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Hippopotamus amphibius, photographed at the Philadelphia Zoo.
Have you ever tried to walk along the bottom of a pool while fully submerged? It isn't easy. Keeping your feet on the bottom is enough of a task, and you would probably need a weight belt to take an underwater stroll. Hippos (Hippopotamus amphibius), though, walk and even prance along the bottom of lakes and rivers with ease. How do they do it?
When compared to a whale or even a manatee (the latter of which I will address a bit later on) a hippo does not look especially well-adapted to life in the water. It has a low, squat body and lacks a broad tail, flippers, or any other broad surface to help propel itself through the water. Neither is this amphibious mammal well-suited to quick movements on land. Hippos can trot a bit, but they are so cumbersome that while walking on dry land they always keep three of their feet in contact with the ground at a time. Read the rest of this post... | Read the comments on this post...... Read more »
Coughlin, B., & Fish, F. (2009) Hippopotamus Underwater Locomotion: Reduced-Gravity Movements for a Massive Mammal. Journal of Mammalogy, 90(3), 675-679. DOI: 10.1644/08-MAMM-A-279R.1
Life restoration of the head of Armadillosuchus. From Marinho and Carvalho (2009).
When I was trying to come up with a title for this post I almost went with "Armadillosuchus: An armored crocodylian you wouldn't want to mess with." Obviously I changed my mind. Not only was the title too long, but it was redundant to boot. All crocodylians are "armored" in that they have little bony plates called osteoderms (primarily on the dorsal, or top, side of their bodies) beneath their scales, which in turn overlay a layer of bony plates called osteoscutes. Crocodylians are tough!
The newly-described crocodylian Armadillosuchus from the Late Cretaceous deposits of Brazil, however, was carrying a more bizarre complement of armor. Right behind its head was an armored dome of hexagonal plates. This bony buckler was rigid, but could be moved independently of the head so that the neck was not always locked in one position. Now comes the really interesting part. Behind this "cervical shield" was a series of about seven mobile armored bands. (What the researchers call "mobile-banded body armor.") This is very similar to what is seen in living armadillos, hence the croc's name Armadillosuchus. This crocodylian had "armadillo-like" armor even before the mammals did! Read the rest of this post... | Read the comments on this post...... Read more »
Marinho, T., & Carvalho, I. (2009) An armadillo-like sphagesaurid crocodyliform from the Late Cretaceous of Brazil. Journal of South American Earth Sciences, 27(1), 36-41. DOI: 10.1016/j.jsames.2008.11.005
Parts of the skull, including the upper jaws (maxillae), of Eritherium azzouzorum as seen from the front (top) and below (bottom). From Gheerbrant (2009).
Yesterday I blogged about the ~27 million year old elephantimorph Eritreum, a creature that stood only about four feet high at the shoulder, but there were once even smaller proboscideans. About sixty million years ago in what is now Morocco there lived a rabbit-sized (~5 kg) hoofed mammal that is one of the earliest known relatives of the modern behemoths of Africa and Asia. Called Eritherium azzouzorum, it was a small mammal that could have a major influence on our understanding of elephant evolution. Read the rest of this post... | Read the comments on this post...... Read more »
Gheerbrant, E. (2009) Paleocene emergence of elephant relatives and the rapid radiation of African ungulates. Proceedings of the National Academy of Sciences, 106(26), 10717-10721. DOI: 10.1073/pnas.0900251106
A restoration of Eritreum compared to the larger Gomphotherium. From Shoshani et al. (2009).
Before I loved dinosaurs, I loved elephants. I would run around the backyard with my little pith helmet on, firing my "elephant mover" to herd the imaginary pachyderms. (At the time I did not understand what guns did. When they went off in the documentaries I saw that the elephants moved, therefore guns were "elephant movers.") It would only be much later, when I could properly appreciate the stout bones I saw in the halls of the American Museum of Natural History, that I would more fully appreciate the past history of some of my favorite animals.
A recent discovery made in ~27 million year old deposits in Ethiopia helps fill in that history. Proboscideans, the group of mammals that contains living elephants and their many bizarre extinct relatives, have been around for about 55 million years, but one of the big questions has involved the evolution of the elephantimorphs (or elephants and extinct groups like mastodons, gomphotheres, and stegodons). How did the early elephantimorphs evolve, disperse, and replace the more archaic elephantiformes (creatures like Palaeomastodon, Moertheritherium, and Barytherium)? The new fossil from the land bordering the Red Sea sheds some new light on this question and may help connect the elephantimorphs to the elephantiformes. Read the rest of this post... | Read the comments on this post...... Read more »
Shoshani, J., Walter, R., Abraha, M., Berhe, S., Tassy, P., Sanders, W., Marchant, G., Libsekal, Y., Ghirmai, T., & Zinner, D. (2006) A proboscidean from the late Oligocene of Eritrea, a "missing link" between early Elephantiformes and Elephantimorpha, and biogeographic implications. Proceedings of the National Academy of Sciences, 103(46), 17296-17301. DOI: 10.1073/pnas.0603689103
Richard Owen's restoration of Glyptodon. From Brinkman (2009).
Perhaps one of the primary reasons that there is so much to say about Charles Darwin is that he left us so much material to scrutinize. Outside of his famous printed works there are numerous notebooks and a staggering amount of personal correspondence which are constantly being parsed for insights into how he formulated his evolutionary ideas. Indeed, there is still scholarly debate about when Darwin embraced the idea of evolution and what observations spurred him to that intellectual turning point, and a new paper by Paul Brinkman examines the role fossils played in the young naturalist's transformation. Read the rest of this post... | Read the comments on this post...... Read more »
Brinkman, P. (2009) Charles Darwin’s Beagle Voyage, Fossil Vertebrate Succession, and “The Gradual Birth . Journal of the History of Biology. DOI: 10.1007/s10739-009-9189-9
Is intelligent design science, or not? Think carefully before you answer. The modern intelligent design (ID) movement is motivated by theological concerns and trades in on religious authority to meet its aims, but stripped of this background, can ID be relegated to the "junk science" bin? While the answer to this latter question is "Yes", in a new paper ("The science question in intelligent design") Sahotra Sarkar argues that proclaiming ID to be non-science without careful consideration does little good. Read the rest of this post... | Read the comments on this post...... Read more »
The face of Anoiapithecus. From Moya-Sola et al. (2009).
