The shortest poem ever written is known as Lines on the Antiquity of Microbes or, more simply, as Fleas. It goes: Adam/ Had ‘em. It’s a cute verse, but we don’t need Blblical references to know what the first fleas fed upon. As with many questions about ancient life, we can turn to fossils.
Among a horde of insect fossils recovered from China and Mongolia, Diying Huang from the Chinese Academy of Sciences has discovered several species of giant fleas. There are nine individuals from three different species, and they hail from the middle Jurassic and early Cretaceous periods. “These outcrops have given us thousands of exquisitely preserved insects, but fewer than ten fleas,” says Andre Nel, who led the study.
The fossilised insects had many features that identify them as fleas – ancient precursors to the ones we know and loathe. But they had many unusual traits too. For a start, they were much bigger. While modern fleas are just a few millimetres long, some of these ancient forms were ten times bigger. The males grew between 8 and 15 millimetres in length and the females reached between 14 and 20 millimetres.
If you think about fossils, you probably picture a piece of bone or shell, turned to stone and buried in the ground. You visit them in museums; some of you may even have found some. But your closest fossils are inside you, scattered throughout your genome. They are the remains of ancient viruses, which shoved their genes among those of our ancestors. There they remained, turning into genetic fossils that still lurk in our genomes to this day.
We’ve known about our viral ancestors for 40 years, but a new study shows that their genetic infiltration was far more extensive than anyone had realised. The viral roots of our family tree have just become a lot bigger.
You can’t go for a month without seeing a claim that some new discovery has rewritten evolutionary history. If headlines are to be believed, phylogeny – the business of drawing family trees between different species – is an etch-a-sketch science. No sooner are family trees drawn before they’re rearranged. It’s easy to rile against these seemingly sensationalist claims, but James Tarver from the University of Bristol has found that the reality is more complex.
Tarver focused on two popular groups of animals – dinosaurs and catarrhines, a group of primates that includes humans, apes and all monkeys from Asia and Africa. Together with Phil Donoghue and Mike Benton, Tarver looked at how the evolutionary trees for these two groups have changed over the last 200 years. They found that the catarrhine tree is far more stable than that of the dinosaurs. For the latter group, claims about new fossils that rewrite evolutionary history (while still arguably hyperbolic) have the ring of truth about them.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Travel back in time to about 50 million years ago and you might catch a glimpse of a small, unassuming animal walking on slender legs tipped with hooves, by the rivers of southern Asia. It feeds on land but when it picks up signs of danger, it readily takes to the water and wades to safety.
The animal is called Indohyus (literally “India’s pig”) and though it may not look like it, it is the earliest known relative of today’s whales and dolphins. Known mostly through a few fossil teeth, a more complete skeleton was described for the first time last week by Hans Thewissen and colleagues from the Northeastern Ohio Universities. It shows what the missing link between whales and their deer-like ancestors might have looked like and how it probably behaved.Whales look so unlike other mammals that it’s hard to imagine the type of creature that they evolved from. Once they took to the water, their evolutionary journey is fairly clear. A series of incredible fossils have documented their transformation into the masterful swimmers of today’s oceans from early four-legged forms like Pakicetus and Ambulocetus (also discovered by Thewissen). But what did their ancestors look like when they still lived on land?
Hooves to flippers
Until now, we had little idea and their modern relatives have provided few clues. According to molecular evidence, the closest living relatives of whales are, quite surprisingly, the artiodactyls, a group of hoofed mammals that includes deer, cows, sheep, pigs, giraffes, camels and hippos.
They all have a characteristic even number of toes on each hoof and not a single one of them bears even a passing resemblance to whales and dolphins. Among the group, the hippos are evolutionarily closest and while they are at least at home in water, their family originated some 35 million years after the first whales and dolphins did.
Enter Indohyus, a small animal about 70cm long that lived 47 million years ago. It was a member of a family of mammals called the raoellids, prehistoric artiodactyls that lived at the same time as the earliest whales and hailed from the same place of origin – southern Asia. By analysing a fossilised skull and limbs collected from India, Thewissen found compelling evidence that the raoellids were a sister group to the ancestors of whales.
Even though Indohyus had the elegant legs of a small deer and walked around on hooves, it also had features found only in modern and fossil whales. Its jaws and teeth were similar to those of early whales, but the best evidence was the presence of a thickened knob of bone in its middle ear, called an involucrum. This structure helps modern whales to hear underwater, it’s only found in whales and their ancestors, and acts as a diagnostic feature for the group.
