Polycotylus - The Good Mother Plesiosaur?
By Brian Switek
Wired
August 13, 2011
http://m.wired.com/wiredscience/2011/08/polycotylus-the-good-mother-plesiosaur/

The opening sentence of F. Robin O'Keefe and Luis
Chiappe's new paper in Science this week is a simple
statement of fact that threw me for a loop. "Viviparity,
or birthing live young," the paleontologists write, "is
common among reptiles, having evolved over 80 times
among extant clades." Think about that for a moment.
According to the traditional typology I was taught in my
elementary school days, reptiles lay eggs and mammals
give birth to live young. Yet monotremes - mammals such
as the platypus and the echidna - lay eggs, and clearly
reptiles have evolved the ability to produce to live
young multiple times. Once again, nature defies our
attempts to squeeze everything into neat conceptual
boxes.

But modern reptile groups are not the only ones to have
shared the evolutionary innovation of viviparity.
Prehistoric reptiles did, too, as exemplified by an
increasing body of fossil evidence documented in a spate
of recent papers. Earlier this summer paleontologists
Yuan Wang and Susan Evans reported on a specimen of the
approximately 125 million year old lizard Yabeinosaurus
which carried at least 15 late-term embryos inside it -
a find which pushed the record of live birth in lizards
back 30 million years earlier than previously found -
and now the paper by O'Keefe and Chiappe has placed
viviparity among a group of marine reptiles whose
reproductive lives have remained mysterious since the
time of their discovery two centuries ago.

How did plesiosaurs go about making other plesiosaurs?
The actual mechanics of plesiosaur intimacy are entirely
unknown to us, and, until now, how baby plesiosaurs were
born was equally opaque. These prehistoric marine
reptiles - four-paddled predators which skulled through
the Mesozoic seas - were clearly adapted from
terrestrial ancestors into creatures which spent their
entire lives at sea, but when it came time to reproduce
it was unclear whether they gave birth to live young in
the ocean, hauled themselves out on the beach to lay
eggs, or if baby plesiosaurs were delivered in bundles
by pterosaurs. (You know, since there weren't storks
around yet.)

That at least some marine reptiles gave birth in the sea
has actually been known for some time. In a 1905 article
written for The Century magazine, American Museum of
Natural History paleontologist Henry Fairfield Osborn
cited two ichthyosaur specimens preserved with embryos
as evidence that these marine reptiles were capable of
"sea-birth" and did not crawl up on to the beach to lay
eggs or went through a "tadpole stage", as other
naturalists had previously proposed. The specimens had
come from Holzmaden, Germany - a beautiful graveyard
where many exquisitely-preserved ichthyosaurs have been
found - and Osborn noted that evidence of fossil embryos
within ichthyosaurs had been suspected since 1828. Other
cases turned up from time to time, and in 1880 the
English anatomist H.G. Seeley published a paper
affirming his conclusion that these creatures were
giving birth at sea.

A skeleton of the Jurassic ichthyosaur Stenopterygius
quadriscissus, showing a late-term embryo sticking out
of the mother's body. The fetus may have been pushed out
during the decomposition process. From Organ et al.,
2009.

There was some initial uncertainty about this. What if
the "embryos" were really the prey of the larger
individuals? It's a reasonable criticism, but the fact
that the little ichthyosaurs were consistently the same
species as the adults, lacked any evidence of having
been fed upon, and were found too far back within the
body cavity to have been contained in the stomach or
intestines eventually ruled out the cannibal hypothesis.
By the beginning of the 20th century, there could be
little doubt that these shark-like marine reptiles were
born into the sea. In some fossils the near-term fetus
even protruded from the birth canal of its mother - a
frozen moment sometimes taken as being the act of birth
but that is more likely to be an effect of gases from
decomposition pushing the embryo out after death. Many
such specimens have been found. While embryos in other
ichthyosaur taxa are relatively rare, there are hundreds
of specimens of the genus Stenopterygius which contain
embryos at varying stages of development.

More recent discoveries have illustrated that
ichthyosaurs were not unique in their ability to
reproduce entirely in the water. In 2001 paleontologists
Michael Caldwell and Michael Lee described a gravid
Carsosaurus - a marine lizard known as an aigialosaur
which was closely related to the impressive mosasaurs of
the Cretaceous. (Imagine a Komodo dragon or monitor
lizard suited to life in the water, and you'll have a
good idea of what these creatures were like.) She was
carrying four embryos, which - based upon their
orientation - were probably delivered tail-first just as
in ichthyosaurs, whales, and manatees. Then, just last
year, researchers Qiang Ji, Xiao-chun Wu, and Yen-nien
Cheng described a Cretaceous reptile called
Hyphalosaurus which contained 18 paired embryos. This
creature belonged to a diverse group of aquatic reptiles
called choristoderes, most commonly represented by the
species Champsosaurus, though it lived in freshwater
rather than in the ocean. (In his recent post at
Tetrapod Zoology, paleontologist Darren Naish mentions
some additional examples, as well.)

