Placoderms evolved penetrative sex

Placoderms are an extinct class of armoured fishes containing several orders. Placoderm comes from Greek words for “plate” and “skin.” They are jawed fishes, but so ancient that they precede both sharks & rays and bony fishes: some elements of the skeleton are cartilaginous while others are bony and there are bony elements in their skin armour. Yes, bones and teeth are derived from skin tissue. They appeared in the fossil record about 420 Ma in the Early Silurian and by 400 Ma in the Devonian all major placoderm orders were present. (See “Australia: The Land Where Time Began: Placoderms“.}

402px-Dunkleosteus_terrelli_2 (From Wikipedia)

Before placoderms, all fertilization of eggs by sperm occurred outside the body, as females released eggs and males released sperm into the water. They might hover near each other, but if sperm found egg it was partly by chance. Female fishes release their eggs through a cloaca, an opening for both eggs and bodily wastes. As the fish that were closest had the most success, eventually males began to position themselves at the cloaca. One group of placoderms, the ptyctodontids, have these external claspers to hold the female, as do sharks. The pelvic fin perhaps developed a tube shape to funnel the male’s sperm into the female’s cloaca. Some modern fish mate in this way. What is known is that some placoderms gave birth to live young, which means that that at the very least, eggs were fertilized while still in the female. In short, these early fishes invented internal fertilization.

Two prehistoric fish swimming

Rhamphodopsis threiplandi, a placoderm with claspers (from Wikipedia)

There are two pathways after internal fertilization. One is ovivipary, where eggs are retained and hatch inside the mother, who then expels the young. Some sharks still do that. In fact, in some sharks the first young to hatch eat the other eggs to nourish themselves until birth. Females may develop areas of nourishing skin that the young can scrape off. The other main path is for the young & the female to cooperatively grow a placenta, which attaches to the her and extracts nourishment from her blood. (Placentas could  not have happened without a retrovirus inserting itself into the genome, but that’s another topic.)

At first, scientists thought that young placoderms inside larger fossils could have been evidence of predation; but at last a tiny fossil placoderm was found with a tiny umbilical cord, still attached to the mother.

Date of first walker pushed back 30 million years

A tiny trackway discovered in sedimentary rock has pushed back the date of the first organism capable of walking to 585 million years ago. This proof is 30 million years older than previously known evidence.It took two years to precisely date the trackway by radiometric dating of igneous rock that intruded into the sedimentary rock. The fossil was found in Uruguay.

The organism was about the size of a grain of rice. A trackway like this shows that it had front and back ends and was bilaterally symmetrical, with limbs that could move it forward. We could call it First Explorer.

See also Study resets date of earliest animal life by 30 million years.

More about the oldest rocks ever

Rocks formed from old mantle were found on Baffin Island in northwestern Canada. The rocks have been seen before, but they awaited dating methods that could distinguish them from the rest of the rocks on Earth. When lava solidifies, elements with their isotopes are frozen into it. We can use them to calculate the age of the rock.

The Earth’s mantle is the hot, stony layer between its crust and the nickel-iron core. Its upper layers can flow, however slowly, but further down it is compressed into a solid by the weight above it.

How can the mantle be modern or primitive? Isn’t it just a ball of molten ooze? Not exactly.

As the Earth formed, it melted from the energy of incoming material. When the loose material in the inner system was mostly accumulated, bombardment slowed and the molten Earth cooled off. In the molten crust, dense material sank to become the core and lighter material floated to the surface. Slow convection currents in the magma probably helped the continents to form in the cooler areas when less dense material was pushed aside by hot, rising plumes. Continents of lightweight silicon-aluminum rock, or sial, float on denser silicon-magnesium rock, or sima. Since the Earth’s crust cooled and solidified it has cracked into several large slabs or plates. The plates slide under or over each other, so that the upper plate is lifted into mountains, as in the U.S. Rockies. They ram into each other, and are compressed, buckling and folding into mountains, as in the Canadian Rockies.  The most spectacular example occurs where India collided with Asia, raising the Himalayas.

Over millions of years, wind, water, ice, and gravity erode the mountains back into rocks and particles, creating new sedimentary rock. However, particles in this rock still have the age signature of the original crust. has the story:

But these rocks contain an early Earth mixture of helium, lead and neodymium isotopes which suggest the mantle rock beneath the crust that yielded them is a virgin pocket of Earth’s original material.That pocket survived for 4.5 billion years under Baffin Island without being mixed by plate tectonics or erupted onto the surface.