Based on these physical similarities, Thewissen suggests that the raoellids are a sister group to the whales. Both of these groups are evolutionary cousins to all modern artiodactyls. (As a note for journalists and creationists, Indohyus is not a direct ancestor of whales, as many news sites are claiming, and nor did whales ‘evolve from deer’!)
Life in the water
Indohyus‘s skeleton also suggests that it was partially adapted for life in the water. Its leg bones were unusually thick, a feature shared by other aquatic animals including hippos, sea otters and manatees. These heavier bones stop swimming mammals from floating by default and allow them to hang in the water and dive more easily.
Because Indohyus had slender legs and not paddle-shaped ones, Thewissen pictures it wading in shallow water, walking hippo-style along the river floor while its heavy bones provided ballast.
Thewissen found more clues about the animal’s lifestyle from its teeth, and particularly the levels of certain isotopes in their enamel. Levels of oxygen isotopes matched those of water-going mammals, providing further support for Indohyus‘s aquatic tendencies. Its large crushing molars are typical of plant-eaters and levels of carbon isotopes in them suggested that Indohyus either came onto land to graze (like hippos) or fed on plants and invertebrates in the water (like muskrats). In terms of behaviour, they were close to the modern mousedeer, a tiny, secretive deer that feeds on land but flees into streams when danger threatens.
Put together, this portrait of Indohyus‘s life also tells us about the changes that drove the evolution of whales, and it looks like it wasn’t a move to water. Whales and raoellids are evolutionary sisters and since early members of both groups were happy in the water, aquatic lifestyles must have pre-dated the origin of whales.
Instead, Thewissen suggests that the key step was a switch in diet. He speculates that whales developed from an Indohyus-like ancestor that fed on plants and possibly small invertebrates on land, but fled to water to escape predators. Over time, they slowly turned into meat-eaters and evolved to swim after nimble aquatic prey.
Video: Have a look at Thewissen talking about Indohyus and the origin of whales.
Images of Indohyus are painted by the extraordinary Carl Buell
Reference: Thewissen, J.G., Cooper, L.N., Clementz, M.T., Bajpai, S., Tiwari, B.N. (2007). Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature, 450(7173), 1190-1194. DOI: 10.1038/nature06343
Around 395 million years ago, a group of four-legged animals strode across a Polish coast. These large, amphibious creatures were among the first invaders of the land, the first animals with true legs that could walk across solid ground. With sprawling gaits and tails held high, they took pioneering footsteps. Their tracks eventually fossilised and their recent discovery yields a big surprise that could rewrite what we know about the invasion of land. These animals were walking around 18 million years earlier than expected.
The evolution of four-legged creatures – tetrapods – is one of the most evocative in life’s history. It has been illustrated by a series of beautiful fossils that vividly show the transition from swimming with fins to walking on legs. These include Panderichthys, a fish with a large tetrapod-like head and a muscular pair of front fins. Tiktaalik expanded on these themes. Its head could turn about a solid neck. Its limbs had the fin rays of its fishy predecessors but clear wrist bones and basic fingers too. Tiktaalik could support itself on strong shoulder bones, bend its fins at the wrists, and splay out its hand-like bones.
These animals – the elpistostegids – have largely been seen as transitional fossils. Tetrapods supposedly evolved from these intermediate forms and eventually replaced them. You could draw all of their skeletons in the corner of a book and flick the pages to see the move from sea to land happen before your eyes. But the new fossil tracks tell a very different story. As one reviewer writes, they “lob a grenade into that picture”.
The earliest true tetrapods so far discovered were around 375 million years old and the earliest elpistostegids hail from around 386 million years ago. But the Polish tracks are 10 million years older still. These dates suggest that the elpistostegids weren’t transitional forms at all. They weren’t early adopters of new biological technology, but late-surviving relics that stayed in their fish-like state while other species had evolved new bodies and, quite literally, run with them. Per Ahlberg, who led the study, says, “I’ve been working on the origin of tetrapods for about 25 years, and this is the biggest discovery I have ever been involved in. It is enomously exciting.”
It would be tempting to cast animals like Pandericthys or Tiktaalik in the role of biological luddites, outstaying their welcome with outmoded bodies. But that would be an injustice. If these animals co-existed with tetrapods for at least 10 million years, it suggests that their bodies were stable, well-adapted structures in their own right. They weren’t just brief flirtations with sturdiness on the way to full-blown walking.
Ahlberg discovered the fossil tracks along with researchers from Warsaw University, led by Grzegorz Niedzwiedzki. The tracks are found in the disused Zachelmie Quarry, nestled among Poland’s Holy Cross Mountains. The area used to be part of a tidal plain. Rocks from the site and a few rare fossils allowed the team to confidently date the track-ridden layer to around 395 million years ago, the middle of the Devonian period.
