species, being proportionally longer in T. barrerae, and rounder in T. aureus (Fig.
Femur. The femur is robust with a straight and cylindrical diaphysis. In the proximal epiphysis, the femoral head is spherical and dorso-medially oriented with a short neck (Fig.
Right femur, anterior, posterior, and distal views. A Tympanoctomys aureus B Tympanoctomys barrerae C Tympanoctomys kirchnerorum D Tympanoctomys loschalchalerosorum. fh, femoral head; gt, greater trochanter; if, intercondylar fossa; lc, lateral condyle; lt, lesser trochanter; mc, medial condyle; pg, patellar groove; tt, third trochanter. Scale bars 5 mm for all views except distal view where the scale bars are 1 mm.
In the distal epiphysis, the lateral condyle is slightly wider than the medial condyle, which is more distally projected. The intercondylar fossa is narrow and deep, being shallower in T. aureus and T. barrerae than the other two species. The patellar groove is narrow and bordered by two parallel ridges.
Tibia and fibula. In the analyzed specimens of T. loschalchalerosorum and T. kirchnerorum, the tibias were broken and distal epiphyses were missing, so that many of the characters could not be described (Figs
Caudal and distal views of the right tibia and fibula. A Tympanoctomys aureus B Tympanoctomys barrerae C Tympanoctomys kirchnerorum D Tympanoctomys loschalchalerosorum. Fi, fibula; lc, lateral condyle; mc, medial condyle; mm, medial malleolus; n, notch; pp, posterior process; Ti, tibia. Scale bars 5 mm for all views except distal view where the scale bars are 1 mm.
In T. aureus, the two foveae on the distal epiphysis are oval, and the medial fovea is narrower and more concave than the lateral one (Fig.
The fibula (Fig.
Indexes. For each species, the results are indicated in Table
Mean, standard deviation, and number of specimens in brackets are indicated for each index and species.
T. aureus | T. barrerae | T. kirchnerorum | T. loschalchalerosorum | Ct. opimus | |
---|---|---|---|---|---|
SMI | 44.43 1.76 (21) |
42.96 2.05 (30) |
41.44 1.52 (2) |
41.77 - (1) |
51.45 2.55 (4) |
HWL | 9.46 0.73 (21) |
9.04 0.57 (30) |
8.71 0.12 (2) |
7.86 - (1) |
11.1 0.62 (4) |
EI | 22.54 0.9 (21) |
21 1.3 (30) |
21.55 0.58 (2) |
21.28 - (1) |
30 0.8 (4) |
IFA | 15.71 3.2 (2) |
13.02 1 (4) |
9.02 0.02 (2) |
8.51 - (1) |
20 0.5 (3) |
FRI | 9.66 0.55 (7) |
9.65 0.53 (28) |
9.18 0.66 (2) |
11.41 - (1) |
10.27 0.8 (4) |
Generally, most studies have been limited to one species of Tympanoctomys (T. barrerae) therefore there is almost no information about morphological change within species. This study provides a detailed description of the postcranial elements of the limbs of all species of the genus Tympanoctomys including the holotype of T. loschalchalerosorum, one of the only two known specimens of this species in the world, and the recently described T. kirchnerorum. As taxonomists know, using descriptions based on a single specimen is not the best protocol; nevertheless, it is useful in phylogenetic reconstructions. Because this unique material is not available in systematic collections and the animals are very rare in natural environments, the descriptions presented here represent the first qualitative approach for the species of the genus Tympanoctomys and for the genus as such. The information here would be useful for future comparisons with the rest of the octodontids.
The morphological differences among the species of the genus are still under revision. Therefore, here is the importance of including new evidence, such as postcranial characters in phylogenetic analysis.
