Posts Tagged ‘Bones’
Seymour Island is located near the northern end of the Antarctic Peninsula. This prolific fossil site has yielded up a massive haul of fossil penguin bones starting over the past century. Thousands of specimens have been collected and there is evidence for about a dozen different species of penguins living side by side in the area roughly 40 million years ago. One of the biggest species is named Anthropornis nordenskjoeldii. Anthropornis means “man bird”, referring to its massive size and “nordenskjoeldii” honors polar explorer Adolf Erik Nordenskiöld. Somewhat ironically for the honoree of a penguin species name, his explorations were primarily in the high North. Among other exploits, he collected fossils on the island of Spitsbergen.
One of the difficulties of studying the Seymour Island penguins is that despite the abundance of bones, almost all of them are isolated. The area seems to have been an estuary, and the frenetic meeting of river and ocean waters may have worked to scatter penguin bones all around. I’ve looked at hundreds of specimens of Seymour Island penguins, and the only two specimens I have ever seen that include more than one bone are a “fossil knee” including the patella, piece of the femur and a piece of the tibiotarsus and a hip socket with a chunk of the femur stuck in it. Isolated bones can be frustrating when there are multiple species that are the same size living in an area. How can we tell whether this sharp beak belongs to that wing? How can can we tell whether the wide foot bone belongs to the penguin with the long neck bones? It poses a particularly difficult challenge to efforts to understand fossil penguin locomotion, because we really want whole flippers or hindlimbs from a single bird if we are going to predict diving or walking style accurately.
As it turns out, a nearly complete flipper from Seymour Island has been known from nearly 60 years. The great New Zealand penguin paleontologist Dr. Brian J. Marples studied the bones in 1953. Marples was a very cautious paleontologist, and avoided naming new species or assigning bones to the same individual unless there was overwhelming evidence. He noted the bones all fit together, but gave them separate numbers and they ended up with their own little tags in the collection. Flash forward to 2012. Dr. Piotr Jadwiszczak at Uniwersytet w Białymstoku revisited the bones in a recent paper. He found evidence from the preservation, siding, and proportions of the bones that they most likely belong to a single individual. This lets us finally get a good idea how the flipper of Anthropornis was built. Leg bones were recovered nearby too, and they probably also belong to this particular penguin.
The flipper is robust – very bulky compared to modern species. One of the strangest things about the flipper is that the tip of the flipper shows a “modern” plan, with the third metacarpal extending past the second (in essence, the bone that would make up the base of the middle finger in a person is longer than the bone that would make up the pointer finger). This advanced feature suggests that Anthropornis may have been more closely related to living penguins than previously thought, although other features of the skeleton would argue that it was very primitive. A primitive feature of the flipper is the great angling between the bones, which results in a somewhat more auk-like wing as opposed to the very straight wings of modern penguins. In the past, this angling has been connected to poorer diving capacities, but as Dr. Jadwiszczak and our own papers have noted this is not necessarily true. Auks can reach tremendous depths, and they are at a disadvantage compared to penguins because of their higher buoyancy and less dense bones. Anthropornis appears to have been a strong diver based on the flipper.
Reference: Jadwiszczak, P. 2012. Partial limb skeleton of a “giant penguin” Anthropornis from the Eocene of Antarctic Peninsula. Polish Polar Research 23: 259-274.
Mandible is the scientific term for the lower jaw. Whereas we humans have a lower jaw made of only a single bone (the dentary), penguins have a more complicated mandible made up of half a dozen different elements (the dentary, splenial, articular, prearticular, angular and surangular). These bones are all connected in penguins, though some of the joints are rather loose, which allows the jaws to flex a bit This process is called kinesis. At the front tip of the mandible, the left and right sides of the jaw meet and connect at the symphysis. This region is often a very firm connection, with no movement possible. One of the many unique things about living penguins is that they have a very short, flexible connection at the symphysis. This allows for more “play” in the jaws, which may be helpful when a bird has a mouthful of thrashing prey. Not all penguins have a short symphysis though. The spear-beaked fossil species Icadyptes salasi, for example, has a long, firm connection which is probably related to a different style of prey capture (e.g., spearing versus biting).
