What Are Ossified Tendons? Soft Tissue That Turned to Bone in Life

A tendon connects muscle to bone and is normally a flexible cord of collagen. In several animal groups tendons mineralise, becoming rigid rods of bone in place, which happens during life rather than after death and is a normal part of development in the species concerned. In dinosaurs the process affected the tendons running along the top and sides of the vertebrae, particularly in the hip region and the tail, producing lattices of thin bony rods lying parallel to the spine and crossing between vertebrae. They fossilise readily because they are bone, and they are common enough in some groups that a well-preserved specimen shows a dense basketwork of them. Living birds ossify tendons extensively, particularly in the legs and along the spine, which supplies the comparison used to interpret the fossil arrangement.

The mechanical consequences follow from replacing flexible cord with rigid rod:

• •Stiffening the spine, which prevents it sagging under the weight of a large body without needing more muscle
• •Reducing the energy cost of standing and moving, since a passive rod holds a position that a muscle would otherwise have to hold actively
• •Locking the tail into a rigid counterweight, which balances the body over the hips in bipedal animals
• •Transmitting force efficiently along a limb, which is what the bird leg arrangement does
• •Allowing a long neck to be held out without continuous muscular effort
• •Constraining movement, which is the cost, since a stiffened section cannot bend

The distribution is informative about what the arrangement was solving. Bird-hipped dinosaurs have them extensively, with duck-billed dinosaurs and their relatives carrying dense lattices along the back and tail, which fits animals that were large, that spent time on two legs and that needed a stiff horizontal body balanced over the hips. Theropods have them in the tail, producing the rigid counterweight that reconstructions of running depend on, and the dromaeosaurs took this furthest with elongated projections from the vertebrae bundled alongside the tendons to make the distal tail essentially a rod. Sauropods have them less conspicuously. The arrangement appears independently in several lineages, which is what would be expected of a solution to a mechanical problem that large animals repeatedly face rather than of an inherited quirk.

Turning flexible tissue rigid to save muscular effort is a solution animals have found repeatedly. Bird legs contain ossified tendons that transmit force from muscles high in the limb down to the toes, which is why a bird can grip a perch while asleep without expending effort, since the arrangement locks passively when the leg bends. Large mammals including horses and cattle have elastic ligaments that hold the head and neck up with minimal muscular work, and the same principle appears in the suspensory apparatus of the leg. Turkeys develop ossified leg tendons with age and are a standard research model for the process, which has attracted medical interest because unwanted tendon ossification occurs in people after injury. The recurrence of the principle is a reminder that a large body is mostly a problem of holding itself up cheaply.

The structures cause confusion in the field and in museums. Isolated rods separated from a skeleton resemble fish bones, plant stems or fragments of rib, and have been catalogued as all three. In articulated specimens the lattice can obscure the vertebrae beneath it during preparation, and preparators have removed them as matrix in the past. They are also occasionally described in popular accounts as extra bones or as a bony rod running down the animal's back, which overstates the case, since they are separate elements lying alongside the spine rather than part of it. The comparison with living birds resolves most of the difficulty, since the same structures can be dissected, and it is a good example of how a living relative settles a question that the fossil alone leaves genuinely ambiguous.

Tendons that mineralised during life, forming rigid rods along the spine and tail, common enough to appear as a dense basketwork in good specimens. They stiffen the body passively, which is cheaper than holding it with muscle, and lock the tail into a counterweight. The arrangement appears independently in several lineages, and living birds supply the comparison that interprets it.