The first named fossil species of Giraffa was not discovered as a complete animal. Giraffa sivalensis was defined from a single neck vertebra found in the Siwalik region of South Asia. Over time, other bones and teeth were assigned to the same species, yet descriptions of its size ranged from smaller than living giraffes to larger.
Sybrand van Sittert and Graham Mitchell revisited those conflicting estimates in a 2015 study. Their question went beyond deciding which number to print beside a reconstruction. Could the defining neck bone and the other assigned remains plausibly represent animals of the same body plan?
The neck vertebra pointed toward an individual of roughly 400 kilograms, with an estimated neck length of about 1.47 metres. Several limb bones implied much larger bodies. The discrepancy remained scientifically useful because it exposed the assumptions that a complete-looking reconstruction could otherwise conceal.
The bone has to be identified before it can be scaled
The defining specimen, or holotype, anchors the species name. It is therefore the starting point when asking whether other fossils belong with it. The vertebra's body measured about 20 centimetres long, but its anatomical position within the neck had been disputed since the nineteenth century.
That uncertainty matters because the cervical vertebrae are not interchangeable. A measurement associated with a particular body size at one position can imply something different at another. The researchers compared the fossil's shape with the neck bones of living giraffes, paying particular attention to how its forward joint-bearing processes projected beyond the vertebral body.
On that comparison, they considered the third cervical vertebra the most likely identification, with the fourth and fifth less likely. This depended on an assumption of broad anatomical similarity to living giraffes; there was no articulated fossil neck in which its position could simply be counted.
When the same fossil measurement was entered into equations for different neck positions, its predicted mass changed substantially. The preferred third-vertebra interpretation gave about 390 kilograms from one cross-sectional dimension, compared with lower estimates if it were the fourth or fifth. Anatomical identity was therefore part of the numerical result, not a preliminary detail without consequences.
The fused ends and developed surface features of the bone also supported an adult animal. A smaller-than-modern estimate could not simply be dismissed as a young calf that had yet to grow.
Two ways to borrow a living comparison
No equation can directly weigh an extinct animal. A scaling estimate first establishes a relationship between bone dimensions and body mass in animals whose sizes can be studied, then applies that relationship to the fossil.
One approach compares different living species. Another follows differences among growing individuals within one species. Van Sittert and Mitchell tested both, including equations derived from the growth of living giraffes. A younger giraffe might preserve a more useful version of the unusual giraffe body plan than an unrelated adult mammal of similar size.
That possibility had to be checked rather than assumed. The researchers tested how well the equations worked in living giraffes and in okapis, the other living giraffid. Okapi skeletons supplied a valuable contrast in proportions, although their masses were represented by an assumed adult range rather than recorded weights for each individual.
Some measurements transferred much better than others. Among the neck dimensions, the height of the rear joint-bearing end produced comparatively modest mass prediction errors in both living forms. Other dimensions could fit giraffe growth well yet give poor okapi estimates.
A close-fitting curve within one species is therefore not enough to justify its use in another. The relevant test is whether it can make useful predictions beyond the animals from which it was constructed.
Neck length and body mass are separate predictions
Scaling from the probable third vertebra produced an estimated bony neck length around 1.27 metres. An estimate including the surrounding tissues gave an external neck length around 1.47 metres. The authors also inferred a reaching height near 3.9 metres, assuming a body plan comparable to that of a living giraffe.
These figures are connected model outputs, not measurements of a preserved fossil silhouette. In particular, the length of one vertebra does not establish body mass on its own. An animal can have a relatively slender neck and a stockier body, or different proportions between its neck and limbs.
The authors favoured roughly 400 kilograms for the individual represented by the defining vertebra. That was a considered estimate based on the better-performing neck measurements, with uncertainty inherited from the living comparisons and the fossil's identity. It should not be treated as the measured adult mass of the species as a whole.
Why the limbs did not agree
The assigned limb fossils were incomplete, so their full lengths could not provide a common basis for comparison. The researchers instead measured shaft circumferences and diameters. The two upper-arm bones, or humeri, implied bodies around 770 to 810 kilograms when estimated from circumference using giraffe growth data.
The radius and metacarpal produced still larger estimates, some exceeding a tonne. But those more distal bones also performed less reliably when the equations were checked against living giraffids. Their larger estimates could not automatically overrule the neck or humerus results.
An independent study by Nicolás Campione and David Evans explains the broader reasoning behind such checks. They compared limb measurements with recorded body masses across hundreds of living mammals and non-avian reptiles. Combined humeral and femoral circumference predicted mass more reliably than limb length in their dataset, while still carrying substantial prediction error.
That supports testing load-bearing dimensions, but it does not make every limb bone an equally reliable scale. The fossil giraffe study had different elements available and faced the additional problem of deciding whether those elements belonged to the same species. A broadly successful method cannot remove uncertainty about the specimen being measured.
A disagreement with several possible explanations
The relatively reliable humerus estimates were still much higher than the preferred estimate from the defining neck vertebra. One explanation is that the species had a more substantial body for its neck than the living giraffe model assumes. Another is that the neck and limb fossils represented different sexes in a species with a large size difference between males and females.
A third possibility is that some of the isolated fossils were assigned incorrectly. The specimens were not associated parts of one skeleton, and historical records of their geological and geographic origins were incomplete. Finding broadly giraffe-like bones in a region does not guarantee that they all represent the species named from one vertebra.
The study could not choose decisively among these alternatives. An associated skeleton would be especially valuable because it would connect the neck, limbs and teeth within one individual, directly testing whether their proportions fit the proposed reconstruction.
The resulting picture is more informative than a single averaged mass. The defining specimen probably came from a smaller, shorter-necked animal than a large living giraffe, under the tested assumptions. The other fossils pose a separate question about variation, body shape and identity. Keeping that question visible is what makes the reconstruction open to improvement when the next bone is found.
Sources
- Sybrand J. van Sittert and Graham Mitchell (2015). “On reconstructing Giraffa sivalensis, an extinct giraffid from the Siwalik Hills, India.” PeerJ 3, e1135. Full paper.
- Nicolás E. Campione and David C. Evans (2012). “A universal scaling relationship between body mass and proximal limb bone dimensions in quadrupedal terrestrial tetrapods.” BMC Biology 10, 60. Related study.