One of the most controversial aspects of the whole Darwinius kerfuffle has been the primate's proposed status as "the ancestor of us all." The fossil, named "Ida", has been popularly touted as the "missing link" connecting us to all other mammals, but how can we really know if Darwinius fits this role? The truth is that we can't, and it is nearly impossible to parse direct ancestor-descendant relationships among fossil vertebrates, especially when we're talking about a fossil that lived over 40 million years before the first hominins evolved.
Indeed, the frenzy over Ida has just been made all the more unfortunate by the announcement of a fossil primate in the journal PNAS that is much, much more closely related to us. Called Anoipithecus brevirostris, this short-faced ape was recently discovered in 11.9 million-year-old (Middle Miocene) strata in Spain. It doesn't have a book, a flashy website, or a prime-time TV slot, but it could have some very interesting implications for the origins of our ape ancestors. Read the rest of this post... | Read the comments on this post...... Read more »
Moya-Sola, S., Alba, D., Almecija, S., Casanovas-Vilar, I., Kohler, M., De Esteban-Trivigno, S., Robles, J., Galindo, J., & Fortuny, J. (2009) A unique Middle Miocene European hominoid and the origins of the great ape and human clade. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.0811730106
The exceptionally preserved skeleton of Darwinius, known popularly as "Ida." From PLoS One.
It has been three days now since an international team of paleontologists promised to deliver the change we need change everything, but when I woke up this morning I was pleased to find that things had still not gone "Bizarro World" around here. There is still a lot going on with Darwinius (better known as "Ida"), though, and while I am sure we will still be talking about her for some time to come I wanted to take a moment to step back and answer a few questions that keep cropping up about this spectacular fossil.
As I attempted to follow the frenzy over this fossil primate I noticed that many people did not understand how scientists knew Ida was a female and that she was not yet fully mature. How can you tell such a thing from a fossil? It is not like they come with a birth certificate. True enough, but there are tell-tale characteristics the paleontologists who studied Ida used to come to these conclusions.
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Franzen, J., Gingerich, P., Habersetzer, J., Hurum, J., von Koenigswald, W., & Smith, B. (2009) Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology. PLoS ONE, 4(5). DOI: 10.1371/journal.pone.0005723
A restoration of the extinct adapid Darwinius, known popularly as "Ida." From PLoS One.
So the big day is finally here. "Ida", a 47-million-year-old primate skeleton from Messel, Germany has finally been unveiled on PLoS One and in a flurry of press releases, book announcements, and general media hubub. Under different circumstances I would be happy to see an exceptional fossil receiving such treatment, but I fear that Ida has become a victim of a sensationalistic media that values audience size over scientific substance.
Before I jump into my criticisms of the paper describing Darwinius masillae, Ida's scientific name, I do want to stress how spectacular the fossil really is. The primate fossil record is extremely fragmentary, and if you want to know anything about fossil primates you are going to have to know your teeth. That's usually all that is left of them. Ida, then, is a paleontologist's dream come true. Not only is it a complete specimen but parts of the primate's last meal were preserved inside its stomach and its body outline was marked by bacteria that fed on the decomposing carcass during fossilization. This is the first time a fossil primate has been found exhibiting such extraordinary preservation.
The exceptionally preserved skeleton of Darwinius, known popularly as "Ida." From PLoS One.
Most of the media reports about Darwinius have only mentioned this point in passing, though. What they are most interested in is its status as a "missing link" between anthropoid primates (monkeys and apes) and their ancient ancestors. As John Wilkins has pointed out the phrase "missing link" is woefully inaccurate, conjuring up images of life ranked in an unbreakable Great Chain of Being put in place by God, but that has not stopped media outlets from running with the idea. Even though the authors of the paper deny making any such statement, the promotional materials they are associated with (most notably the "Revealing The Link" website) play up this angle to a ridiculous degree.
So what is all the hubub about? Why is the History Channel falling all over itself to promote this fossil? It all goes back to a long-standing debate over the origins of anthropoid primates that, until now, has mostly gone on in academic journals and scientific meetings.
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Franzen, J., Gingerich, P., Habersetzer, J., Hurum, J., von Koenigswald, W., & Smith, B. (2009) Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology. PLoS ONE, 4(5). DOI: 10.1371/journal.pone.0005723
About four million years ago, in the shallows of an ocean that once covered what is now southern Peru, a large shark bit into the jaw of a baleen whale. The whale had been dead for some time, but it was kept afloat but the gases building up in its body as it decomposed. It was absolutely rotten, but it still presented a free meal to the scavenging shark. As the shark bit down, however, one of its teeth became stuck in the whale's jaw bone and broke off. No matter. The lost tooth would soon be replaced by another.
The above scenario does involve a bit of speculation, but such events certainly did happen. Sharks fed on whale carcasses in ancient times just like they do today, and when sharks feed they often lose teeth. In the latest edition of the journal PALAIOS scientists Dana Ehret, Bruce MacFadden, and Rodolfo Salas- Gismondi describe a 4-5 million year old piece of whale jaw with a shark tooth stuck in it, direct evidence of interaction between predator and prey.
