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Pregnant Plesiosaurs and Baby Bones: Bone histology reveals ontogeny in polycotylid plesiosaurs

Pregnancy in the fossil record is an exciting find. Setting aside the sad fact that an unfortunate mother met her demise while carrying a baby, these one in a million specimens provides some key insight into the behavior and lifestyle of organisms unlike any living today.

One such specimen is on display at the Natural History Museum of Los Angeles County. It reveals that a species of plesiosaur, Polycotylus latipinnus, was in fact pregnant when it died, revealing information about reproduction and birth in these marine reptiles. But a new study is looking closer at this specimen, in part to to confirm this miracle mother and its baby, but also to address further questions and hypotheses about maternity in marine reptiles.

According to the study, published in the journal Integrative and Comparative Biology by co-authors Dr. Robin O’Keefe, research associate of the Natural History Museum of Los Angeles County and professor at Marshall University; Martin Sander, professor at Bonn University in Germany; Tanja Wintrich, Ph.D. candidate at Bonn University; and Sarah Werning, assistant professor at Des Moines University, new evidence obtained via bone histology supports the hypothesis that plesiosaurs gave birth to live young, and that these young animals went through rapid growth that might have hindered their swimming performance.

According to O’Keefe, “Our study…reaches the novel conclusion that plesiosaur fetal bone grew extremely quickly, sacrificing bone strength for growth rate. Plesiosaur babes may have needed maternal care for protection.”

And these babies weren’t just growing quickly, they were already born large! The data obtained in this study shows that at least some plesiosaurs gave birth to live young that were about 40% the length of the mother, the equivalent of a human mother birthing a six-year old.

Co-author Tanja Wintrich takes research samples from the pregnant plesiosaur on display at the Natural History Museum of Los Angeles County. Image Courtesy NHMLA.

The team sampled the Pregnant Plesiosaur by drilling directly into the specimen on display and obtaining samples suitable for a histological analysis. These were compared it to histological samples of juvenile specimens of closely-related plesiosaur, Dolichorhynchops bonneri, to illustrate possible ontogenetic changes in plesiosaurs and gain a bigger understanding of bone development in this marine reptiles.

I had a chance to ask lead author Robin O’Keefe some questions related to the paper, and he provided some great additional insight into this exciting research!

PPC: First of all, tell me a little bit of the background on this paper. Who first suggested that the LACM specimen was pregnant, and who thought of utilizing histological methods?

FRO: Because the LACM mother is so complete, and because most of the fetus is there, it was obvious when the specimen was collected in the 1980s that it might be mother and child. It is a spectacular fossil. But preparation was never finished and no paper written until Luis Chiappe decided to include the specimen in the then-new displays at the Natural History Museum in L.A. Luis brought me in to advise on the mount and to write up the fossil (O’Keefe and Chiappe, 2011); that paper received a lot of attention. But there was a little backlash at the time; people made a valid point that the fossil was a single occurrence and so could be some random event.

Good science is repeatable, and makes predictions, and this got me thinking: how could the notion that plesiosaurs gave birth to large, live young be tested? I discussed this at SVP with Hans Larsson, and he suggested looking for birth lines and other histological evidence for ontogeny. The LACM fetus would not have a birth line, but other juveniles might. So I identified a good growth series from a single species and made sections. Meanwhile, my colleagues Sander and Wintrich sampled the histology of the LACM specimen. We we able to use these data from the fetus to make predictions about the other material. And everything checks out: our young juvenile has a clear birth line and is about 40% of maternal size. It is great in science when you come at a question from another direction and get the same answer. It gives you confidence in your findings.

Why are young polycotylid fetuses so large? 40% of the maternal length seems massive, wouldn’t that be more of a danger to the mother when birthing?