Plesiosaurs were briefly on the list, too. In 1887, a
few years after he considered the reproductive habits of
ichthyosaurs, H.G. Seeley delivered a paper "On the mode
of development of the young in Plesiosaurus" at the
British Association for the Advancement of Science.
Seeley had been given a big nodule of Jurassic mudstone
and shale which seemed to contain several plesiosaur
specimens, primarily some very small ones which appeared
to show "only a slight budding of the fore limbs." There
even seemed to be some vestige of the placenta. These
must be embryos, Seeley proposed, meaning that
Plesiosaurus was also viviparous. Despite this, however,
it seems other paleontologists never took more than a
passing interest in the discovery. This may have been
for the best. As it turned out, Seeley had been misled
by shrimp burrows.

In 1982 paleontologist Richard Thulborn published a
redescription of the specimen Seeley was so struck by.
What the 19th century naturalist considered to be
embryos were truly "rounded masses of grey-brown
mudstone protruding from a core of flaky grey shale."
These bits weren't skeletons at all - Seeley had thought
the specimens were embryos because of their shape alone
- but, surprisingly, they were actually still fossils.
Though a definite identification was difficult to
ascertain, Thulborn proposed that Seeley's plesiosaur
embryos were truly the infilled burrows of prehistoric
shrimp.

It turns out that Seeley's conclusion was right, though,
even if his evidence was crap. At least some plesiosaurs
delivered their young at sea. The evidence comes from a
Late Cretaceous plesiosaur named Polycotylus latippinus
which swam the Western Interior Seaway which once
cascaded down the middle of North America.

Two views of the pregnant Polycotylus latippinus
specimen: the top shows the reconstructed skeleton, and
the lower image indicates reconstructed elements
(white), adult bones (tan), and embryo bones (dark
brown). From O'Keefe and Chiappe, 2011.

Polycotylus wasn't your stereotypical plesiosaur. Since
the time of their discovery in the early 19th century,
plesiosaurs have primarily been represented by long-
necked, small-headed forms, but Polycotylus was a short-
necked, large-headed variety. The specimen which is the
focus of the new paper - LACM 129639 - is a nearly-
complete adult splayed out on its bed of Pierre Shale,
but near the pelvic region there is a dense accumulation
of smaller bones from a young individual of the same
species. The smaller animal did not hold together like
the embryonic ichthyosaurs of Holzmaden - "The
juvenile", O'Keefe and Chiappe write, "consists of a
mass of poorly ossified and largely disarticulated bones
spilled from the body cavity of the adult, intermingled
with phalanges of the adult right fore-paddle."

Frustratingly, the specimens were excavated, jacketed,
and broken up before anyone drew up a quarry map. Even
though the finished fossil looks as if fossil hunters
simply brushed off the overlying rock, the whole thing
is actually a reconstruction put together on the basis
of how the plaster jackets related to each other and
observations of the excavators. Still, there are a few
clues that indicate that the smaller animal is, in fact,
an embryo preserved close the position it occupied
inside its mother.

Parts of the smaller plesiosaur's hips appear to be
preserved in the position they had in life. Two portions
of the hip, in particular, were attached to the internal
surface of one of the adult's shoulder bones, indicating
that fact that the smaller animal was inside the adult
at the time of burial and had not somehow become
accidentally mixed up with the larger animal.

So far, so good, but might the small animal have been a
meal for the adult? Paleontologist Ken Carpenter raised
that possibility in a brief comment printed at
ScienceNOW, and almost every case of fossil embryos
found inside their possible mothers has raised the
possibility of cannibalism. (For another recent example,
see the discussion about the small bones found inside
the early whale Maiacetus published at PLoS One.) As
O'Keefe and Chiappe point out, though, the development
of the bones is consistent with the hypothesis that the
animal was a fetus and there is no sign that the bones
were chewed or etched by digestive acids. The newly-
described find is consistent with what has been reported
for other pregnant marine reptiles, and there does not
appear to be any direct evidence that the small
Polycotylus was the last meal of the adult.

There's more to the paper than a confirmation that
plesiosaurs likely gave birth at sea, though. The
Polycotylus fetus was large and all by itself - the
mother was not carrying a litter of puny plesiosaurs. In
terms of natural selection, this is putting all your
eggs in one basket. A single, large offspring is a big
energetic investment and the sole genetic representative
of its parents, and therefore it would be expected that
parents which provided some amount of extra care would
better ensure the survival of their offspring and the
continued replication of their genes. The authors are
tentative about the idea, but single, big offspring are
often associated with additional maternal care after
birth - much like whales provide to their young - and
there is the possibility that plesiosaurs did the same.