Different types of penguins exhibit different mandible shapes. The depth of the mandible can be an important clue to the type of food penguins eat. Many fish and squid specialists have low, slender mandibles like most other birds. Krill-loving species like the Adélie Penguin often show much deeper jaws. This difference is interpreted as an accommodation for the larger, spikier tongue of those penguins, which helps them capture shoaling prey. For this reason, I am always eager to measure the jaw dimensions of fossil penguin specimens that I stumble upon in the field (or in museum drawers). Its quite interesting to note that so far, none of the ancient penguin species that have been discovered had deep jaws. This suggests they had not yet adapted specializations to catching krill. Perhaps this feeding strategy was acquired only recently in penguin evolution, as Antarctic ice sheets spread and gave rise to new ecosystems.
In this installment of our tour of the penguin skeleton, we will take a look at the phalanges. Phalanx (plural phalanges) is the word anatomists use for a finger bone. We have 14 total phalanges in our own hands, 3 in most fingers but just 2 in the thumb. You can tell where two of these bones meet because the connection forms a knuckle joint. Birds, of course, are not running around with visible fingers (not since the Cretaceous Period anyway), but they do have phalanges embedded within the wing. Most birds have one phalanx remaining in their first digit (equivalent to our “thumb”), two phalanges in their second digit (equivalent to our “pointer finger”), and one phalanx in their third digit (equivalent to our “middle finger”). These are of no use for manipulating objects, but do serve a purpose by anchoring some feathers. In particular, the phalanx of the “thumb”, in birds called the alular phalanx, is important because it anchors the alula, a special feather that is important in controlling flight speed during landings.
In penguins, the phalanges are weird. They look a bit like a normal birds phalanges got run over – this is part of a general pattern of flattening seen in the penguin wing skeleton, which makes it more flipper-like. The third digit is typically tiny in birds, but in penguins it is huge. The phalanx is long and tapers to a pointed tip, and has a sharp backward pointing projection. One of the really cool things about this bone is that you can trace its evolution in the fossil record. In more basal fossils penguins like Icadyptes, the phalanx is large compared to flying birds, but still much smaller than in living penguins, and also lacks the projection. Extending this bone to modern lengths happens about 30 million years into penguin evolution, and results in a slight decrease in aspect ratio of the flipper. Most importantly, penguins lack an alula altogether. This may be related to the lack of differentiation in penguin feathers. Certainly, they don’t need to “land” anymore so a special alula feather is probably superfluous. In the image below, you can see the major differences in shape between the wing of a shearwater (a flying relative of penguins) and an Emperor Penguin.
One of the most difficult penguin bones to identify in isolation is the patella. This element looks like a misshapen cube, with one smooth surface and several rough faces. If found by itself outside a box of penguin bones, or eroding out of the surface in fossil form, it would be difficult to be sure that a patella was even a bone. The bizarre appearance of the patella is in part due to the fact that it is a sesamoid, or a bone that is embedded within a tendon. In life, the patella sits at the joint between the femur and tibiotarsus. One of its main functions is to help guide the tendon of the ambiens muscle, which either travels through a hole in the patella (in most extinct penguins and in the living stiff-tailed penguins of the genus Pygoscelis) or across a groove in the surface (in most living penguins).
Humans have a patella too, and it is sometimes referred to as the kneecap. This is a fairly apt name, as the bone looks somewhat like a smashed lid. It sits between the same two bones in humans (although we have a plain tibia, rather than a tibiotarsus). Presence or absence of a patella varies in birds – some families have a large patella like penguins, others have a very tiny version, and some have none at all. Perhaps the most interesting patella is that of the loon, which is very large and helps these birds with their unique kick-diving mode of locomotion.
It’s time to continue out tour of the penguin skeleton. Today, we will look at the pygostyle. This is a special element that is formed by multiple caudal (tail) vertebrae that fuse together as birds reach adulthood. Whereas the dinosaurian ancestors of modern birds had long bony tails with dozens of individual vertebrae, living birds only have a few individual “normal” caudal vertebrae with a pygostyle at the end. This structure is usually somewhat plate shaped – that is, flattened in the vertical plane. The pygostyle is very important in flight because it serves as the attachment site for muscles the raise and lower the tail, and those that move the tail from side to side. This allows volant birds to change the angle of the tail feather fan, which is critical in landing and turning.