Among this layer, Niedzwiedzki found several tracks of different shapes and sizes. He also found several isolated handprints and footprints that clearly show signs of toes and ankles. Many of these tracks (such as in the diagram at the top) were clearly made by an animal walking with powerful, diagonal strides, powered by sprawling right-angled limbs.
They weren’t the work of any elpistostegid, whose straight limbs and backward-pointing shoulders and hips would have created far narrower tracks. Elpistostegids would also have dug long central troughs as they laboriously dragged themselves along. No such troughs exist at the quarry. This tells us that the track-makers strode along using strong hips and shoulders to hold their bodies and tails off the ground.
Niedzwiedzki thinks that they were undoubtedly tetrapods, and big ones too. The animal that created the tracks above was just 40-50 cm long. But some of the footprints were 15cm wide and hinted at creatures that were around 2.5 metres in length. The largest print is 26cm wide and its maker was probably a giant.
The implications of the Polish tracks are so controversial that reactions from other palaeontologists have been, understandably, mixed. Ted Daeschler and Neil Shubin, who discovered Tiktaalik, both find the study intriguing, but not definitive.
For Shubin, the deal-breaker would be identifying the animals that made the trackways and establishing where they sit on the evolutionary tree. He says, “The skeletal anatomy, let alone evolutionary relationships, of a trackmaker is hard to interpret from a track or print.” For example, he says that a model of Tiktaalik‘s skeleton would produce a print much like the one in the paper if it’s mushed into sand, and different consistencies or angles would produce an even closer match. He adds, “There is nothing in Tiktaalik’s described anatomy that suggests it didn’t have a stride.”
Daeschler agrees that “trace fossils such as these presumed tracks… are a notoriously difficult class of evidence to interpret with full confidence”. Nonetheless, he’s keeping an open mind and a keen eye on future developments. “Paleontology is a lively field in which new discoveries constantly refine our knowledge of the history of life on earth,” he says.
Jenny Clack, the Cambridge scientist who discovered Acanthostega, has seen the Polish tracks for herself and finds them more convincing. Her only reservation is that the detailed prints don’t have any trackways to show how their maker moved, while the trackways themselves consist of blobs. “But so do lots of previously known tracks,” she says. “If you’d found those in other deposits in the last part of the Devonian, you wouldn’t have any qualms about them.” She’d like to see trackways of the detailed prints but she’s nonetheless excited. “It’s going to change all our ideas about why tetrapods emerged from the water, as well as when and where.”
On the question of where, many scientists have suggested that the invasion of land began at the margins of freshwater – at river banks, deltas, lakes or flooded forests. But the Zachelmire quarry wasn’t any of these. Most likely, it was a shallow tidal flat or perhaps a saltwater lagoon. The first tetrapods didn’t lurk in rivers, but trampled the mud of coral-reef lagoons.
Niedzwiedzki thinks that this revised locale makes a better staging ground for the invasion of land. Twice a day, the zone between high and low tide is awash with stranded marine animals that would provide a feast for marine creatures experimenting with life on land. He argues that life was driven aground by the rich availability of snacks.
This new setting for the rise of the tetrapods also helps to answer a big question raised by the tracks. If tetrapods were walking around 18 million years earlier than we thought, they and the elpistostgids must have large “ghost lineages” – periods when they must have existed but for which no fossils have been found. Actually, very few fossils have been found at Zachelmie Quarry at all. These sites, where tetrapods first marched onto land, may have been good at preserving footprints, but they haven’t been equally kind to bones.
But why do the fossils that have been found make it look a lot like the elpistostegids preceded the tetrapods? That’s more of a stumper, but Niedzwiedzki has a possible answer. He thinks that elpistostegids may have colonised new environments before their tetrapod peers (or at least those environments that would preserve their bones). It’s a nice hypothesis, but for the moment, it’s just that. There’s no clear answer, although the hunt for new fossils or tracks will hopefully provide one.
Clack is certainly excited by the new doors opened by this discovery. “People are now going to start looking in different places from where they traditionally looked,” she says. “The Polish trackways were only discovered by accident. Nobody had ever looked at these Devonian deposits in detail before. Now the same team are starting to look for body fossils and they’ve started to find some, but no tetrapods yet. I’m expecting stuff to come out from other parts of the world too, like China.”
And here, even the sceptics agree. “All scenarios are intruiging, but we simply do not know for sure,” says Shubin. “All the more excuse to continue to go out in the field and find skeletons!”
