Convergent evolution occurs when two distantly related organisms on different continents appear to be morphologically similar. An analysis of several groups of desert mammals that exemplify convergent evolution is presented.
The newly discovered section at Inchasi, located about 50 km southeast of Potosi, Bolivia, in the eastern Cordillera, consists of about 120 m of undeformed terrestrial sediments containing fossil mammals. Paleomagnetic analysis of 54 sites indicates a polarity pattern with an estimated duration of about 0.64 Ma. The rich Inchasi local fauna indicates a Montehermosan and/or Chapadmalalan land mammal age (Pliocene). Given these constraints, Inchasi correlates from the interval between the late Gilbert (within the Cochiti subchron) to the early Gauss (within the Mammoth subchron) chrons; that is, between about 4.0 and 3.3 Ma. The distinct lack of North American mammals in the Inchasi local fauna provides some of the first well-calibrated evidence that the Great American Interchange occurred after about 3.0 Ma, as has been previously stated based on other calibrations of the earliest immigrant (Uquian) faunas.
To some potential readers of this book the description of Biological System atics as an art may seem outdated and frankly wrong. For most people art is subjective and unconstrained by universal laws. While one picture, play or poem may be internally consistent comparison between different art products is meaningless except by way of the individual artists. On the other hand modern Biological Systematics - particularly phenetics and cladistics - is offered as objective and ultimately governed by universal laws. This implies that classifications of different groups of organisms, being the products of systematics, should be comparable irrespective of authorship. Throughout this book Minelli justifies his title by developing the theme that biological classifications are, in fact, very unequal in their expressions of the pattern and processes of the natural world. Specialists are imbibed with their own groups and tend to establish a consensus of what constitutes a species or a genus, or whether it should be desirable to recognize sub species, cultivars etc. Ornithologists freely recognize subspecies and rarely do bird genera contain more than 10 species. On the other hand some coleopterists and botanists work with genera with over 1500 species. This asymmetry may reflect a biological reality; it may express a working practicality, or simply an historical artefact (older erected genera often contain more species). Rarely are these phenomena questioned.
The two extant genera of tree sloths, Bradypus and Choloepus, exhibit highly similar suspensory postures and modes of locomotion. Fossil sloths are far more abundant and diverse, but all are morphologically distinct from extant sloths. Thus, a perceived functional and phylogenetic dichotomy arose between the small, suspensory, tree sloths and the mostly large, robust, "ground" sloths. While various workers have proposed arboreality in certain fossil genera, or have questioned monophyly of extant sloths based on cranial differences, little attention has focussed on the evolution of postcranial features associated with arboreality from both a functional and phylogenetic perspective. To ultimately infer function from skeletal morphology in fossil sloths, anatomical correlates of locomotor mode were identified in a morphologically and behaviorally diverse sample of extant xenarthrans (sloths, anteaters, and armadillos). Shape indices of the limb skeleton successfully discriminated suspensory, climbing, terrestrial, and fossorial behaviors in extant taxa, and were subsequently used as a comparative data base to investigate locomotor adaptations in two groups of fossil sloths. First, morphological indices were quantified in the early radiation of Santacrucian sloths from Argentina. There is evidence for locomotor heterogeneity among these Miocene sloths, and the ancestral sloth morphotype exhibited generalized climbing adaptations. Next, the Pleistocene sloths of the Antillean islands were studied because the clade has been postulated to be closely related to Choloepus based on cranial data. The Pleistocene taxa exhibit more postcranial specializations than do the Santacrucian taxa, and practiced a variety of behaviors including arboreal and terrestrial quadrupedalism, vertical climbing, suspension, browsing, and digging. Particularly Acratocnus manifests features suggestive of a transitional form of suspensory behavior, especially like that of Choloepus. Postcranial and cranial data were collected from both fossil groups for phylogenetic analyses designed to (1) compare conflicting implications of postcranial and cranial data for tree sloth monophyly, and (2) trace morphological transitions of locomotor adaptations. Results indicate that tree sloths are not monophyletic, their suspensory adaptations are convergent, and Choloepus is the sister taxon of Antillean sloths. Functional character analysis, and the use of fossils and multiple outgroups, are crucial for inferring polarity, recognizing homology, and resolving extant relationships.
Based on 200 hours and 5424 observations of a zoo colony of 11 two-toed sloths, Choloepus hoffmanni, an ethogram of 16 behaviors was developed, with subsequent construction of a time budget that quantified the frequency of each behavior and the percentage of time spent engaged in these behaviors. Behaviors were classified as either energy-conserving or energy-intensive maintenance (individual), or as social (interactive). Results of the study indicate that: (1) members of both sexes spend 64-76%, and all age groups spend 62-79% of their time engaged in energy-conserving behaviors; (2) in comparison to males females spend more time sleeping (65 vs. 51%) and less time resting (9 vs. 11%); (3) in comparison to females males engage more of their time in energy-intensive (37 vs. 24%) and less in energy-conserving activities (64 vs. 76%); (4) more time is spent by males and females of all age groups in maintenance than in social behaviors (97 vs. 3%); (5) more energy-intensive and social behaviors are engaged in during the night than dusk; (6) location plays a significant role in influencing behavior while cage does not.