Penguins have a pygostyle, but it is quite different in shape from the standard avian pygostyle. In penguins, the element is more elongated and less flattened. Rather than being plate-like, it is almost triangular in cross-section with a flattened base. Penguins also have a very different set of tail feathers. Rather than forming a fan, penguin tail feathers are very stiff and quill-like, and stick out somewhat like the bristles of a broom. This is especially true of penguins from the genus Pygoscelis – the Linnean name Pygoscelis actually translates to “stiff tail”. These penguins are prone to be caught slouching around, partially propped up on their tail feathers. It seems that without the necessity of maintaining a “fan” of tail feathers, penguins have gone ahead and modified their pygostyle to a shape more suited to supporting themselves on land than steering in flight.
One of the new themes that has appeared on March of the Fossil Penguins is a tour of the penguin skeleton – looking at each bone and discussing how it is different from flying birds. A map might be handy, so here is a King Penguin (Aptenodytes patagonicus) skeleton with the bones we’ll visit labeled. Check the list below the image, and I will link in each bone as new posts occur.
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Penguin bones are common in the fossil record. Trillions of penguins have lived over course of the Cenozoic, and a tiny portion of these died in environments favorable to fossil preservation. Some fossil penguins are giant and others tiny, some are complete skeletons and others single bones, some pristine and others badly damaged by erosion or chewed on by sharks. However, almost all of them are bones of adult birds. This is actually not too surprising when you think about penguin life cycles. Adults spend most of their time in the water, so it is easier for animals that die and sink to the bottom to be covered by sediments and have a chance at fossilization. Hatchling and juvenile penguins stay on land though, and if they don’t make it to adulthood it often means they were gobbled up by a predator and leave no trace in the fossil record.
Hatchling penguin fossils can be really informative, because they provide unequivacal evidence that a breeding colony existed in the area. Usually, this is very hard to prove in the fossil record, because modern penguins are infamous drifters. Some travel more than a thousand miles over the course of a normal year before returning to their breeding grounds, and a few random individuals always end up on the wrong continent annually (like New Zealand’s stray Emperor Penguin). Because of these penguin proclivities, it is hard to be sure whether adult fossil penguin bones we find at some sites belong to penguins that bred there, or instead belong to a bird that was just passing through.
Unfledged birds are those that haven’t yet left the nest. In dramatic cases, fledging occurs when a baby bird is pushed out of the nest by its parent, and instinctively takes its first flight. For penguins, fledging involves a trip into the ocean instead. That’s why hatchling penguin bones can be so informative to paleontologists. If we find a fossil from an unfledged bird, we know with certainty that bird was hatched in the area because it was too young to have started swimming.
Part of the story in my recent paper with Dr. Daniel Thomas is based on tiny fossils from hatchling penguins. There are quite a few fossils of juvenile animals in the South African records from Langebaanweg. These bones show a very spongy texture because they have not completely ossified yet and parts remain cartilaginous (just like our own bones when we are children). There are also fossil parts of compound bones that remain separate in hatchlings but fuse together in adults. For example, the three small bones on the right side of the photo are parts of the foot that join completely together in fully grown penguins. In order to figure out how old these penguins were when they perished, we looked at modern skeletons of hatchling, juvenile and adult penguins in museums. Based on the patterns we observed, we aged the bones in the photo to a very young bird, which would not yet have started moulting into its adult feathers (and therefor wouldn’t be ready to take its first plunge). It’s sad to think these birds never had a chance to enter the marine realm, but studying their fossil remains pins down the location of their species breeding ground pretty precisely. That’s a rare and valuable peek into a vanished ecosystem.
Remarkable is a fair way to describe the penguin skeleton. Each bone seems perfectly molded to the complex mission of the penguin – to swim, dive, scramble across rocks, or catch prey as the situation calls. One obvious difference between penguin bones and those of other birds is their density. Pick up box of penguin bones (most species skeletons will pack neatly into the average tennis shoe box) and a box of chicken bones and you’ll instantly notice the weight difference. Increased density and reduction of air space helps penguins maintain negative buoyancy while diving. While almost every bone in the penguin skeleton has undergone increased osteosclerosis for density, the shape of each bone also tells its own story. Some are identical to the comparable element in a “normal” bird, while others would barely be recognized as avian by most non-specialists.
