Identifying the Involvement of Multiple Carnivore Taxa with Archaeological Bone Assemblages

1. What they did

Many early archaeological bone assemblages carry both stone-tool butchery marks and carnivore tooth marks. Which carnivores fed on the bones, and in what order with the hominids, matters for understanding how early hominids got their meat. If cats (flesh specialists) ate first, hominids probably came later as scavengers; if only hyaenas (bone crunchers) were involved, hominids may have had first access. The authors made a first attempt to tell, from tooth marks alone, whether bones were chewed by one carnivore species or by several.

They measured tooth pits: marks shaped like a whole tooth crown or a single cusp. They compared three samples:

• Modern carnivores (221 pits). Long bones chewed by a single known species: spotted hyaena (77), leopard (67), lion (38), cheetah (35) and jackal (4). Apart from the hyaenas, which were captive animals at the University of California, Berkeley, the marks came from free-ranging carnivores of both sexes and various ages in Tanzania.
• The FLK Zinjanthropus assemblage (61 pits). Moulds of tooth pits on bovid long bones from this 1.8 million year old site, made at the National Museum of Kenya.
• Extinct carnivores (27 impressions). Latex impressions of undamaged tooth crowns and cusps of Dinofelis, Megantereon, Percrocuta, Homotherium and an extinct Felis from the Omo fossil assemblage. These were used for shape only, because the soft latex made the impressions larger than real marks.

For each pit they measured two things on video images: the area in square millimetres, and the shape, as the ratio of the longest axis to the shortest. They sorted the marks by bone type (soft spongy cancellous bone at the ends, thinning cortical bone near the ends of the shaft, and hard cortical bone of the shaft), because softer bone lets teeth sink in further. They also tested whether moulds gave the same measurements as real bone, and dropped moulds of shallow pits from the Zinj sample because shadows distorted their outlines.

2. What they found

Size of tooth pits

• For each modern species, and for the Zinj sample, pits on cancellous bone were larger than pits on cortical bone. For example, spotted hyaena pits averaged 21.54 mm² on cancellous bone and 2.36 mm² on cortical bone.
• Pit size on thinning cortical bone varied the most of the three bone types.
• Size alone cannot identify the carnivore: cheetah pits on cancellous bone (3.18 mm²) were similar in size to hyaena and leopard pits on cortical bone (2.36 and 2.21 mm²), and lion pits on cortical bone (5.48 mm²) were similar to leopard pits on cancellous bone (6.26 mm²).
• In the Zinj sample, pits on cancellous bone (21.34 mm²) were similar in size to those of hyaenas and lions; pits on cortical bone (1.77 mm²) were most similar to those of cheetahs, leopards and spotted hyaenas.
• Jackal pits were the smallest (0.26 mm², cortical bone only).

Shape of tooth pits

• The Zinj pits varied much more in shape (mean axis ratio 2.41, standard deviation 1.83) than the pits of any single modern or extinct species (standard deviations 0.14 to 0.73).

3. What they concluded

• Bone density affects the size of tooth marks, so large marks on spongy bone ends should not automatically be put down to hyaenas. Bone density and other factors must be considered before naming the carnivore.
• The wide range of tooth pit shapes at FLK Zinjanthropus shows that more than one kind of carnivore fed on these bones; hyaenas can be ruled out as the only one.
• Marks on cortical bone point to medium-sized carnivores. The authors infer that medium-sized cats removed the flesh first, hominids then took the remaining scraps and marrow (many bones carry butchery marks), and hyaenas came last for the grease, removing many of the bone ends.
• This fits the idea that the hominids at this site scavenged carcasses left by leopards or extinct medium-sized carnivores, or carcasses cached in water by hyaenas.
• Most studies have looked only at the spotted hyaena, leading to what the authors call the erroneous conclusion that hyaena ravaging wipes out traces of earlier feeding; in palaeoanthropology “carnivore” has become almost a synonym for “hyaena”. Tooth marks of other carnivores can survive in the archaeological record.
• Identifying carnivores from their tooth marks could show which carnivores were at a site even when their skeletons are not preserved, helping to reconstruct past environments and test ideas about hominid and carnivore competition for carcasses.
• Similar work is needed at other early sites, and further tooth mark features may yet identify individual species.

4. How tracking was or was not used

There was no tracking in the field. The study is about one kind of sign: tooth marks left on bones by feeding carnivores. It measures the size and shape of these feeding signs to work out which carnivores, and how many kinds, chewed a set of bones.

5. Tracker or observer certification

Not reported. The paper does not say how each modern chewed bone was linked to the species that made the marks (the method is referred to an earlier thesis, Selvaggio 1994a), and the comparison depends on those attributions being right.

6. Other technologies, skills or innovations

• Video imaging: each tooth pit was viewed through a video camera with a magnifying lens, traced by mouse on the monitor, turned into a black and white image, and measured by software calibrated in pixels per millimetre. Faint outlines on real bone were lightly traced with a lead pencil to improve contrast.
• Moulds of archaeological marks in “Xanthopren”, and latex (“Cuttersil”) impressions of fossil carnivore teeth.