“I was surprised that any of the (original) mantle survived,” said geoscientist Matthew Jackson of Boston University. He is the lead author on a paper announcing the discovery in this week’s issue of the journal Nature. “Finding a piece of the original mantle has been a holy grail. The original Earth was a big ball of magma. That’s our (planet’s) original composition.”

The discovery has surprised other researchers as well.

“Even if a vestige of such material remained, it seems unlikely that it would be found in any samples from Earth’s surface or the shallow subsurface that are available to geologists,” observed David Graham of Oregon State University in Corvallis, who wrote a commentary in the same issue of Nature. “Yet that is what (this) new evidence suggests.”

One of the obstacles in finding rocks from such ancient mantle, up to now, has been that researchers had assumed early Earth was composed of rocks with helium and lead isotope matching those of a type of ancient meteorite called a chondrite.

That may be true up to a point, said Jackson. Some recent research by scientists at the Carnegie Institution of Washington has suggested that the Earth’s early mantle would also have tell-tale neodymium isotopes that are unlike chondrites.

“That turns out to be the same as we find in these lavas (from Baffin Island),” said Jackson.

The other signs of untouched ancient mantle material — which has not before lost any of its material to Earth’s surface or been otherwise tainted — is a large amount of the isotopes helium-3 relative to helium-4. There is also an very old lead-isotope signature.

It was these three criteria — the helium, lead and neodymium — that led Jackson and his team to the conclusion that the Baffin Islands’ massive volcanic cliffs are made of the oldest material on the planet.

As for how much of this original mantle might be around, the only way to tell is to look at lava rocks and see if they came from such stuff, said Jackson.

It occurs to me that if there are longstanding downward currents of cooled magma in the mantle, there are places that are between those “downdrafts” and furthest from their influence. Those would be the most likely places for old magma to stay in place near the surface.

U-Haul graphics

Nevada has an Area 51 stealth plane, Newfoundland has a giant squid, and Illinois has a Cambrian Explosion monster: uhaul-sg-illinois


“llinois once lay near the equator on the supercontinent of Pangea and was home to unique creatures. How did the strip mining of Illinois’ coal deposits reveal the secret of the Tully Monster?”

The Tully Monster, discovered in 1958 in the Mazon Creek Lagerstaaten and named Tullimonstrum gregarium in 1966, is the state fossil of Illinois. Many have been found, but so far the Tully Monster is unique to Illinois. It dates back about 300 million years. We do not know what phylum it fits into. Its shape recalls the Anomalocaris, but that disappeared 100 million years earlier. Of course, with fossilization of soft-bodied organisms being so rare, perhaps it is a descendant of Anomalocaris!

Hesperornis, the great toothed diver

2008-01-16_ROM-wingless-penguin, originally uploaded by monado.

Hesperornis, or “western bird,” was discovered by O.C. Marsh in 1872. At one location he found dozens of well-preserved specimens from what had formerly been a shallow sea in the middle of North America. He name the species Hesperornis regalis. The birds are four to five feet tall, or long you might say, as they likely never got farther from the water than seals. Their hind legs were turned backwards to act as strong paddles, so they couldn’t really stand up. And they certainly couldn’t fly–their wings are entirely gone!

Hesperornis fossils date back to about 70 million years ago. The North American fossils are found all up and down the central continental sea. Several species are known. Marsh found another species of Hesperornis, but he was so reluctant to consider that it might be found in a different range that he gave it another name, and it took later paleontologists to sort out the new fossil, which they called Hesperornis montana.

When I first saw it, at the Page Museum at La Brea, I thought “wingless penguin.” Other people call it a large, wingless loon, and that might be more accurate. It was an early example of “use it or lose it” in evolution.

hesperornis-70myaWhen Birds Had Teeth

By FREDERIC A. LUCAS, Evolution: A Journal of Nature, August, 1929. –from Internet archive

SEPARATED by millions of years from that earliest of all known birds, the toothed Archaeopteryx of the Jurassic period (described last month), the next birds that we know come from the chalk beds of western Kansas. Time enough had passed for members of one group to have quite lost their wings, yet they still retained teeth, the most bird-like of them being quite unlike any modern bird in this respect. The first specimens were obtained by Professor Marsh in his expeditions of 1870 and 1871, but not until a few years later, after the material had been cleaned and was being studied, was it ascertained that these birds were armed with teeth. The smaller of these birds was not unlike a small gull…. The larger, however, was remarkable: The Toothed Diver, Hesperornis Regalis [sic].

