Were Dinosaurs Good Parents?
Designed & maintained by Daniel John Elvidge.
Parental care includes any investment that the parent or parents of an animal make toward their offspring. Until recently, birds and mammals were thought to be the only groups of creatures to show true parental care and that reptiles simply laid their eggs then abandoned them to their fate. However, during the 1970s field observations showed crocodiles helping their young to hatch and carrying them to the water. It was no surprise then when in the 1980s evidence of parental care in dinosaurs was unearthed.
There is no evidence to suggest that any dinosaurs were outside the confines of eggs, yet there are very few eggs that can be demonstrated that have been laid by particular dinosaur species. Identifiable embryonic remains are the only conclusive evidence that can be used to identify particular eggs to a particular dinosaur species. Discoveries of eggs and nests in North America and Mongolia have shown that many Dinosaurs may have cared for their young after hatching. Including brooding, some laid their eggs in earth nests scooped in the soil, and returned to feed the young after they emerged.
Evidence to support the idea that some dinosaurs may have cared for their young after hatching has been gained from looking at the bone structure of the embryos found in the known dinosaur eggs and that of juvenile dinosaurs found in the nests. The hind legs of juvenile Maiasaurs (see "The North American nests" showed that their bones were too weak just after hatching to have left the nest to look for food this means that in order to survive adults must have brought food to them in these early days. Also, if the indications of their biology are correct, Pterosaurs (although not strictly speaking a dinosaur) could not have flown as soon as they hatched and so must have been cared for while they grew rapidly to fledging size. Young Tyrannosaurs are unusual for dinosaur in that they had long snouts and large teeth suggesting that they may have been independent hunters from the time they hatched.
The nest too can tell us a lot about whether parental care was likely. From what we see in today's reptiles, eggs broadcast in simple hole nests are most likely to be abandoned after burial. Mound and open nests were most likely to be guarded by adults.
The size of the dinosaur is also important. Large dinosaurs like Sauropods would have been too heavy to brood their young. The extreme size difference between adults and their young in some species would have made parenting impractical. The young juveniles would also have been too small to join herds (presuming that there were herds) of adults, where they could have been trampled. They would have to have been cared for by their parent(s) until they were large enough to join the others.
The only cases of direct evidence of parental care in dinosaur, however, are the association of the adult Oviraptor on a clutch of 24 eggs discovered in Mongolia (see links below) and a partial Troodon skeleton found atop of a clutch of Troodon eggs. Both of these dinosaurs had larger relatively more complicated brains that the other known dinosaurs. Could the activity of parental care be limited to those dinosaurs that had a large enough brains to carry out such activity? It is unclear.
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The Mongolian nests.
The North American nests.
The Spanish nests.
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The Mongolian nests.
The famous American Museum expeditions to Mongolia in the 1920's discovered complete fossilized Protoceratops nests. Some of them contained several concentric rings of up to 18 eggs. Many of the nests were also found with the skeletons of adults, juveniles, and hatchlings. However, later in 1990 many of the eggs and nests had to be reassigned to another dinosaur called Oviraptor when a embryo of Oviraptor was found in one of the eggs.
Oviraptor was badly named. It's name means "egg thief". This was because the first type skeleton was found in 1923 lying on the top of a nest of eggs. At the time it was assumed that the dinosaur was feeding on the eggs in the nest using it's parrot like beak. This was until another Oviraptor was found in UKHAA-TOLGOD (Southwest Mongolia) in the same position in 1993, but this time an unhatched Oviraptor was found inside one of the covered eggs. It is now believed that the Oviraptor was far from an egg thief, but was in fact brooding their own eggs in such nests!
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The North American nests.
Dr. Jack Horner (who actually discovered the bones of the aptly named "Egg Mountains" baby hadrosaurs not in the field, but in a rock shop down the road) has been making excavations of Late Cretaceous hadrosaur and sauropod nests in Montana around "Egg Mountain". These nests showed some very important aspects of dinosaur "child care." The hadrosaur nests occurred in colonies, which suggests communal nesting. Horned found that these dinosaurs returned to the same site year after year to lay their eggs.
The nests are low mounds that Dr Horner believes were dug out by the female hadrosaur using her hind feet. He said that then the female would lay approximately 24 ellipsoidal eggs in concentric rings, covered them in sand, and then tended the nest until they hatched. There may also have been some trace of vegetation perhaps lining the nests, but this nest has since been lost. He made the suggestion after finding skeletons of adult and juveniles hadrosaurs of the genus Maiasaurs ("the mothering dinosaur") all around the nests. These findings strongly suggest some kind of parental care within these dinosaurs. The hatchlings may have been offered food for their first days out of the shell. It was suggested that the young remained in the nests for some time after hatching because the shell fragments that were found were very small presumably because they were trampled by the hatchlings. The presence of adults would also have deterred predators. The baby hadrosaur found also had worn teeth. Suggesting that they must have been fed in the nest or made short trips from the nest to find food. In order to do this the defenceless young must have been looked over by the adults.