The fossil that is the subject of the study was discovered in 2007 from the Pisco Formation in Peru. It is a site well-known for its excellent preservation of fossils, enough to put together a fairly clear picture of what the area was once like. Around 4-5 million years ago the site was a shallow, coastal environment inhabited by turtles, crocodiles, fish, sharks, whales, and even a giant aquatic sloth. Of the shark species present there some got to be quite large; the older part of the Pisco Formation contains the remains of Carcharocles megalodon and Isurus hastalis while the younger part contains the fossils of a species of Carcharodon (the genus to which the living great white shark belongs).
The tooth crown of Carcharodon broken off in a baleen whale jaw. The boxes on the side indicate the presence of tooth scrapes. From Ehret, et al 2009.
The tooth that was broken off in the whale jaw most closely resembles that of Carcharodon, and this is supported by comparing it to a complete set of jaws from the same locality just described in the Journal of Vertebrate Paleontology. The question is, however, whether the whale was killed by the shark or already dead when the Carcharodon came along. It is not an easy question to answer, but there are ways to rule out some of the possibilities.
If the shark had attacked but the whale survived it would be expected that the whale bone would show signs of healing. No such signs were found. This leaves two main possibilities: that the shark killed the whale or the shark scavenged the whale carcass. The latter hypothesis seems more likely. Observations of sharks, even large ones, attacking living baleen whales are extremely rare, and when they do attack they often target the belly or sides of the whale. More often sharks are seen feeding on the decomposing carcasses of whales which often float on the surface of the ocean for extended periods of time. During these feeding events the postcranial part of the whale is often preferentially stripped of blubber and meat, but on at least one occasion juvenile great white sharks were seen to feed around the head region of a dead baleen whale. Perhaps the fossil from Peru is a remnant of a similar behavior.
Unfortunately the paper does not have much of a satisfying conclusion. The fossil probably represents a scavenging event but it is not possible to know for sure (unless, of course, you have a time machine, some scuba gear, and about a million years to kill). Perhaps the specimen is not as exciting as some other recently announced fossils but it is interesting for another reason. It is a specimen that illustrates an interaction between two organisms, a little piece of paleobiology frozen in time.
DANA J. EHRET, BRUCE J. MACFADDEN, and RODOLFO SALAS-GISMONDI (2009). CAUGHT IN THE ACT: TROPHIC INTERACTIONS BETWEEN A 4-MILLION-YEAR-OLD WHITE
SHARK (CARCHARODON) AND MYSTICETE WHALE FROM PERU PALAIOS, 24, 329-333 DOI: 10.2110/palo.2008.p08-077r Read the comments on this post...... Read more »
DANA J. EHRET, BRUCE J. MACFADDEN, and RODOLFO SALAS-GISMONDI. (2009) CAUGHT IN THE ACT: TROPHIC INTERACTIONS BETWEEN A 4-MILLION-YEAR-OLD WHITE SHARK (CARCHARODON) AND MYSTICETE WHALE FROM PERU. PALAIOS, 329-333. DOI: 10.2110/palo.2008.p08-077r
Of all the evolutionary transitions that have ever taken place few have received as much attention as the origin of whales. (See here, here, here, here,and here for a few of my posts on the subject.) The story of how terrestrial hoofed mammals gave rise to the exclusively aquatic leviathans has been highlighted in headlines over and over again, but other marine mammals have not received the same amount of public attention. In the case of pinnipeds (seals, sea lions, and walruses) this may be at least partially due to the fact that their origins have been difficult to tease out.
It has long been known that seals and sea lions are carnivoran mammals closely related to weasels or bears, but just how pinnipeds first became adapted to a semi-aquatic life has been more difficult to figure out. This is largely due to the fact that their fossil trail stopped rather abruptly. The 24-22 million year old Enaliarctos, the oldest pinniped well-represented by fossils, was already a sea-lion like creature that swam in the sea. Surely there had to be even older fossils connecting it to its progenitors, but for years the trail was cold.
The skull of Puijila from the underside (a) and left side (b). From the Nature paper.
The gap between Enaliarctos and its forebears did not represent a real void in nature. Instead it pointed to a gap in our knowledge. That gap has now been partially filled with the announcement in the journal Nature of Puijila darwini, an early Miocene seal that represents a more terrestrial stage of pinniped evolution.
Bear in mind, however, that Puijila was probably not ancestral to Enaliarctos or living pinnipeds. Given that evolution is a branching process it can be extremely difficult to tell whether you are dealing with, as T.H. Huxley put it, "fathers" and "sons" (linear types) or "uncles" and "nephews" (intercalary types). Puijila appears to be one of the latter, a side branch that preserved some of the tell-tale traits than can inform us about pinniped evolutionary transitions. Indeed, the significance of Puijila is that it is representative of a stage of pinniped evolution that scientists could only hypothesize about previously. It is the fossil confirmation of the idea that seals did evolved from fully terrestrial ancestors.
The restored skeleton of Puijila. From the Nature paper.
Puijila was found on Devon Island, a chilly island well inside the Arctic Circle in Nunavut, Canada. About 21 million years ago there was a freshwater lake there where rabbits, rhinoceros, antelope, and other creatures sometimes came to drink.Puijila also lived in and around the lake. It was about three feet long with a long tail and feet that, while webbed, could certainly not be called flippers. Indeed Puijila was very similar in many respects to the extant river otters of North America, yet it differed in some important ways. It had upper arms and shoulders, for instance, that were slightly more expanded and robust than the same bones in otters. These differences would have provided more area for muscle attachment, and this could mean that Puijila was propelling itself with its webbed front and hind feet (possibly with an emphasis on the front feet).
So how does Puijila relate to other mammals? Given that it lived just prior to Enaliarctos and a radiation of marine pinnipeds it is probably more of a "persistent type" of early pinniped than an actual ancestor of creatures like Enaliarctos. Still, the phylogenetic analysis grouped it with Enaliarctos and another pinniped called Potamotherium. From what is known of these genera and the locations in which they have been found the authors of the paper suggest that pinnipeds may have originated by about 33 million years ago somewhere in the Arctic.
