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| 文件类型： | 文章 |
|---|---|
| 所有的著者/提供者： | Jeffrey S Willey; Richard W Blob |
| ISSN: | 0022-1511 |
| OCLC号码： | 480383929 |
| 语言注释： | English |
| 注意： | Fig. 1. Evolutionary history of tail length in turtle lineages, evaluated for two different phylogenetic hypotheses using MacClade (Vers. 3.0, W. P. Maddison and D. R. Maddison, Sinauer Assoc., Sunderland, MA, 1992). Extinct taxa are indicated by dagger symbols (†). The character state for a terminal taxon is indicated by the open or shaded rectangle at the top of its branch; this rectangle is absent for fossil clades in which caudal vertebrae adequate to evaluate tail length are not preserved. (A) Tail length mapped onto the phylogenetic hypothesis of turtle relationships proposed by Gaffney (1996), based on morphological characters. Gaffney's "Trionychoidea" is divided into its component clades (trionychids and kinosternids), and his "Testudinoids" is divided into its component groups (testudinids and batagurids); the undescribed specimen TMP 87.2.1 from his analysis is omitted here. (B) Tail length mapped onto a phylogenetic hypothesis in which the relationships of extant cryptodires are derived from results of Shaffer et al. (1997), an analysis based on combined molecular and morphological data in which trionychids (softshells), rather than chelydrids (Common Snapping Turtles), are the basal clade of extant cryptodires. In both (A) and (B), open bars indicate a long tail and black bars indicate a short tail. In (B), grey shading indicates an equivocal optimization of the ancestral state of tail length. Tail length data were compiled from the following sources: Proganochelys-Gaffney (1990); pleurodires-Gaffney (1985), Ernst and Barbour (1989); Kayentachelys-Gaffney et al. (1987); Kallokibotion-Gaffney and Meylan (1992); pleurosternids-Gaffney (1979); baenids; Gaffney (1985); plesiochelyids-Peng and Brinkman (1993:2022); Xinjiangchelys-Peng and Brinkman (1993); meiolaniids-Gaffney (1985); Sinemys-Brinkman and Peng (1993b); Dracochelys-Gaffney (1996:128); Ordosemys-Brinkman and Peng (1993a); extant cryptodires-Ernst and Barbour (1989), pers. obs. Tails of Sinemys and Ordosemys were scored as long based on partially preserved tails that are at least one half of shell length; this scoring is conservative in the context of evaluating whether a long tail might be a derived feature in chelydrids, rather than a primitive retention. Fig. 2. Schematic diagrams of tail muscles and skeleton in Chelydra serpentina, based on Walker (1973) and specimen dissections. (A) Lateral view. Bones are filled in black, except for the ischium and pubis, which are outlined by a solid black line and shown as transparent for clarity. The relative location and attachments of three muscles are illustrated: caudi-iliofemoralis (diagonal line fill pattern), pubococcygeus (grey shading), and ischiococcygeus (horizontal line fill pattern). To simplify the figure and improve clarity, the epaxial muscles, superficial caudal hypaxial muscles, and distal tip of the tail have been omitted. (B) Ventral view, with plastron and distal tip of the tail removed. The same three major muscles are illustrated using the same shading patterns as in (A). Fig. 3. Diagrammatic model illustrating expected hip and tail movements for one cycle of aquatic walking in Chelydra serpentina, if tail movements are a passive consequence of ipsilateral contraction of the caudi-iliofemoralis muscle during limb retraction. If tail movements toward one side of the body were solely a consequence of caudi-iliofemoralis shortening on the same side of the body during limb retraction, then the timing of tail flexion toward that side would coincide closely with hind-limb retraction on the same side. For example, as illustrated here, tail flexion toward the left would coincide with retraction of the left leg (during the last 50% of the cycle in this figure). Fig. 4. Mean kinematic profiles for the hind limb and tail of juvenile Common Snapping Turtles during aquatic walking (N = 3 individuals, six cycles per individual), with still images from high-speed video indicating the positions of the legs and tail at landmark times during the locomotor cycle. Error bars indicate ± 1 SE. In the upper graph (hip angle), black circles represent data for the right hind limb and white circles represent data for the left hind limb. For both limbs, an angle of 0° indicates that the femur is perpendicular to the sagittal plane (i.e., body midline), a positive slope indicates the limb is protracting, and a negative slope indicates the limb is retracting. These portions of the locomotor cycle are designated for the left hind limb by the bold dashed line (labeled "C") separating protraction (labeled "P") from retraction (labeled "R"). In the lower graph, data for the tail are represented by circles filled with a dot patterning; an angle of 0° indicates the tail is straight, negative angles indicate flexion to the left, and positive angles indicate flexion to the right. Dashed lines illustrate (A) maximally retracted angle of left hind limb (= start of left leg protraction); (B) maximum tail flexion toward the left (= start of tail flexion toward the right); (C) maximally protracted angle of left hind limb (= start of left leg retraction); and (D) maximum tail flexion toward the right (= start of tail flexion toward the left). For clarity, white lines are superimposed on the still video images to highlight segments defining the body axis, tail, and left femur. White dots with black centers indicate positions of digitized points; solid white dot indicates calculated position of the sacrocaudal joint. |
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摘要：
Chelydrids (including snapping and big-headed turtles) are unusual among extant turtles in possessing long, robust tails. In other lineages of quadrupedal reptiles, long tails perform critical functions during both terrestrial and aquatic locomotion, and the tails of Common Snapping Turtles have been shown to help stabilize juveniles as they ascend terrestrial slopes. However, Common Snapping Turtles live primarily in aquatic habitats, and the function of the tail in these environments has not been examined. The first step to evaluating the role of the chelydrid tail in water is to evaluate its pattern of motion; therefore, we collected high-speed digital video of tail kinematics from juvenile Common Snapping Turtles (Chelydra serpentina) during aquatic walking. Common Snapping Turtles hold the tail off the substrate and move it as a nearly rigid strut during aquatic walking, cyclically flexing it side to side by 11-12° from the body midline. These motions occur one-quarter cycle out of phase with the motions of the limbs; thus, the timing of tail movements suggests that they are likely not a passive consequence of hind-limb retraction and are likely controlled by one (or a combination) of tail muscles, rather than ipsilateral hind-limb retractors. The potential for tail movements to contribute to aquatic thrust in Common Snapping Turtles is uncertain. However, Common Snapping Turtle tail movements resemble those of salamanders and lizards in many respects, suggesting that Common Snapping Turtles might retain primitive tetrapod or sauropsid features of tail motor control despite possessing a radically divergent body plan.
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