This is a very important
discovery. We are so used to associating
all reptile orders with egg bearing and mammals with placental birthing that no
one thought to look to the sea for the evolution of placental birthing.
After all crocodiles do place
their eggs in decaying plant material and that is a reasonable expectation for
a sea going dinosaur. Suddenly that is
not the case. Then what is to stop a
small placental dinosaur reestablishing itself on land to establish a new line
of land based placental creatures?
I suspect that a lot of our best
adaptations occurred at sea in the first place.
Thus this is quite plausible. A
land based line experimenting with a placental system is unconvincing while a
sea based evolution is quite plausible and rather natural.
Fossil 'suggests plesiosaurs did not lay eggs'
The fossil of the pregnant Polycotylus latippinus was almost complete
11 August 2011 Last updated at 23:53 ET
Scientists say they have found the first evidence that giant sea
reptiles - which lived at the same time as dinosaurs - gave birth to live young
rather than laying eggs.
They say a 78 million-year-old fossil of a pregnant plesiosaur suggests
they gave birth to single, large young.
Writing
in Science, they say this also suggests a degree of parental care.
The fossil, the first of a pregnant plesiosaur found, is at the US Natural History Museum of Los Angeles
County
After being excavated from a ranch in Kansas ,
US, the 5m-long fossil skeleton Polycotylus latippinus lay for two
decades in the basement of the Los
Angeles
Immature skeleton
Two years ago when researchers began to piece the bones together they
quickly realised that they were in fact dealing with two separate animals; an
adult plesiosaur and a smaller juvenile.
The study's authors report that the juvenile was unlikely to have been
eaten by the larger reptile because its tiny bones showed no evidence of bite
marks, and its soft, immature skeleton suggested an animal only two-thirds of
the way through its development.
For more than 200 years palaeontologists have speculated about how
these colossal Cretaceous animals reproduced.
Many believed the plesiosaur was too cumbersome to drag itself up the
beach to lay eggs, and so must have given birth to live young.
"[The find] provides the first direct evidence for live birth in
plesiosaur," said palaeontologist Adam Smith from the Thinktank Centre, Birmingham Science Museum , UK
"It's a very interesting find...[and] has been a long time
coming."
Doting parents?
"The lack of fossil evidence of a pregnant plesiosaur was
frustrating," explained the study's lead author Frank O'Keefe, from Marshall University
in Huntington , US
But he added: "What is really surprising about this fossil [is]
that plesiosaurs [reproduce] differently to other marine reptiles... they give
birth to one big baby instead of a lot of little babies."
By making comparisons with modern animals, such as whales, which give
birth to larger, single young and then go on to care for them, Dr O'Keefe and
his colleague, Luis Chiappe from the museum, attempt to infer something about
plesiosaur behaviour.
In a similar way to land-dwelling dinosaurs, which are thought to have
provided food for their nest-bound young, plesiosaurs, the authors suggest,
might have been doting parents.
But Dr Smith was less convinced. He said that it was "certainly
quite possible... but is very speculative".
P. latippinus's close relatives had three young at one time. It is
possible the P. latippinus evolved to have only one, and care for it,
he told BBC News, but more pregnant specimens are needed to be more certain of
this.
Dr O'Keefe agreed, explaining that unlike on land where nests are
preserved, studying the behaviour of ancient marine creatures is very
difficult.
"When you get right down to it, behaviour doesn't fossilise, so we
are stuck trying to make these inferences using modern animals where we can
observe their behaviour," he explained.
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