If the authors of the paper are correct the transition of pinnipeds to an aquatic mode of life would have started with Puijila-like mammals that lived in freshwater. Eventually, though, these amphibious mammals would have extend their range to the sea shore where they would have been further adapted to life in the water into forms more like Enaliarctos. Whether this hypothesis is correct, however, will rest on further studies of the fossil evidence.
Puijila is also important to illustrating the contingent nature of evolution. Even though it is extremely otter-like it did not swim like an otter. It primarily used its limbs to swim and, it probably did not incorporate up-and-down motions of its spinal column like transitional whales (i.e. Rodhocetus) did, either. If Puijila really does represent an important transitional stage in pinniped evolution, then, they way it swam can tell us much about why pinnipeds swim by using their limbs while other marine mammals exhibit different types of locomotion. The past is the key to the present.
I truly hope that Puijila gets the public attention it deserves. It is a wonderful, nearly-complete transitional form that answers some of our evolutionary questions while raising new ones. I hope it will inspire vertebrate paleontologists to look into pinniped origins with renewed vigor, and perhaps in a few decades we can talk about transitional pinnipeds with as much excitement as that with which we discuss transitional cetaceans.
Links: The official Puijila website.
Not Exactly Rocket Science: Puijila, the walking seal.
Rybczynski, N., Dawson, M., & Tedford, R. (2009). A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia Nature, 458 (7241), 1021-1024 DOI: 10.1038/nature07985 Read the comments on this post...... Read more »
Rybczynski, N., Dawson, M., & Tedford, R. (2009) A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia. Nature, 458(7241), 1021-1024. DOI: 10.1038/nature07985
A giraffe, photographed at the Bronx zoo.
Why do giraffes have long necks? We know that modern giraffes must have evolved gradually, but figuring out what selection pressures influenced giraffe evolution is another story altogether. One of the most popular recent explanations is that giraffes have long necks as a result of sexual selection.
The "necks for sex" hypothesis is primarily inspired by the contests between male giraffes. In these duels the males stand side by side and whack each other with their necks and ossicones ("horns"). This can be seen in the video below;
What does this behavior have to do with the evolution of long necks? According to the "necks for sex" hypothesis males with longer necks in an ancestral population of giraffes would have won more of these contests and thus been more successful at mating. It seems simple enough, but there is a problem: the selective pressure would just be on the males. Indeed, if this kind of competition between males was driving the selection for long necks we would expect to see more sexual dimorphism. Females, who don't engage in these contests, would probably have shorter necks than males. If little to no sexual dimorphism can be seen in living giraffes it is unlikely that sexual selection was the main cause for the evolution of the modern forms. To test this hypothesis G. Mitchell, S.J. van Sittert, and J.D. Skinner examined 17 male and 21 female giraffes that were killed during culling in Zimbabwe and reported their results in the Journal of Zoology.
What Mitchell and colleagues found was there were virtually no differences between males and females in terms of body size, neck length, and leg length that could be attributed to sexual selection. There was one interesting minor difference, though. The zoologists found that female giraffes had proportionally longer necks compared to foreleg length than males when the sexual selection hypothesis would predict that males would have longer necks. Both sexes also had necks that grew faster than the rest of their body, cutting down the idea that males "invested" more in their necks than females.
Simply put, the "necks for sex" hypothesis fails because there is no evidence of sexual dimorphism in giraffes that would result from male-male competition. The competitions between male giraffes are a consequence, rather than a cause, of neck elongation. Giraffe necks became elongated for some other reason.
The debate over giraffe necks illustrates the pitfalls of trying to figure out past evolutionary pressures based almost solely upon living animals (and just one living species, at that). Studies involving the benefits of having a long neck to feeding or intrasexual competition might be informative, but what giraffes do now might not tell us much about what caused their ancestors to evolve long necks. We should not confuse what an organ is used for now with what led to its origin: they are not always the same.
Strangely, a discussion of fossil evidence is almost always missing from hypotheses about the evolution of giraffe necks. Perhaps this is because we cannot observe the behavior or feeding patterns of extinct creatures, but studies of fossil giraffes could do much to inform discussions of giraffe evolution. In fact I recall seeing a fossil giraffe with a neck intermediate in length between ancestral and living forms in Don Prothero's Evolution: What the Fossils Say and Why it Matters. I do not know if a study of this animal has been formally published yet, but it could certainly be important to figuring out how giraffes evolved.
Mitchell, G., van Sittert, S., & Skinner, J. (2009). Sexual selection is not the origin of long necks in giraffes Journal of Zoology DOI: 10.1111/j.1469-7998.2009.00573.x Read the comments on this post...... Read more »
Mitchell, G., van Sittert, S., & Skinner, J. (2009) Sexual selection is not the origin of long necks in giraffes. Journal of Zoology. DOI: 10.1111/j.1469-7998.2009.00573.x
An artist's restoration of Hurdia. From the Science paper.
It is not easy working on Cambrian fossils. The petrified treasures are found in only a few places in the world, and even though many exhibit exquisite preservation they come from a time when life on earth would have looked very unfamiliar. One such creature, Anomalocaris, was a three foot long invertebrate that swam by undulating a series of lobes on either side of its body. In front of its mouth were two spiked tendrils that may have helped situate prey items to be processed by its conveyor belt of crushing plates that was its mouth. There is nothing quite like it alive today.
