What Can a Claw Tell You? Shape, Keratin and the Missing Outer Layer
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A fossil claw is only the bony core of what the animal actually had, since the horny sheath covering it rarely survives. That missing layer was longer, sharper and differently curved, which changes every conclusion drawn from the bone alone.
What is preserved and what is not
The structure at the end of a digit has two parts. The bony core, technically the last bone of the finger or toe, is what fossilises, and it carries grooves, ridges and a roughened surface where the covering attached. Over that sits a sheath of keratin, the same material as fingernails, horn and beak, which is produced continuously from the base and worn or shed at the tip. That sheath is longer than the bone beneath it, frequently by a third or more, and its curvature continues beyond the bone rather than matching it, so a reconstruction using only the bony core underestimates both length and sharpness substantially. Comparison with living birds and reptiles supplies the correction, since the relationship between core and sheath can be measured directly in animals where both are available, and the ratio varies with what the claw is used for.
What shape indicates
Curvature and proportion correlate with function in living animals, which allows inference in fossils:
- •Strongly curved and laterally compressed claws are associated with climbing and with gripping prey
- •Shallow curves and blunt tips are associated with walking on the ground, where claws contact the substrate constantly
- •Deep flexor tubercles, the attachment point for the tendon that closes the digit, indicate the force available
- •Asymmetry between the digits of one hand indicates the hand was doing something other than bearing weight
- •Wear patterns record use, since a claw that contacted the ground wears differently from one that did not
- •Size relative to the limb indicates how much investment the animal made in the structure
The famous ones
Several dinosaurs are known primarily for their claws and the interpretations have shifted considerably. The enlarged sickle claw on the second toe of dromaeosaurs was long described as a slashing weapon, and mechanical testing on models found that it cut poorly and gripped very well, which supported a reinterpretation as a holding device used to pin prey while the animal fed, on the analogy of modern birds of prey. The enormous hand claws of therizinosaurs, among the largest known, belonged to animals whose teeth and gut suggest a plant diet, so the claws were probably used for pulling vegetation rather than for anything predatory. A famous set of giant arms known for decades without an associated body caused prolonged speculation before a complete specimen showed the animal was an unremarkable plant eater with a very odd build. In each case the bone alone supported more than one story.
The material itself
Keratin explains several features of how these structures behave and why they rarely fossilise. It is a protein produced by cells that die and compact into a tough layer, so the working surface is dead tissue with no blood supply and no capacity to repair, which is why a damaged claw grows out rather than healing. Growth is continuous from the base, matching wear at the tip, which means a claw kept off the ground grows longer than one in constant contact and that captive animals frequently develop overgrown claws. The same material builds horn, beak, hair, feathers, scales and hoof, differing in arrangement rather than composition, which is why a group that evolves one of these has the machinery for the others. Being protein, it decays readily and survives only in exceptional preservation, and the specimens that do preserve it are correspondingly valuable for calibrating everything inferred from bone.
The limits of inference
Reading function from shape is more uncertain than the cleanness of the correlations suggests. Living animals use their claws for more than one thing, so a single shape serves several functions and the categories overlap considerably. Behaviour changes with age, and a juvenile may climb while an adult does not, with the claw shape reflecting a compromise. Some structures are used for display or for combat with rivals rather than against prey, which leaves no distinguishing signature. Statistical studies assigning fossil claws to functional categories by curvature return substantial overlap between categories in the living reference animals themselves, so a fossil falling in the overlap cannot be assigned. And the missing sheath introduces an error that has to be estimated rather than measured. The honest conclusion for most specimens is a range of plausible uses rather than a single one.
The takeaway
Only the bony core fossilises, and the keratin sheath over it was longer and differently curved, so reconstructions from bone alone understate length and sharpness. Curvature and the tendon attachment indicate climbing, gripping or walking. The sickle claw grips far better than it cuts, which reinterpreted it as a holding device, and living animals overlap enough that many fossil claws cannot be assigned at all.