Reference: Niedzwiedzki. 2009. Tetrapod trackways from the early Middle Devonian period of Poland. Nature doi:10.1038/nature08623
All images: copyright of Nature
More on transitional fossils:
Meet Raptorex, the “king of thieves”. It’s a new species of dinosaur that looks, for all intents and purposes¸ like the mighty Tyrannosaurus rex, complete with large, powerful skull and tiny, comical forearms. But there’s one very important difference – it’s 100 times smaller. Unlike the ever-shrinking world of music players and phones, it seems that evolution crafted tyrannosaur technology with much smaller specifications before enlarging the design into the giant predators of the late Cretaceous.
Raptorex is a new species of meat-eating dinosaur, discovered in northwest China by Paul Sereno from the University of Chicago. The specimen is a young adult, but it wouldn’t have grown to more than 3 metres in length. It stood about as tall as a human, and wouldn’t have weighed much more. And yet Raptorex looked very much like a scaled-down version of its giant future relatives. All the features that made tyrannosaurs so recognisable and such efficient killers (except their enormous size) were present in this animal.
It really is a beautiful transitional fossil. As Sereno says, “Raptorex really is a pivotal moment in the history of the group where most of the biologically meaningful features of tyrannosaurs came into being, and the surprising thing is that they came into being in such a small animal.” Raptorex clearly shows that natural selection initially honed the distinct body shape of these giant predators at a 1/100th scale. This design was then scaled up with remarkably few modifications.
It had a skull that had clearly been developed into the animal’s primary weapon. It was unusually big for its body size (40% of its torso length), it was structurally reinforced against the stresses of heavy bites, it had large places where powerful jaw-closing muscles attached and it was armed with sharp teeth.
Its limbs also had classic T.rex proportions – strong hind legs that that were fit for running, but miniscule forearms. In contrast, other early tyrannosaurids, such as Guanlong, Dilong, Eotyrannus and Stokesosaurus, looked very different with arms that were long and useful, and proportionally smaller heads (just 30% of its torso length). Only a few distinctive parts of their skeleton mark them out as early tyrannosaurids.
Raptorex brain was also large for its size. For comparison, the Jurassic predator Allosaurus had a brain that was just 60% bigger, despite having a body that was 10 times heavier! Raptorex‘s sense of smell was particularly well-developed, just as Tyrannosaurus‘s was. A scan of its skull showed a large area for its olfactory bulbs – the parts of its brain devoted to smell. These bulbs take up a full 20% of the brain’s total volume, a proportion that exceeds that of all meat-eating dinosaurs except the giant tyrannosaurs.
Despite what many newspapers will assuredly tell you, Raptorex isn’t the ancestor of Tyrannosaurus although it probably looked very much like what this hypothetical animal would have done. It’s more like an early cousin, but one that’s clearly more closely related to T.rex and its giant kin than any of the other smaller species so far discovered.
Based on his new fossil, Sereno tells a three-act story of tyrannosaur evolution. Act One was set in the Jurassic and early Cretaceous periods, with a cast that included Eotyrannus and Dilong. Their snouts had become stronger and their jaws more powerful, but they were typical of other predators of the time. It was only during Act Two, around 125 million years ago, that this dynasty of predators started to become truly specialised, enhancing the skull, lengthening the legs, and shrinking the forearms.
All of these features were present in Raptorex, setting the stage of the final act in tyrannosaur evolution – getting really big. The lineage grew in bulk by around 100 times. By the end of the Cretaceous, the meat-eating scene in the northern continents was dominated by tyrannosaurids – predators such as Albertasaurus, Gorgosaurus, Daspletosaurus and Tarvosaurus, each weighing in at 2.5 tons or more.
It would be fascinating to see if the same story could be told for other lineages of predators, if the abelisaurids, carcharodontosaurids and spinosaurids all had their own mini-prototypes.
Reference: Science 10.1126/science.1177428
Images: Reconstruction by Todd Marshall; other images from Science/AAAS
More on dinosaurs:
Seals and sea-lions gracefully careen through today’s oceans with the help of legs that have become wide, flat flippers. But it was not always this way. Seals evolved from carnivorous ancestors that walked on land with sturdy legs; only later did these evolve into the flippers that the family is known for. Now, a beautifully new fossil called Puijila illustrates just what such early steps in seal evolution looked like. With four legs and a long tail, it must have resembled a large otter but it was, in fact, a walking seal.