Indeed, Anomalocaris was so unusual that it was misidentified multiple times. Parts of it were taken as representing jellyfish, crustaceans, or other strange Cambrian creatures. Now we known differently, but similar problems still affect other Cambrian fossils. In the book Wonderful Life the late paleontologist Stephen Jay Gould wrote that "Three genera (Hurdia, Tuzoia, and Carnarvonia) are bivalved arthropod carapaces with no soft parts preserved; they cannot be properly allocated to any arthropod subgroup, and remain unclassified today." Twenty years later Tuzoia remains a mystery and I can only imagine that the name of Carnarvonia has changed as that genus name is also used for a flowering plant, but in the journal Science a team a researchers has just revealed a new identity for Hurdia.
Admittedly I am a little late to the game on this one (see what others have had to say at Ediacaran, The Dragon's Tales, and Deep Sea News), but it is hard to resist talking about such a strange creature. It has a story as complex as, and entwined with, that of Anomalocaris.
As Allison Daley and colleagues note, Hurdia has had a complex history. Parts of it were identified as belonging to jellyfish and various hard-shelled invertebrates, but during the major revisions that occurred during the 1980's these parts were assigned to two particular genera: Anomalocaris and Laggania. This placed the pieces of Hurdia in the right ballpark, but it was not until the 1990's that Desmond Collins saw that Hurdia deserved its own genus designation. A reinvestigation of the material relating to all three genera has supported this conclusion and given us an image of a creature even stranger than Anomalocaris.
Generally speaking Hurdia looked a lot like Anomalocaris. Being close relatives they shared a common body plan with anterior tendrils, stalked eyes, and a trash-compactor-of-doom type mouth. A major feature that made Hurdia distinct, though, was a large head shield that stuck out in front of its eyes. These shields were a little over three inches long, making up for about half the body length of the entire animal.
The systematic placement of Hurdia. From the Science paper.
Hurdia has also provided increased insight into the place of creatures like Anomalocaris among invertebrates. Together Hurdia, Anomalocaris, and Laggania belong to a group called Radiodonta, and this group is placed close to the Euarthropoda, or "true" arthropods like trilobites. This makes the members of the Radiodonta very significant to questions of arthropod origins, and the recent discovery of another Anomalocaris-relative called Schinderhannes has provided researchers with even more material to investigate. These creatures were strange enough to start with, but with each new discovery they seem get a little bit stranger.
Daley, A., Budd, G., Caron, J., Edgecombe, G., & Collins, D. (2009). The Burgess Shale Anomalocaridid Hurdia and Its Significance for Early Euarthropod Evolution Science, 323 (5921), 1597-1600 DOI: 10.1126/science.1169514 Read the comments on this post...... Read more »
Daley, A., Budd, G., Caron, J., Edgecombe, G., & Collins, D. (2009) The Burgess Shale Anomalocaridid Hurdia and Its Significance for Early Euarthropod Evolution. Science, 323(5921), 1597-1600. DOI: 10.1126/science.1169514
The extinct whale Dorudon, from the new PLoS One paper.
When the English anatomist William H. Flower proposed that whales had evolved from terrestrial ungulates in 1883 he cast doubt upon the notion that the direct ancestors of early whales chiefly used their limbs for swimming. If they did, Flower reasoned, whales would not have evolved their distinctive method of aquatic locomotion, typified by vertical oscillations of their fluked tails. Instead Flower suggested that the stock that gave rise to whales would have had broad, flat tails that paved the way for cetacean locomotion as we know it today, and he closed his lecture with a vision of such creatures shuffling about the water's edge;
We may conclude by picturing to ourselves some primitive generalized, marsh-haunting animals with scanty covering of hair like the modern hippopotamus, but with broad, swimming tails and short limbs, omnivorous in their mode of feeding, probably combining water plants with mussels, worms, and freshwater crustaceans, gradually becoming more and more adapted to fill the void place ready for them on the aquatic side of the borderland on which they dwelt, and so by degree being modified into dolphin-like creatures inhabiting lakes and rivers, and ultimately finding their way into the ocean.
Flower's hypothesis was criticized by other naturalists and eventually forgotten, but it proved to be prescient in at least one respect. During Flower's time whales were thought to have evolved from terrestrial carnivorous mammals, perhaps going through a seal-like stage. Flower differed in that he favored ungulates, or hoofed mammals, and he made particular references to both pigs and hippopotamus (note that his hypothetical cetacean ancestor is an omnivore).
Both of these latter creatures are artiodactyls, mammals marked by an even number of hoofed toes and a distinctive ankle bone (the astragalus) with a "double pulley" shape to it. This is significant because studies undertaken during the past decade have confirmed that whales not only evolved from artiodactyls, but are derived artiodactyls adapted to life in the sea. Even though Flower did not have access to the majority of data we presently have his hypothesis held the hint of a relationship that would be confirmed over 100 years later.
Flower's hypothesis that the ancestors of whales used their tails to swim right from the start, however, has not stood up so well. In early whales there was a transition between using limbs, the limbs and undulations of the spine in combination, finally oscillations of the tail as locomotor methods. A new archaeocete, Maiacetus, is a creature "caught in the act" of this transition.
A reconstruction of Maiacetus, from the PLoS One paper.
Just announced in the journal PLoS One, Maiacetus was a member of the Protocetidae, an extinct group of early whales that beautifully illustrate an important phase of whale evolution. Previously known members of the group include Rodhocetus and Protocetus from Pakistan and Georgiacetus from the southern United States, the latter animal illustrating that this group included the first whales able to cross oceans.
Unlike Basilosaurus, these whales still had hips that were attached to their vertebral column. Many members of this group probably swam by undulating their spine while paddling with their limbs, perhaps similar to the way an otter moves through the water. The fusion of the hips to the spine limited their range of movement, but it did allow them to support themselves on the shores of the estuaries and coastlines they inhabited.
