Who Lies Sleeping?

Must all dinosaurs have laid eggs?

Not even all modern reptiles lay eggs. Snakes, skinks, some amphibians (salamanders) and even some fish (sharks, guppies and sea horses) keep their eggs within themselves until birth.

• The sea snakes of the Indian Ocean give birth to live young (viviparity). They do not have a placenta like the mammals to nourish the embryo while it is developing, but nevertheless they do keep their eggs within their bodies until they are hatched.
• Rattlesnakes are even more advanced. They retain their eggs and have even dispensed with a hard shell. The embryo not only gets nutriment from a yolk but also by diffusion of sustenance from the mother across the thin outer membrane of the egg, a support system very suggestive of a placenta. At birth the mother continues to look after the young.
• One species of frog, gastrotheca, keeps its young in a pouch until they hatch into tadpoles when they are released into the water, but another species keeps them in its pouch until they emerge as baby frogs.
• A West African toad, nectophrynoides, retains its eggs in its oviduct. When the tadpoles hatch they feed upon particles released from the walls of the oviduct and at the next wet season live baby toads are born, the mother having provided a pseudo pond for them within her oviduct.
• Amazingly, the coelacanth, a fish thought to have been extinct for 70 million years until found again in 1938, gives birth to live young. This creature is believed not to have evolved in any significant way for hundreds of millions of years indicating that creatures had live offspring before the dinosaurs even appeared on earth.

Following a discovery made by an assistant in 1947 in New Mexico, Edwin Holbert found several fossilized skeletons of a small early dinosaur called coelophysis. Coelophysis was an early type of coelurosaurus living at the end of the Triassic period about 210 million years ago. A remarkable graveyard of these specimens was found in New Mexico. Bones were weathering out of a rock stratum in a hillside and some had been recovered by a collector as long ago as 1881. Thus the species had been known for 60 years but previous specimens had been poor. These were excellent. When the site was rediscovered it was agreed to dig away the overlying strata and look at the layer containing the fossils. An amazing lode of coelophysis bones was found, young and old together. The amazing feature of one of them, Colbert noted, was that it seemed to have inside it the bones of a tiny juvenile. Colbert could not accept the obvious inference that the dinosaur gave birth to live young, especially as the pelvic bones seemed too narrow. He deduced that the “baby” was actually the adult’s last meal.

Live birth did occur in ichthyosaurs, the dolphin-like dinosaurs. At first, paleontologists, faced with the idea of sea-dinosaurs, thought they must leave the sea to lay their eggs like turtles. But the ichthyosaurs were far too whale-like for that to happen. An ichthyosaur would be no more able to crawl up a beach than a porpoise or a killer whale—on land it would be literally stranded. It also seemed odd that no ichthyosaur eggs could ever be found. Even though masses of ichthyosaur fossils were found at Holzmaden in Germany there were no signs of any eggs. And this despite the discovery of fossilized ichthyosaur droppings (called coproliths) that would plainly have been less suitable for fossilization than eggs.

The answer came from the noted amateur, Bernard Hauff, who owned those productive quarries in Holzmaden and made a name for himself by the skill he put into the delicate process of extracting the imprint from the rock matrix. He conclusively showed that some of the ichthyosaurs had smaller specimens inside them. As we might expect, this triggered off a controversy about the “baby” ichthyosaurs. “They are not unborn babies but part of the larger creature’s last dinner”, was the cry. It is far from unknown for vertebrates, especially fish, to eat their own young. The small specimens inside the body of the larger specimen were always facing forwards, in the direction of motion of the larger fossil. “If the animal were to be born it would have its head to the rear—animals are always born head first”, pronounced the critics.

What’s more, a swimming creature being pursued and finally swallowed by another would be swallowed tail first and would be bound to be ’head forward’ in the predator’s stomach.

Hauff countered by showing what the larger ichthyosaurs had had for dinner—mainly a variety of types of swimming shelled molluscs having a lifestyle similar to modern squids.

The experts remained unmoved. Hauff responded by providing the ultimate proof. He had had a slab of rock needing cleaning for a long time but had constantly sidelined it as more promising finds were brought to him. When he did remove the extraneous rock, he was amazed to find that the impression was one of an ichthyosaur in the act of giving birth. The smaller specimen was hanging below the body of the parent yet with its foreparts still evidently within the mother’s body. It is now known that whales give birth in this fashion, tail first, to allow the tail of the foetus to get strong in the stream of water flowing over the mother’s body. The infant whale will dangle thus for four to six weeks, the birth only being completed when the baby is strong enough to swim alongside its mother. Once again the similarity of function in similar environments and lifestyles demonstrates itself. Efficient evolution into a particular ecological niche generates the same solution to problems of adaptation. We shall have reason to remember this when the question of the evolution of intelligence is considered in more detail.

A lumbersaur, Apatosaurus, formerly Brontosaurus

The sauropods like brontosaurus also could have given birth to live young. Tracks of sauropods indicate that they moved about in groups, if not herds. Bakker has found that dinosaur herds were structured such that the young were protected in the middle by a surrounding circle of adults, showing that the young were evidently cared for after birth, viviparous or otherwise. Yet, if they laid eggs, there are several problems to answer. Did the herd stop in one locality while the eggs hatched? If they did, would not such dim-witted animals trample all over the eggs before they had time to hatch. If not, how did the young rejoin the herd, which had presumably moved on after the egg laying? Furthermore, eggs cannot be larger than a certain maximum size since beyond that size they would either collapse under their own weight or they would have to be so tough that the hatchling would not be able to crack the shell to emerge. The maximum size is small for such huge dinosaurs as brontosaurus and its relatives, which reached 50 tons or more at maturity. Even if the eggs were three feet across like those of the extinct bird, the aepyornis, the hatchlings would be still likely to be crushed underfoot.

All these problems are answered if the young were carried until they had reached a reasonable level of maturity. At birth they would then have been able to keep up with the wanderings of the herd and avoid the clumsy feet of their elders. They would also have been big enough not to lose heat to their surroundings. Bakker believes the sauropods’ live young weighed as much as 500 pounds at birth, solving most problems, but if they were smaller the problems remained.

Could sauropods have carried their young in pouches rather like a kangaroo? The problem then is what they could have fed on. Kangaroos are mammals with teats to provide nourishing milk. One assumes that we are on safe ground in believing that not even hot blooded dinosaurs had mammalia! Could the young have snuggled into a pouch near to the sauropod’s tail feeding upon the parent’s dung? Since they were too small to avoid rapid heat loss, they would also be kept warm by their mother’s body heat. The large herbivorous dinosaurs probably had to allow their food to ferment in their stomachs because the cycads and ferns they ate were tough and fibrous. Their droppings would therefore be effectively predigested food for the infants. Many smaller creatures live on the dung of larger ones and some, like rabbits and mole rats, eat their own to make sure no nutrition is wasted. Perhaps some of the many dinosaurs that undoubtedly did lay eggs also carried their young like marsupials, particularly to keep them from dying of heat loss when they were tiny. The upright posture of many dinosaurs is reminiscent of the posture of the kangaroo and wallaby. Though marsupials do not necessarily adopt this erect stance, it might be convenient for erect animals to adopt a marsupial method of protecting their young. Admittedly there has been no quoted instances of this, but the dinosaurs were still vigorously adapting even shortly before their final demise. Is it possible that they anticipated other vertebrate systems for protecting their young, millions of years ago? The marsupial system? The human system?

Stumble! for WP

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