Archaeoindris - Anatomy and Physiology

Anatomy and Physiology

Though similar to Palaeopropithecus, Archaeoindris was significantly larger and more robust. Archaeoindris was one of the largest primates to ever evolve, and was the largest-known prosimian primate, weighing an estimated 160 kg (350 lb). It was roughly the size of an adult male gorilla, which was first noted by Lamberton.

Since its discovery, size estimates have varied from "larger than a human" to "possibly the largest primate ever". In a study by Jungers from 1990, the area of its molar teeth predicted a mass of 230.5 kg (508 lb), while the femoral head diameter predicted a mass of 244.1 kg (538 lb). In 1995, Laurie Godfrey estimated a mass of 197.5 kg (435 lb) using the midshaft circumferences of the humerus and femur. Based on multiple regressions of the cortical area of the femur in 2008, Jungers and colleagues generated the current best estimate of 161.2 kg (355 lb) with a possible range of 150–187.8 kg (330–414 lb). These estimates were considered to be more accurate since the harder cortical bone in the midshaft of the femur supported an animal's weight, and its thickness better correlated with the animal's mass than the midshaft diameter (which includes both hard cortex and spongy bone). The only fossil primate that was probably larger than Archaeoindris was Gigantopithecus blacki, a close relative of orangutans.

Like all three species of Palaeopropithecus, Archaeoindris exhibited derived traits not seen in the two other less specialized genera of sloth lemur. These traits included deflated auditory bullae and paired protrusions of bone around the nasal opening consisting of part of the premaxilla and the nasal bones. Its skull was wider than that of Megaladapis, but shorter, measuring 269 mm (10.6 in). Its face was shorter than that of Palaeopropithecus, with its eyes directed further forward. The neurocranium (braincase) was small and elevated relative to the face, unlike Palaeopropithecus. Postorbital constriction (narrowing of the skull behind the eye sockets) is pronounced. The skull also bore a low, broad sagittal crest (a ridge of bone on the top of the skull to which jaw muscles attach) and robust but smaller nuchal crests (ridges of bone on the back of the skull to which neck muscles attach). Unlike Palaeopropithecus the rims of the orbits (eye sockets) were not as thick. The area of the orbit was 946 mm2 (1.466 sq in), comparable to that of gorillas. Based on the ratio of its orbit area to the size of its optic canal, Archaeoindris had low retinal summation, meaning its eyes were less sensitive to light (like living diurnal lemurs). Yet the ratio was not as low as in comparably sized apes, suggesting Archaeoindris had low visual acuity and lacked trichromatic color vision.

The jaw exhibited a long, robust mandibular symphysis (joining of the two halves of the lower jaw), which fused early during development. Its palate (bones on the roof of the mouth) was rectangular. Like other sloth lemurs, it likely experienced accelerated dental development, and had an adult dental formula of . Its teeth were also similar to that of Palaeopropithecus, both in morphology and proportions. The four lower incisors that would have made up the toothcomb were modified, being shorter and more robust, possibly for feeding purposes. The canines were short and stout, but sharp. Also, there was a diastema (gap) between the lower premolars (p2 and p4). Other dental similarities with Palaeopropithecus included small third upper and lower molars (M3 and m3), the first and second molars were narrow and long, and the enamel of its cheek teeth was crenulated (low and rounded), though not as wrinkled and slightly higher-crowned.

Most bones of the postcranial skeleton, including the bones of the hands, feet, vertebral column, ribs, radius (lower arm bone), tibia, and fibula, have not been found for Archaeoindris. As with many cranial features, the postcranial bones that have been discovered are similar to that of Palaeopropithecus, but significantly larger and more robust. The head of the femur was large and lacked a fovea capitis femoris (a small depression in the head of the femur). The femur was short and extremely robust, had a very high collodiaphyseal angle (the angle of the neck and shaft of the bone), and the greater trochanter was small. In the adult, the humerus was significantly longer than the femur, while in the immature specimen, both the humerus and ulna were much longer than the femur, making the arms considerably longer than the legs, as also seen in Palaeopropithecus. The relative length of the arms to the legs was shorter in Archaeoindris, so although its intermembral index was over 100, it was lower than that of Palaeopropithecus.