As to why plesiosaurs in general, and polycotylids in particular, had such large babies is a difficult inference. The evolutionary tradeoff between making many cheap offspring vs. a single expensive offspring is complex and has different drivers in different species. It probably had something to do with how dangerous the ocean was (and is); most whales and other marine mammals have single progeny, while that is not the case with all terrestrial mammals. That’s a suspect comparison because mammals are so different physiologically, but we know from analogy with other reptiles that have large, single young that they tend to have maternal care. As to the size of the fetus being a danger to the mother, that is less of a concern than it might seem. The Solomon Islands skink, a lizard, can have single progeny that are over half the length of the mother when born. It sounds nuts, but they pull it off. Also, the hip bones in plesiosaurs are reduced because they are aquatic. So there was plenty of room for the baby to make it out. Humans are actually quite unusual in that birth is so constricted; because we are bipedal, there is a great evolutionary pressure for the hips to be close together, while at the same time there is great evolutionary pressure for increased head size. Birth is much more dangerous in humans than most other animals; in a quadrapedial animal the hips can get wider to accommodate larger offspring.

You reference Kastschenko’s Line frequently in the study. What is that?

Kastschenko’s Line is a histological feature that delineates the two zones of ossification in a developing limb bone. The bone begins as a cartilaginous precursor that then ossifies; at the same time this is happening, additional layers of bone are being deposited around the ossifying cartilage. So you have a cortex deposited around a medulla, and Kastschenko’s Line is the boundary between the two. The cortex is deposited from this line outward, giving a record of bone deposition over ontogeny.

As you mention in the study, the propodials of the LACM fetus are not preserved, and so you used the scapula as a proxy. Is there any concern that the histology of the scapula of the specimen might be misleading regarding the histology of the distal portion of the flipper?

There is some concern. It is a valid criticism. I would be more concerned if the histology was vastly different from a limb bone; however, it is very similar, with identical bone types. Also the scapula is still a limb bone and has the same development pattern as a humerus or femur. This is not true of vertebrae or ribs and I would not make that comparison.

The microstructure of a newborn plesiosaur bone in polarized light. Differences in color show differences in bone fiber direction, deposited rapidly around canals for blood vessels. The small black dots are individual bone cells.

What did you find surprising or most interesting in this study?

The histology indicates that the fetus grew extremely rapidly in utero. I think that is very interesting; where did the energy come from? The high growth rate probably required a high body temperature, and possibly full warm-bloodedness, to support it. This is in accord with isotopic data that suggest that plesiosaurs were warm-blooded. So I think we we are really starting to flesh out a picture of plesiosaurs as active, warm-blooded animals that lived in social groups and cared for their young. A good modern analog would possibly be an orca.

How applicable/accessible are histological methods to other researchers?

Very. Paleohistology is really exploding right now, and its methods are being applied across all vertebrate clades. The insight it can give us about growth biology in extinct animals is unprecedented. We have a long way to go, this research in plesiosaurs is just beginning, and we don’t understand as much as we would like about growth in plesiosaurs in general.

It is interesting that so much info regarding aquatic locomotion can be elucidated histologically. Have you learned anything else about these specimens by examining histological data?

We are learning it right now. There is a lot of histological variability among plesiosaur clades, and patterns might be linked to lifestyle, phylogeny, or both. I am working on a bunch of elasmosaur data right now and they look like they are doing something different; stay tuned for more.

Where does this project go from here? Do you have more plans to sample similar specimens?

See above; we’ve got to document the similarities and differences among plesiosaur clades to understand how their life cycles varied.

Anything else you’d like to share to the PLOS Paleo Community?


Thank you Robin!



O’Keefe FR, Chiappe LM. 2011. Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science 333:870–3.

O’Keefe FR, Sander PM, Wintrich T, Werning S. 2019. Ontogeny of Polycotylid Long Bone Microanatomy and Histology. Integrative Organismal Biology 1(1):oby007. https://doi.org/10.1093/iob/oby007

Featured Image: Visitors to the Natural History Museum can see the pregnant plesiosaur on display in the Dinosaur Hall. The specimen is 15.5 feet wide and 8 feet tall. It is the only pregnant plesiosaur fossil ever discovered. Image courtesy NHMLA.

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