I’d like to review the entire penguin skeleton bone by bone. This will take a long time – there are over one hundred free elements in the adult penguin skeleton (some are formed by multiple bones that fuse together as the penguin grows). Let’s start with the scapula, one of the most unusual.
The scapula is commonly known as the shoulder blade in humans. In fact it is even more blade-shaped in birds. Most living birds have a scapula that looks somewhat like a curved sword. The “blade” extends over the front part of the birds back. The “handle” contacts two other bones, the furcula (wishbone) and coracoid, forming the an opening in between called the triosseal canal. This canal is very important because the tendon that helps birds lift their wings travels through it. Lifting the wing is of course part of the wingbeat cycle – up, down, up ,down and there goes the bird through the air.
In volant (flying) birds, the upstroke is really used only to re-position the wing for the next downstroke, which does all of the actual thrust generation to push the bird through the air. In penguins though, the upstroke is much more important. Because water is so much denser than air, a penguin can push against the water as it lifts its flipper up. Therefore, it can generate thrust on both the upstroke and the downstroke.
This high density of water also creates a problem though – it requires a lot more force to flap a flipper in water than a wing in air. So penguins have maxed out the muscles that lift the wing. One of these, the scapulohumeralis caudalis, attaches to the scapular blade. In most birds, this muscle does not have to be very large, so the thin blade provides more than enough room for it to attach. In penguins, the muscle needs more room and the blade is greatly expanded to accommodate this. A penguin scapula looks almost like a tennis racket, with a normal thin “handle” and a flattened, paddle-like blade region. Interestingly, the blade is only slightly widened in the earliest fossil species, suggesting that penguins gradually improved their wing upstroke strength over time.
Penguin feet are very distinct. Perhaps the most important bone in terms of giving penguins their cachet with the public is the tarsometatarsus (the ever-present foot bone). Thanks to their almost comically short feet, penguins move on land with an endearing waddling gait rather than with the more serious step of birds with longer, more gracile legs. Aside from length, another major difference between the feet of penguins and those of most other birds is that penguins have a very tiny hallux, or first toe. Most birds have four toes, instead of the five typical of mammals like ourselves. A few have only three toes (for example, some kingfishers) and the ostrich is unique in having only two. In most living birds, the first toe quite large and is modified for perching. It is reversed, facing the opposite direction as the remaining three toes, to help grasp branches more firmly. In some aquatic birds, the first toe is connected to the second toe by a web which makes the foot a more efficient paddle.
Penguins certainly don’t perch. They also don’t paddle with their feet, instead using their flippers to propel themselves through the water. Penguin feet are made for walking and steering. These birds, although much more graceful at sea, are quite capable of marching across challenging terrain. Adelie Penguins can march up to 100 kilometers (about 60 miles) across sea ice to get to their breeding grounds. Penguins can also jump surprising distances. The aptly named rockhopper penguins can maneuver dangerously jagged stacks of wave-beaten rocks on windswept islands by bounding from one to another with striking skill. Penguin feet may not be good for running or perching in trees, but they are well designed for this type of workload. The tarsometatarsus and phalanges (small bones of the toes) are wrapped in a thick layer of blubbery fat to cushion them, and covered with rough, thick scales to stand up to wear and help gain purchase on slippery surfaces. Steering is the other locomotory task of the penguin foot. If you have a chance to observe penguins swimming through a glass divider at an aquarium or zoo, pay attention to their feet. They employ their feet like little rudders, angling them to help control their dive direction.
The fossil record shows us that penguins have developed a more compact foot over time. Waimanu, the oldest known fossil penguin, has a relatively short tarsometatarsus by the standards of flying birds, but it cuts an elegant figure compared to the squat tarsometatarsus of living penguins. We see a more square shape and some shortening in the Eocene penguins from Seymour Island. By the Oligocene (25 million years ago) most of the penguins preserved in the fossil record have a foot of modern proportions. So far, no fossil has preserve the hallux. This suggests that even the oldest penguins already had a very small hallux, as such a tiny bone can easily be lost during the fossilization process or even go unrecognized during fossil collection and preparation. Aside from being a bit of trivia, many ornithologists consider the fact that both Procellariiformes (tubenose seabirds like albatrosses and petrels) and penguins have a miniscule hallux to be strong evidence linking these two groups of birds to a common ancestor.