These are generalities, of course, because not all members of the Protocetidae were the same. In particular they were marked by differences in limb proportions, and Maiacetus fits snugly into the continuum of anatomical types. Rodhocetus, for example, has very large hind limbs ended with flat, paddle-like feet. Protocetus, by contrast, had much smaller hind limbs that were probably not very important to locomotion; its tail was more important to its swimming style. Maiacetus is closer to Rodhocetus in form but its hind limbs are smaller and do not show the same adaptations for hind-foot propulsion. What does this mean?
There is more than one possible answer, especially since Rodhocetus and Maiacetus were contemporaries. Maiacetus could represent the form of the more-terrestrial stock from which Rodhocetus evolved, thus making the big feet of Rodhocetus an adaptation to an aquatic lifestyle. This is consistent with the fact that the range of undulatory motion in the spine of Maiacetus would have been limited as the sacral vertebrae of this animal, those that articulated with the hip, were still fused. (They are unfused and difficult to discern from neighboring vertebrae in whales that swim chiefly through tail oscillations.)
On the other hand, Maiacetus could represent the stage in protocetid evolution after Rodhocetus and thus represent a reduction in the hind limbs as tail propulsion became more important. In this scheme of transitional types a reduction in hind limb size would be seen from Rodhocetus to Maiacetus to Georgiacetus and Protocetus. No systematic analysis or phylogenetic tree was included in the paper, but it seems that in either scenario Maiacetus represents an important link between the form of Rodhocetus and other early whales.
What is clear is that Maiacetus, like Rodhocetus, had hips attached to its spine. Did this genus spend some time on dry land? It is certainly possible but there is a key complication needs to be addressed. Early whales had heavy, dense, hypermineralized bones that acted as a kind of "bone ballast" when they were in the water. (This adaptation would later be adapted-out when heavy bones became a liability to whales of the open ocean, like Dorudon.) This allowed them to spend more energy swimming and less on staying submerged, but it also made their skeletons more fragile on land. They would not have been able to run quickly along the shores and, like modern pinnipeds, probably stayed close to the water.
A female Maiacetus, with fetus (in baby blue). From the PLoS One paper.
One of the Maiacetus skeletons contained an important clue that may support the idea that early protocetids hauled out onto the beach on occasion. The authors of the paper reported that a highly-developed Maiacetus fetus was found inside one of the skeletons. Its position (nearly inside the ribcage of its mother) illustrates that it may have shifted position between the time its mother died and when it became fossilized, but its orientation inside the mother might have been preserved in its natural state.
The fossil fetus faces towards the tail-end of the mother, and if this was its natural orientation (as the authors suggest) then it might be a clue that Maiacetus gave birth on land. Modern whales are born tail-first, an adaptation to life in the sea that prevents them from drowning. Terrestrial mammals, by contrast, are often born head first. The authors use this difference to hypothesize that Maiacetus came out onto land to give birth, meaning that these early whales were still tied to the coast.
This hypothesis cannot be accepted unequivocally, however. The orientation of the fetus is consistent with the author's hypothesis but if the baby's bones moved due to some taphonomic process then we have to be careful in considering what it might (or might not) mean. It should also be remembered that some protocetids were capable of crossing oceans, as shown by the presence of Georgiacetus in the United States. This is far from the hub of whale evolution in the region of present-day Pakistan, and it suggests that at least some protocetids were able to spend extended periods at sea. Given this information it is possible that Maiacetus had behavioral and physiological mechanisms that allowed it to give birth in the water.
Just as the skeletal anatomy of whales changed due to life in the water their physiology would have changed as well. There would have been physiological transitions just as there were skeletal ones, and it is possible (if not likely) that Maiacetus possessed a physiology different from both modern terrestrial artiodactyls and whales. Unfortunately it is difficult, if not impossible, to test this because there are no living Maiacetus to examine. Further research will be required, and it may take many years to find another protocetid skeleton with a preserved fetus inside it for comparison. For now, the hypothesis that Maiacetus gave birth on land is certainly open to question.
A male Maiacetus. From the new PLoS One paper.
The fossil fetus was important for another reason, though. It showed that the individual that it was found inside was a female, and this provided a key to identifying the sexes of Maiacetus. It's all in the hips. The hips of female mammals are shaped differently than those of males and this is related directly to childbirth. Since the researchers were able to see what the hips of female Maiacetus looked like they could then compare those bones to the hips of another nearly complete skeleton to see if they matched. They were different, and it appears that the paleontologists have found both a male and female Maiacetus.
As in humans, the place where the left and right hip bones of the male Maicetus met on the ventral side created a narrow V shape. In females this area was wider and makes a U shape, which is a consequence of the presence of a birth canal. In this way the authors were able to make a strong case that the second skeleton belonged to the male.
The overall differences between the male and female skeletons were slight (the male was slightly longer and had slightly larger canine teeth), reflecting a moderate amount of sexual dimorphism. Males may have competed for mates but there was no evidence that these whales had an elephant seal-like harem arrangement where large, highly dimorphic males dominate a group of female and drive off competitors. The amount of dimorphism between the sexes, however, will have to be continually reviewed as new skeletons come to light. Variation between adults of both sexes will have to be accounted for in order to more precisely pin down the extent of sexual dimorphism in Maiacetus, although the authors are most likely correct that it was relatively minor.
There will be plenty to discuss about Maiacetus in the coming weeks, months, and years, but regardless of how hypotheses might change Maiacetus is a fantastic creature. it sat right at the nexus of the land-to-water transition in whale evolution and can tell us much about how that transition was affected. Although paleontologists were frustrated for decades by the dearth of early fossil whales there is now an abundance of material to analyze, with new discoveries being made every year. I wonder what William Flower would think of all this.