On the other hand, the postcranial elements of the forelimbs and hind limbs of the four species of the genus show an anatomical plan related to the mode of terrestrial life, consistent with what was observed in other rodents and marsupials, for example the posterior extension of the humeral head forming a “peak”, the tubercles not surpassing the head, the separation between the trochlea and the capitulum, the flat or just concave articular surface of the radial notch in the ulna, the diaphysis of the radio curved, the extension of the greater trochanter above the femoral head, the posterior or posteromedial position of the third trochanter, and the asymmetry between the lateral and medial condyles, with the lateral wider (
Almost all indexes analyzed in Tympanoctomys have values below those calculated in Ctenomys, except for the robustness of the femur where T. loschalchalerosorum shows a higher value. Moreover, the results for SMI, HWL, EI and IFA in T. aureus has higher values among Tympanoctomys. These scores and a lowest value for the robustness of the femur can be related with allometric changes in T. aureus, the largest species of the genus. Elissamburu and Vizcaíno, (2004) and
Although these results are preliminary, data obtained in this study are consistent with observations made by other authors for other members of the family Octodontidae, and provide information for species that are poorly known or recently described. The family Octodontidae is highly specialized and adapted to living in desert habitats with a wide range of habits in just a few genera, so it is expected that the limbs have modified structures for that purpose. According to our descriptions, there are not too many intrageneric morphological variations regarding to the configuration of the limbs, probably due to phylogenetic proximity, and not related to specific variations in response to different use of substrates or habits. With regard to ecological aspects, just one species (T. barrerae) is well known about their burrow system, which is considered as an intermediate level of complexity compared with Ctenomys (see
Some authors (
We thank M. Fernanda López Berrizbeitia for her assistance with the change by Colección Mamíferos Lillo–Universidad Nacional de Tucumán, Ulyses Pardiñas (Centro Nacional Patagónico, Puerto Madryn, Chubut) and Benjamín Bender (IADIZA, Mendoza) for allowing access to the collections. Thanks also to Dr. Emily Oaks who improved the English of the manuscript. This study was founded through a scholarship from CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas).
List of specimens analyzed detailing the number of individuals by species in brackets, collection localities, type specimens, and collection numbers are indicated. See materials and methods by collections acronyms.
Ctenomys opimus (4): Jujuy Province, Susques Department: Borde de Ecalón, 31 km al SO de Susques, por ruta 52 y 1 km SE de ruta, 1 (CML 9353). Salta Province, Los Andes Department: Vega Cortadera 1 km W de ruta 53, 3910 m, 2 (CML 7243, 7244); 36 km N San Antonio de Los Cobres, 11,600 ft., 1 (CML 8438).
Tympanoctomys aureus (22): Catamarca Province, Pomán Department: Salar de Pipanaco, 10 km de Pío Brizuela (Est. Río Blanco), km 96 sobre R46, 35 km S de Andalgalá, 3 (CMI 6565, 6818, 6888); en los bordes del Salar Pipanaco, 3 (CMI 6562, 6563, 6564); 5 km del puesto de Pío Brizuela, entrada km 96 sobre R46, 2 (CMI 7188, 7189); a 13 km de la entrada Establecimiento Río Blanco, 4 (CMI 6558, 6559, 6560, 6561); Establecimiento Río Blanco, 28 km S, 9.3 km W Andalgalá, 4 (CML 6137, holotype; 4136, 4137, paratypes; 10110); Pipanaco, Salar Pipanaco, 6 (CMI 6846, 6848, 6849, 6850, 6851, 6856).
Tympanoctomys barrerae (35): La Pampa Province, Limay Mahuida Department: Gran salitral, 6 (CMI 6877, 6878, 6879, 6880, 6882, 6883). Mendoza Province, La Paz Department: 27 km N Desaguadero, 556 m app, 2(CMI 4485, CML 3438), Desaguadero, El Tapón 37 km, 1 (CMI 3314); Lavalle Department: 34 km al N de Desagüadero camino a Arroyito, 1 (CML 10111); Malargüe Department, 6(CMI 7263, 7264, 7266, 7267, 7268, 7269), a 8.5 km camino a Llancanelo, 1(CMI 7098), Camino Llancanelo 7(CMI 7270, 7271, 7272, 7273, 7274, 7275, 7276), Laguna Llancanelo, 2(CMI 7248, 7250); San Rafael Department: 10 km S El Nihuil 2 (CMI 3845, 3846), El Nihuil, 2 (CMI 6290, 6291). San Juan Province, Valle Fértil Department: Parque Provincial Ischigualasto, 4 (CMI 6842, 6843, 6853, 6889). San Luis Province, 1 (CMI 3821).
Tympanoctomys loschalchalerosorum (1): La Rioja Province, Chamical Department: 26 km SW Quimilo, 1 (CML 3695, holotype).
Tympanoctomys kirchnerorum (2): Chubut Province, Sarmiento Department: Ea. La Porfía, 2 (CNP 2503, 2505).