David DeGusta

Macrovertebrate paleontology and the pliocene habitat of ardipithecus ramidus

A diverse assemblage of large mammals is spatially and stratigraphically associated with Ardipithecus ramidus at Aramis. The most common species are tragelaphine antelope and colobine monkeys. Analyses of their postcranial remains situate them in a closed habitat. Assessment of dental mesowear, microwear, and stable isotopes from these and a wider range of abundant associated larger mammals indicates that the local habitat at Aramis was predominantly woodland. The Ar. ramidus enamel isotope values indicate a minimal C4 vegetation component in its diet (plants using the C4 photosynthetic pathway), which is consistent with predominantly forest/woodland feeding. Although the Early Pliocene Afar included a range of environments, and the local environment at Aramis and its vicinity ranged from forests to wooded grasslands, the integration of available physical and biological evidence establishes Ar. ramidus as a denizen of the closed habitats along this continuum.

The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus

Sediments containing Ardipithecus ramidus were deposited 4.4 million years ago on an alluvial floodplain in Ethiopia’s western Afar rift. The Lower Aramis Member hominid-bearing unit, now exposed across a >9-kilometer structural arc, is sandwiched between two volcanic tuffs that have nearly identical 40Ar/39Ar ages. Geological data presented here, along with floral, invertebrate, and vertebrate paleontological and taphonomic evidence associated with the hominids, suggest that they occupied a wooded biotope over the western three-fourths of the paleotransect. Phytoliths and oxygen and carbon stable isotopes of pedogenic carbonates provide evidence of humid cool woodlands with a grassy substrate.

A method for estimating the relative importance of characters in cladistic analyses.

The method of character importance ranking (CIR) is proposed here as a means for estimating the relative importance of characters in cladistic analyses, especially those based on morphological features. CIR uses the weighting variable to incrementally remove one character at a time from the analysis, and then evaluates the impact of the removal on the shape of the cladogram. The greater the impact, the more important the character. The CIR method for determining which characters drive the shape of a particular cladogram has several applications. It identifies the characters with the strongest (though not necessarily most accurate) signal in a cladistic analysis; it permits the informed prioritization of characters for further investigation via genetic, developmental, and functional approaches; and it highlights characters whose definition, scoring, independence, and variation should be reviewed with particular care. The application of CIR reveals that at least some cladograms depend entirely on a single character.

New data on enamel thickness in Homo sapiens

Understanding the relationship between bone strain and bone growth is critical for interpreting variations in skeletal robusticity. Recently we presented a model for interactions between estrogen, strain, and periosteal bone growth, in which high estrogen (E2) increases, and low (E2) decreases, osteogenic responses to strain. We compared cortical growth in expercised and sedentary sheep (Ovis aries) with higher vs. lower estrogen levels, and showed that exercised animals with high E2 added substantially more bone than those with lower E2. However, without normal controls, it was unclear whether exercise-induced cortical growth was stimulated by high E2, suppressed by lower E2, or both. Here we present a broader test of interactions between E2 levels (normal, low, high) and loading (exercised and sedentary). Low E2 animals were vaccinated against GnRH to suppress estrogen, while high E2 animals received estrogen implants. After 45 days, periosteal bone growth was measured at hindlimb midshafts. The results support the hypothesis that estrogen upregulates strain-induced cortical bone growth: exercised, high-E2 animals grew 6-27% more bone than exercised animals with lower E2 levels, or sedentary animals (p<.05). The effects of the anti-GnRH vaccine on bone growth are more complex. Assays showed that vaccinated animals had normal, not decreased, E2 levels, but grew 34-39% less bone in response to exercise than normal controls (p<.05). This suggests the vaccine affected strain-induced bone growth without changing circulating E2, an unexpected finding. These results demonstrate that variation in E2 levels may produce differential growth response to similar mechanical loading through complex mechanisms.