Philip D. Gingerich, Munir ul-Haq, Wighart von Koenigswald, William J. Sanders, B. Holly Smith, Iyad S. Zalmout (2009). New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004366
If you'd like to know more about whale evolution, see these posts;
Ancient Armored Whales
Ancient Toothed Whales Had Baleen
Shaking the Cetacean Evolutionary Bush
The Legacy of Basilosaurus
The Whereabouts of Buckley's Basilosaurus
Who was the first to mount Basilosaurus?
The Rise and Fall of Alabamornis
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Philip D. Gingerich, Munir ul-Haq, Wighart von Koenigswald, William J. Sanders, B. Holly Smith, & Iyad S. Zalmout. (2009) New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism. PLoS ONE, 4(2). DOI: 10.1371/journal.pone.0004366
This past February I wrote about a new giant sengi (or elephant shrew) described in the Journal of Zoology. When attempts to capture live animals failed, researchers used camera traps to get a better look at these previously undescribed creatures. A new paper in the Journal of Mammalogy has announced the discovery of another (although smaller) species of elephant shrew, but it wasn't so easy to spot.
The new species of sengi (bottom, EPI), compared with Elephantulus edwardii (EED), and E. rupestris (ERU). From Smit et al., 2008.
In the Western and Northern Cape Provinces of South Africa, there lives a small elephant shrew named Elephantulus edwardii. Previously there appeared to be the only endemic species, but the new study reports that there are in fact two species of sengi endemic to this region of South Africa. The new species has been named Elephantulus pilicaudus. The differences between it and its close relative, E. edwardii, however, are subtle.
"Species" as a popular term does not often match the scientific understanding of the concept. To the public, a species should be able to be visually distinguished from other animals, and what a non-scientist may recognize as different "species" may, in fact, be entirely different genera. To a scientist, however, the issue is much more complex, and a variety of information is required to determine whether animals that look very similar are truly distinct species or exhibit variation within one species.
Wild-captured and stuffed specimens of E. pilicaudus did show some morphological differences from the other species of sengi that live in its range (and its close relative, E. edwardii), but this alone was not enough to determine that it was a distinct species. Genetic information was also required, and researchers compared the DNA of the cytochrome-b gene from several sengi species to confirm that the new species was distinct.
It is not know how large or small the population of E. pilicaudus is. It appears to be rare, and its visual similarity to E. edwardii does not make identification easy. (A tell-tale sign of E. pilicaudus appears to be a larger tuft at the end of the tail.) The authors of the new paper report that it probably has a more restricted range than the other sengis that live in the same area, but it is unknown how prevalent it is within that range.
The announcement of a new species has something of a romantic mythology associated with it. With so much of the world explored, it would seem that the few remaining unknown species are only to be found in steaming jungles criss-crossed by lianas or in the darkest ocean trenches. Yet the truth is even more fantastic than such adventurous imagery. There are plenty of previously undescribed species virtually right under our noses, and it takes only a bit of careful attention to sniff them out.
H. A. Smit, T. J. Robinson, J. Watson, B. Jansen van Vuuren (2008). A New Species of Elephant-shrew (Afrotheria: Macroscelidea: Elephantulus) from South Africa Journal of Mammalogy, 89 (5) DOI: 10.1644/07-MAMM-A-254.1 Read the comments on this post...... Read more »
H. A. Smit, T. J. Robinson, J. Watson, & B. Jansen van Vuuren. (2008) A New Species of Elephant-shrew (Afrotheria: Macroscelidea: Elephantulus) from South Africa. Journal of Mammalogy, 89(5), 1257. DOI: 10.1644/07-MAMM-A-254.1
"Leonardo," the mummy dinosaur, courtesy of the HMNS.
Although it got a brief treatment in the book Horns and Beaks, many people have been waiting for more information on the exceptionally-preserved Brachylophosaurus skeleton named "Leonardo." Due to be unveiled next week at the Houston Museum of Natural Science (the date was pushed back due to Hurricane Ike; the museum and Leonardo were unharmed), the fossil provides a unique look at the soft tissues of this particular dinosaur.
Dinosaur "mummies" have been found before, dating back to the 19th century, but in many cases little more than skin impressions were preserved. Leonardo, by contrast, is so well preserved that paleontologists are getting a look at the internal anatomy of the dinosaur, and the new paper (out in Palaios) focuses on gut contents.
Within the body cavity of Leonardo there is a large amount of plant material. The question is, however, whether those plants represent gut contents or found their way into the specimen by some other route. If the body cavity was preserved intact (i.e. there were no gaping holes in it) then there could be little doubt that the plant material represents gut contents, but if the cavity was punctured other possibilities open up. The gut contents might have stayed in the body and been reworked, they could have been mixed with outside plant material, or the original contents could have been removed and then replaced. The actual soft tissue of the digestive tract would have decomposed fairly quickly after death and so the contents would not have been in their original position even if they were preserved, but the first task of the researchers was to determine whether they were dealing with gut contents or washed-in plants (or both).
A reconstruction of "Leonardo," courtesy of the HMNS.
The paleontologists were not looking at whole leaves, stems, or other plant parts, though. The preserved plant material was very fragmentary, best described as "fragile, black crumbs." These little bits were substantially different from the woody material found in some hardosaur coprolites (or fossil poo), meaning that (if the fragments were true gut contents) shortly before death Leonardo had a lunch of leaves. The fragments were consistent with what would be expected for partially-digested hadrosaur meals, of course, but it did not make identification of the plants especially easy. This became even more vexing when the researchers realized that the high amount of clay inside the body cavity most probably meant that the body cavity was breached. There was a possibility that some of this material came in from the outside.
[As is pointed out in the paper, many animals have clay or soil inside their digestive tract as a result of purposefully eating it or the material coming in with the food. In Leonardo's case, though, the sheer amount of clay showed that it was probably not intentionally swallowed. If the hadrosaur did so then it would have eaten more clay than plants!]