Natalia Rybczynski unearthed the new animal at Devon Island, Canada and worked out that it must have swam through the waters of the Arctic circle around 20-24 million years ago. She named it Puijila darwini after an Inuit word referring to a young seal, and some obscure biologist. The skeleton has been beautifully preserved, with over 65% of the animal intact, including its limbs and most of its skull.
Puijila is a massive boon for biologists trying to understand the evolution of pinnipeds, the group that includes seals, sea lions and walruses. It’s not itself a direct ancestor, having branched off the evolutionary path that led to modern pinnipeds. It did, however, retain many of the same features that a direct ancestor would have had. “Puijila is a transitional fossil,” Rybczynski explains. “It gives us a glimpse of what the earliest stages of pinniped evolution looked like, before pinnipeds had flippers. And it suggests that in the land-to-sea transition, pinnipeds went through a freshwater phase.”
This familiar group evolved from land-dwelling carnivores and their closest living relatives are the bears and the mustelids (otters, weasels, skunks and badgers). For other marine mammals like whales and dolphins, the fossil record has given us dramatic visuals for the gradual transformation from land-dweller to full-time swimmer. But for pinnipeds, that transition is much murkier because until now, the earliest known seal Enaliarctos already had a full set of true flippers. Puijila changes all of that.
In the Origin of
the Species, the ever-prescient Darwin wrote, “A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean”. This year, on the 150th anniversary of the book’s publication, the walking seal that bears his name pays a fitting tribute to Darwin’s insight.
Nine years ago, a team of fossil-hunters led by Philip Gingerich from the University of Michigan uncovered something amazing – the petrified remains of an ancient whale, but one unlike any that had been found before. Within the creature’s abdomen lay a collection of similar but much smaller bones. They were the fossilised remains of a foetal whale, perfectly preserved within the belly of its mother. Gingerich says, “This is the ‘Lucy‘ of whale evolution.”
The creatures are new to science and Gingerich have called them Maiacetus inuus. The genus name is an amalgamation of the Greek words “maia” meaning “mother” and “ketos” meaning “whale”, while Inuus, the Roman god of fertility, gave his name to the species.
The foetus’s teeth were the first to be uncovered and only as the surrounding (and much larger) bones were revealed, did Gingerich realise what his team had found – the first ever foetal skeleton of an
ancestral ancient whale (see video). Alongside the mother and calf, the group also discovered another fossil of the same species in even better condition. Its larger size and bigger teeth identified it as a male.
This trio of skeletons is so complete and well-preserved that Gingerich likens them to the Rosetta Stone. They provide an unparalleled glimpse at the lifestyle of an ancient whale before the group had made the permanent transition to the seas. How it gave birth, where it lived, how it competed for mates – all these aspects of its life are revealed by these beautiful new finds.
Maiacetuswasn’t quite like the whales we know and love. It was an intermediate form between the group’s earliest ancestors and the fully marine versions that swim about today. For a start, still had sturdy hind legs that were good for swimming but would have allowed it to walk on land.
Another piece of evidence tells us that Maiacetus was definitely amphibious – its foetus was facing backwards in the womb. If the mother had lived long enough to give birth (and judging by the foetus’s size, that wasn’t far off), the infant would have greeted the world face-first. No living whale or dolphin does that – all of their young emerge backwards, leading with their tails, to minimise the risk of drowning in the event of a prolonged labour. A head-first delivery means that Maiacetus gave birth as a landlubber.
Beipaiosaurus was among the strangest of dinosaurs. It looked like a fusion of body parts taken from several other species and united in the unlikeliest of proportions. It had a stocky body, long arms adorned with massive claws, a long neck topped by an incongruously small head, and a beaked mouth. Bizarre as this cocktail of features is, it’s the animal skin that has currently warrants attention.
Fossils of Beipaiosaurus include impressions of its skin and these clearly show long, broad filaments clumped around its head, neck, rump and tail. They are feathers, but most unusual ones. Traces of feathers have been found on other dinosaurs (including the infamous Velociraptor) and they come in a variety of shapes and forms. But all are composite structures consisting of several slender filaments that either sprout from a single base or branch off a central stem.
Beipaiosaurus was also covered in some of these complex feathers, but it had other plumes that were far simpler. Each of these was a single, unbranched, hollow filament – exactly the sort of structure that palaeontologists had predicted as the first step in the evolution of feathers.
Until now, their existence was merely hypothetical – this is the first time that any have actually been found in a fossil. Other, more advanced stages in feather evolution have been described, so Beipaiosaurus provides the final piece in a series of structures that takes us from simple filaments to the more advanced feathers of other dinosaurs to the complex quills that keep modern birds aloft.