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Restoration of a rearing Diplodocus by Charles R Knight, 1911.

Restoration of a rearing Diplodocus by Charles R Knight, 1911.

A team of scientists believe they have solved the puzzle into why the largest dinosaurs ever to have walked the Earth actually produced the smallest eggs.

The researchers, led by Professor Graeme Ruxton at the University of St Andrews, say the sauropods eggs ended up so small because they took such a long time to incubate.

The new study, published this month, provides fresh insights into the dinosaur suborder which includes the famous Diplodocus from the late Jurassic era.

The team say their work explains why the giants of the dinosaur world ended up producing eggs no larger than an ostrich egg.

Despite the vast size of sauropods (estimated to measure up to 33 metres and weigh up to 16 tonnes), their eggs only weighed around 1.5kg. Until now scientists have been puzzled at the relatively small size of eggs, given sauropods weighed 50 times more than an ostrich.

The new research concludes that the substantial incubation time required for sauropod embryos to develop and hatch may have been an important constraint. This, the scientists say, could explain the small individual size of sauropod eggs.

Professor Ruxton, of the School of Biology at St Andrews, explained, “The living bird with the largest eggs, the ostrich, has to incubate its eggs for 42 days; during which time many eggs are lost to predators. An ostrich weighs about 100kg and lays a 1.5kg egg; a sauropod dinosaur might be 50 times heavier than an adult ostrich but its eggs were only a little heavier than an ostrich egg.

“Both individual egg size and clutch size in sauropods are smaller than might be expected for such enormous creatures, relative to modern egg-laying animals. Some people might find it a bit disappointing that giant dinosaurs didn’t lay equally giant eggs – but it’s very satisfying to think that we might finally understand why.”

The team, which included biologists from the University of Lincoln and George Mason University in Virginia, used data from modern birds and reptiles to investigate factors affecting clutch size in this group of dinosaurs.

They estimated that the time from laying to hatching of eggs, which were incubated in underground nests, was between 65 and 82 days. This long incubation time increases the risk of predation, which coupled with the relatively low temperatures expected in the nest, may have been a significant factor in limiting the egg and clutch size.

The researchers say that although having larger eggs may have been advantageous because of larger hatchling size, this may have been outweighed by the increased risk of predation during the egg stage. They also believe that by laying their eggs in small clutches, possibly in different nesting sites, female dinosaurs found a way of having a better chance of protecting their offspring from predation.

Dr Charles Deeming, from the School of Life Sciences at the University of Lincoln, explained: “We think that a long incubation period of sauropods is likely to have led to very high mortality through predation. We suggest that the females laid their eggs in small clutches, possibly in different nesting sites, as an adaptive strategy to mitigate the high predation risk associated with long time of exposure in the egg stage.”

The team believe their conclusions could be extended to other groups of dinosaurs.

The findings are published in the summer 2014 issue of the Paleontological Society’s journal, Paleobiology.

Note to Editors

The researchers are available for interview:

Professor Graeme Ruxton (St Andrews): 01334 464825 or email graeme.ruxton@st-andrews.ac.uk.

Dr Charles Deeming (Lincoln): 01522 835452 or email cdeeming@lincoln.ac.uk.

‘Incubation time as an important influence on egg production and distribution into clutches for sauropod dinosaurs’ by Graeme D Ruxton, Geoffrey F Birchard and D Charles Deeming is published by Paleobiology 40(3):323-330. 2014 DOI: http://dx.doi.org/10.1666/13028

Issued by the Press Office, University of St Andrews
Contact Gayle Cook, Senior Communications Manager on 01334 467227 or email gec3@st-andrews.ac.uk
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