Tour of the Penguin Skeleton II: The Tarsometatarsus
Ah, the tarsometatarsus. Certainly this is the single most important bone in fossil penguin taxonomy. Thousands of words have been spent describing the arcane details of the difficult-to-pronounce tarsometatarsus in monographs, reports of phylogenetic analyses and diagnoses justifying new species. So what is the purpose of this famous element? The tarsometatarsus is essentially an ankle bone. In the human foot, there are five metatarsals and they make up the arch. In birds, there are three full sized metatarsals (and one tiny one). However, the three full-sized elements all fuse together, joining also with some of the tarsal elements – little cube-like bones that form our own ankles. Thus the complex name – tarso(for the tarsals)metatarsus(for the three metatarsals). Because birds walk on their toes, most of the tarsometatarsus is held off the ground, rather than forming the sole of the foot as it does in humans. Because multiple individual bones combine to form the tarsometatarsus, it has a very complex morphology. For this reason, characteristics of the bone are extremely useful for distinguishing species and determining their evolutionary relationships. In fact, more than 10 extinct penguin species have been named based on a single fossil tarsometatarsus, including the first one ever discovered, Palaeeudyptes antarcticus. One of the key features of interest are the proximal vascular foramina, small openings between the fused metatarsals that transmit blood vessels. Sometimes there are two, sometimes only the medial one is present, and sometimes only the lateral one is present. This simple feature can hint at which group of species a new fossil might belong to – for example, giant Anthropornis penguins tend to have only the medial foramen, while Palaeeudyptes penguins tend to have only the lateral. Another feature is the set of grooves that partially separate the three metatarsals – if they are very deep, this can help identify a penguin of the genus Spheniscus. Finally, there is the shape of the trochleae, the three pulley-shaped ends projections at the end of the bone where the toes attach. There are quite robust (penguins have thick toes). The one for the fourth toe is straight, rather than deflect outwards, which gives the foot a more “pigeon-toed” alignment – that is, except in Waimanu, the most primitive penguin genus.
If we compare the tarsometatarsus of a penguin to that of a close relative like a petrel, the first difference that pops out is shape. A penguin tarsometatarsus is much shorter and wider than a normal birds, appearing very stout and blocky. This is part of the reason penguins have such an endearing waddle on dry land – their feet are very stubby! Another interesting feature is that there are clear grooves marking the boundaries of the three metatarsals, whereas there is no trace of separation in normal birds. This feature may be a form of neotony, the retention of juvenile characteristics in to adulthood – we suspect this because hatchling birds have separate metatarsals which fuse together as they grow. The observation of this neotenous character, along with the mistaken belief that penguins were always flightless, led early ornithologists to consider penguins as the most primitive type of birds. We now know this is not true, and that penguins secondarily evolved these characteristics from a volant (flighted) ancestor.