If there was a mix of gut contents and material from outside, however, it would be expected that there would be different characteristics between the partially digested food and the washed-in plants. Being that the material was generally uniform, the alternatives became narrowed down to either preserved gut contents or outside material. While both options were plausible, the retention of gut contents seems more likely.
A life illustration of Leonardo by Bob Bakker, courtesy of the HMNS.
Exceptional preservation is typically a result of rapid burial; the longer a carcass is left out to rot the more it will be picked apart or otherwise broken down before preservation. Given that Leonardo is perhaps the most exceptionally-preserved large dinosaur yet found, the skeleton was probably buried very quickly after death. This means that if the plant material in the body cavity came from the outside it would have had to find a way in after the body wall was breached, something that is unlikely. Indeed, the type of plant material inside the body was not found outside the body in the surrounding sediment; it is more probable that the crumbly plant fossils were actual gut contents.
About 75 million years ago, in what is now Montana, a subadult Brachylophosaurus died. While it was not killed or consumed by a predator, the exact cause of death is still a mystery. Either in death or shortly afterward the body of the dinosaur was quickly covered by sediment, perhaps by a flooded river. Piled beneath the mud and clay the dinosaur was beyond the reach of the large predators (except, perhaps, its tail, which may have been tugged on by some scavenger), and its body slowly started to decompose. The internal organs went first, with a breach in the body wall letting enough water and sediment in to wash the gut contents around inside the body cavity. This process preserved some aspects of the anatomy but obscured others, but tens of millions of years later the descendants of the tiny mammals that lived in the corners of the Mesozoic world would unearth Leonardo's remains. Paleontologists have only just begun to understand what the skeleton can teach us about dinosaurs, and I'm sure we will be talking about Leonardo for many years to come.
J. S. Tweet, K. Chin, D. R. Braman, N. L. Murphy (2008). Probable Gut Contents Within A Specimen Of Brachylophosaurus Canadensis (Dinosauria: Hadrosauridae) From the Upper Cretaceous Judith River Formation Of Montana PALAIOS, 23 (9), 624-635 DOI: 10.2110/palo.2007.p07-044r Read the comments on this post...... Read more »
J. S. Tweet, K. Chin, D. R. Braman, & N. L. Murphy. (2008) Probable Gut Contents Within A Specimen Of Brachylophosaurus Canadensis (Dinosauria: Hadrosauridae) From the Upper Cretaceous Judith River Formation Of Montana. PALAIOS, 23(9), 624-635. DOI: 10.2110/palo.2007.p07-044r
Hadrosaurs are often called the "cows of the Cretaceous." They were common, had few defenses compared to their armored ornithischian kin, and were a favorite prey for predatory dinosaurs. Natural selection appears to have applied sufficient pressure for at least one genus of hadrosaur, Hypacrosaurus, to change, however. It did not develop horns or spikes or a club, but instead ontogenetically outpaced their predators.
The key to determining how theropods and Hypacrosaurus grew can be found inside their bones, and a new study in the Proceedings of the Royal Society B using histologi... Read more »
Lisa Cooper, Andrew H Lee, Mark L Taper, & John R Horner. (2008) Relative growth rates of predator and prey dinosaurs reflect effects of predation. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1--1. DOI: 10.1098/rspb.2008.0912
The type skull of Velociraptor mongoliensis. From Osborn, et al. 1924.
By the summer of 1993 Velociraptor had become a household name. Although Deinonychus had long been my fleet-footed favorite the olive-green "clever girls" of Speilberg's film soon outshone all of their relatives and gave Tyrannosaurus a run for it's money.* Velocriaptor is hardly a new dinosaur, however. It was discovered during the famous expeditions to Mongolia made by the AMNH in the 1920's, the team setting out to find the "birthplace" of all mammals and coming back with loads of new di... Read more »
Pascal Godefroit, Philip Currie, Li Hong, Shang Yong, & Dong Zhi-Ming. (2008) A New Species Of Velociraptor (Dinosauria: Dromaeosauridae) From the Upper Cretaceous Of Northern China. Journal of Vertebrate Paleontology, 28(2), 432. DOI: 10.1671/0272-4634(2008)28[432:ANSOVD]2.0.CO;2
Of the few courses of value I have enrolled in while at Rutgers, one of my most favorite was the paleontology class taught by William Gallagher from the NJ State Museum (which, coincidentally, has just re-opened!). Much of the course dealt with invertebrates, the lectures being more oriented towards geologists than paleontologists, but there were still a few juicy lectures towards the end that involved vertebrate diversity and evolution. During these lectures he briefly mentioned the Permian temnospondyl Eryops, and he noted that it was probably an aquatic ambush predator, or "crocodiling b... Read more »
Per Ahlberg, Jennifer A Clack, Ervīns Lukševičs, Henning Blom, & Ivars Zupiņš. (2008) Ventastega curonica and the origin of tetrapod morphology. Nature, 453(7199), 1199-1204. DOI: 10.1038/nature06991
When I wrote about the new sauropod Futalognkosaurus dukei last October, I noted that the authors of the paper describing the animal also included a brief summary of the other animals found nearby. Remains of crocodiles, fish, and pterosaurs provided some clues as to the paleoecology of the area about 90 million years ago, but one of the big surprises was a big honkin' claw from Megaraptor. At first the remains of Megaraptor were thought to represent a coelurosaur, but the complete hand has shown that it is probably either a spinosaurid or carcharodontosaurid. A recent study of the... Read more »
Nathan Smith, Peter J Makovicky, Federico L Agnolin, Martín D Ezcurra, Diego F Pais, & Steven W Salisbury. (2008) A Megaraptor-like theropod (Dinosauria: Tetanurae) in Australia: support for faunal exchange across eastern and western Gondwana in the Mid-Cretaceous. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1--1. DOI: 10.1098/rspb.2008.0504
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