In many ways, Hesperornis was a great diver, in some ways the greatest of the divers, slender and graceful in general build, looking somewhat like an overgrown, absolutely wingless loon.

The penguins, as everyone knows, swim with their front limbs — we can’t call them wings — which, though containing all the hones of a wing, have become transformed into powerful paddles. Hesperornis, on the other hand, swam altogether with its legs — swam so well with them, indeed, that through natural selection the disused wings dwindled away and vanished, save one bone. Hesperornis was large, upwards of five feet long, and if its ancestors were equally bulky, their wings were quite too big for swimming under water as do the short-winged Auks which fly under water quite as they fly over it. Hence the big wings were closely folded upon the body to offer the least possible resistance, and it was advantageous that they and their muscles dwindled, while the bones and muscles of the legs increased by constant use. By the time the wings were small enough to be used in so dense a medium as water, the muscles had become too feeble to move them, and so degeneration proceeded until but one bone remained, a mere vestige. The penguins retain their great breast muscles, as did the Great Auk, since it takes even more strength to move a small wing in water than a large wing in the thinner air. [Without evo-devo and mathematical analysis to confirm it, this is guesswork.]

As a swimming bird, one that swims with its legs and not with its wings, Hesperornis has probably never been equalled, for the size and appearance of the bones indicate great power, while the bones of the foot were so joined to those of the leg as to turn edgewise as the foot was brought forward, thus offering less resistance to the water. It is remarkable that these leg bones are hollow, because as a rule the bones of aquatic animals are more or less solid [inherited from large, land-dwelling dinosaurs], their weight being supported by the water; but those of the great diver were almost as light as if it had dwelt on dry land. That it did not dwell there is conclusively shown by its feet.

The most extraordinary thing about Hesperornis is the position of the legs relative to the body, and this is something that was not even suspected until the skeleton was mounted in a swimming attitude. As anyone knows who has watched a duck swim, the usual place for the feet and legs is beneath and in line with the body. But in our great extinct diver, the joints of the leg bones are such that this was impossible, and the feet and lower legs must have stood out nearly at right angles to the body, like a pair of oars. This is such a peculiar attitude for a bird’s legs that, although apparently indicated by the shape of the bones, it was at first thought to be due to the crushing and consequent distortion to which the bones had been subjected, and an endeavor was made to place them in the ordinary position, even at the expense of a dislocation of the joints. But when the mounting of the skeleton had advanced further, it became evident that Hesperornis was no ordinary bird and could not swim in the usual manner, since this would have brought his knee-caps uncomfortably up into his body. And so, at the cost of much time and trouble, the mountings were so changed that the legs stood out at the sides of the body, as shown in the picture, a position verified later by the discovery of the specimen now in the American Museum of Natural History, in which the limbs lay in just the position given them by the artist, Mr. Gleeson. [See image below.]

Hesperornis, by Lucas after Gleeson

Hesperornis, by Lucas after Gleeson

New Dinosaur Gallery – now with added Barosaurus

At long last, I’ve posted to flickr my images of the Royal Ontario Museum’s new dinosaur gallery, with several shots of their new exhibit, the Barosaurus.

Darwin was right

Sea sponge

Sea sponge

A new fossil discovery had proven that another of Darwin’s hypotheses about evolution was correct. During his lifetime, no animal fossils were known earlier than the Cambrian Explosion [of diversity], 540 million years ago. It occurred when animals developed hard body parts that could be fossilized, so some people call it an “induration.” It’s mostly in rare, fine limestones that you find the outlines, and sometimes the pigments, of soft body parts. Darwin, however, reasoned that evolution must have occurred for millions of years before the Cambrian Explosion for that diversity to develop. And he was right.

Fossils push animal life back millions of years

A novel technique used to date fossils buried in rock sediment in Oman shows that sponges, among the most primitive of animal organisms, flourished there more than 635 million years ago.

Softer structures in Pikaia?

Maybe it’s just my imagination, but when I look at Understanding Evolution’s image of a fossil Pikaia, the first known chordate,  from the Precambrian era, I see more than they do.

Here’s their image from Is Pikaia an arthropod?

Enlargement of head of a fossil Pikaia at right

Enlargement of head of a fossil Pikaia, facing right

In the enlargement, I see a couple of feathery antennae or mouth-parts, suitable for sensing and sweeping small bits of food towards the mouth.

Feathery extensions

Feathery extensions?

What do you think?

Links to more fossils are welcome.

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