The ability to give birth to live young in the water is
something that has evolved over and over again among
disparate groups of marine organisms. Even if we
restrict ourselves to the marine reptiles listed in this
past - ichthyosaurs, aigialosaurs, and plesiosaurs are
all representatives of distantly-related lineages which
became adapted to life in the sea independently. Mammals
- whales and manatees - also became adapted in similar
ways due to their shared way of life. Why, then, are
there no penguins or crocodiles that can give birth to
live young in the water?

Both penguins and crocodiles are members of a group
called the Archosauria - "the ruling reptiles."
(Dinosaurs are included in this group; penguins are the
modern day descendants of feather-covered theropod
dinosaurs, after all.) There are no live-bearing
archosaurs today, but perhaps there were in the past.
Fossil evidence has discredited the idea that dinosaurs
may have delivered live young, but there was once a
long-lived and widespread group of entirely aquatic,
sea-dwelling crocodiles. Based on their anatomy and mode
of life alone, it is a reasonable hypothesis that these
archosaurs may have become adapted to giving birth at
sea. Further consultation of the fossil record will be
needed to test this idea.

Extinct marine crocodiles aside, though, the ability to
bear live young has evolved so many times that it is
peculiar that there are no living archosaurs can do the
same. Researchers Daniel Blackburn and Howard Evans
considered why this might be so in a 1986 paper called
"Why are there no Viviparous Birds?" They discounted the
notion that there might be some unknown, live-bearing
species of birds hiding somewhere in New Guinea,
Madagascar, or the Amazon Basin. There must have been
some kind of evolutionary constraint which prevented the
evolution of the trait.

A variety of different constraints had been considered
before. Maybe carrying internal offspring would have
weighed down mother birds too much and made them more
vulnerable to predators, or perhaps the type of hard-
shelled egg birds lay is not able to be adapted into a
live-birth reproductive system, among other
possibilities. Yet Blackburn and Evans rejected the
previously-aired hypothesis. (In the flight hypothesis,
for example, the scientists pointed out that female
birds do internally carry the eggs before they are laid,
and that pregnant bats fly around just fine while
carrying the extra mass of their offspring.) Instead,
they proposed that there was never any kind of
evolutionary pressure that would have favored the
internal retention of the egg and internal hatching (one
of the modes of live birth). If it ain't broke, don't
fix it.

Asking about the evolutionary road not taken is tricky
business. We can approach the puzzle of why a certain
trait or adaptation appeared through the records
organisms contain in their anatomy and - among living
and recent species - their genes, but how can we really
know why this or that feature did not appear? Was there
some kind of anatomical or physiological constraint? Did
the proper selective pressure just never come into play?
Was there more than one cause? These are the kinds of
questions that can keep an evolutionary theorist up at
night, dreaming of penguins delivering their chicks in
the tranquil blue of the sea.

References:

Blackburn, D., & Evans, H. (1986). Why are there no
Viviparous Birds? The American Naturalist, 128 (2) DOI:
10.1086/284552

Caldwell, M., & Lee, M. (2001). Live birth in Cretaceous
marine lizards (mosasauroids) Proceedings of the Royal
Society B: Biological Sciences, 268 (1484), 2397-2401
DOI: 10.1098/rspb.2001.1796

Ji, Q., Wu, X., & Cheng, Y. (2010). Cretaceous
choristoderan reptiles gave birth to live young
Naturwissenschaften, 97 (4), 423-428 DOI:
10.1007/s00114-010-0654-2

Maxwell, E., & Caldwell, M. (2003). First record of live
birth in Cretaceous ichthyosaurs: closing an 80 million
year gap Proceedings of the Royal Society B: Biological
Sciences, 270 (Suppl_1) DOI: 10.1098/rsbl.2003.0029

O'Keefe, F., & Chiappe, L. (2011). Viviparity and K-
Selected Life History in a Mesozoic Marine Plesiosaur
(Reptilia, Sauropterygia) Science, 333 (6044), 870-873
DOI: 10.1126/science.1205689

Organ, C.; Janes, D.; Meade, A.; Pagel, M. (2009).
Genotypic sex determination enabled adaptive radiations
of extinct marine reptiles Nature, 461 (7262), 389-392
DOI: 10.1038/nature08350

Thulborn, R. 1982. Liassic plesiosaur embryos
reinterpreted as shrimp burrows. Palaeontology 25 ( 2),
351-259

Wang, Y., & Evans, S. (2011). A gravid lizard from the
Cretaceous of China and the early history of squamate
viviparity Naturwissenschaften DOI:
10.1007/s00114-011-0820-1

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