Also found in this area was a bone bed containing the remains of hundreds perhaps thousands of adult and young hadrosaur that had been killed in some sort of catastropy. With this in mind it is tempting to say that these animals migrated to this area in huge herds. Following the rich pastures throughout the year to rest, lay their eggs, and rear their young.
Hadrosaurs were apparently not the only animals to nest in this area. Associated with the hadrosaur nests and eggs were the eggs and nests of another dinosaur which the hadrosaurs must have shared the area. The hypsilophodontid Orodromeus was also found to have laid eggs in the area. The eggs were found in "Egg Mountain" and a little further away on "Egg Island". Some of the Orodromeus eggs and nests were extremely well preserved. Much less of the supposed trampling observed in the hadrosaur nests was seen. One nest found by Dr Horner and his team was complete with nineteen eggs laid in a spiral. One egg was placed vertically in the center of the clutch and the others leaning progressively farther outward along the spiral.
Comparison between the hadrosaur and hysilophondontid nests and eggs arrangements also threw some light on the varieties of parental care in dinosaurs. Many of the hysilophondontid nests contained opened eggs that were not trampled. This suggests that the young did not remain in the nests after the hatched-but left promptly. This could mean that the hysilophondontids of "Egg Mountain" indulged in little if any parental care of their young as in the case of modern reptiles.
Weather this was true or not can be tested by looking at the bones in the legs of these embryonic Orodromeus. Their legs seemed to be well formed and ready for use at the time of hatching. However the Maiasaur embryos had extremely poorly developed joints in their legs, making them confined to the nest for sometime after hatching. This means they would have adults would have had to bring them food! It is possible that hadrosaur nestling were semi-nestbound and that they had just enough locomotary ability to flee the nest if a predator penetrated the parental defenses.
Also found in Montana were a group of four associated specimens of Troodon. They were found to be two juveniles, a sub-adult, and an adult. Some ask, "can this represent a family unit that perished together on the shores of a freshwater lake?" It is uncertain.
However, Dr Horners' views of the finds in "Egg Mountain" are not universally accepted. Many are keen to point out that there are many faults in his reports. Firstly that the nests originally documented to have been lined with vegetation have since been "lost" before they could be fully described. Secondly that the nests themselves could simply be sedimentary impressions that have later filled with the egg-shell fragments (which are incidentally found all over the site not simply in the supposed nests). Thirdly the nests prescribed to be apart of a nesting colony can not be proved to be on the same level, and could be that of single dinosaurs nesting in that area over a long period of time. Also only a few of the nests have been proved to be that of Maiasaurs or Orodromeus. All of these problems must be kept in mind when considering parental care in the dinosaurs of this area.
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The Spanish nests.
The Bastus dinosaur nesting site is found in the Arenisca de Aren Formation in the province of Lerida (northeastern Spain). Here a huge volume of egg shell fragments have been found in the red sandstone sediments that were laid down in a shoreline environment during the Late Cretaceous period. The egg shell material represents about 0.5% of the total volume (12,000 cubic meters) of the deposit.
Estimated 300,000 fragmented dinosaur eggs lay in the Bastus site, and each egg has a diameter of approximately 20cm, with an egg shell thickness of 1.45mm. The area contains 24 nests most of them contain two or three eggs, with a maximum of seven. The eggs are subspherical in shape.
The sediments and the large number of eggs found, in this site tell us that a large group of dinosaurs (some kind of nesting fidelity) were nesting in the exposed sand of a beach-ridge plain. The nests are well preserved so the cause of the egg fragmentation can not be transport. It is better explained in the same way as the fragmentation found in North American Maiasaur nests.
That is, that the eggs were fragmented by trampling and nesting activities of the dinosaurs. Meaning that the young must have remained in the nest after hatching and that, in order to do this the adults would have had to tend to them, meaning that the dinosaur that made these nest displayed parental care of their young.
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Click on any of the links below to learn more about a particular find or topic.
Troodon nests on "Egg Mountain"
The 'brooding' Oviraptor
What were the dinosaurs?
Were the dinosaurs warm or cold blooded?
What else was alive?
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(Click on highlighted definition to take you back to text)
Egg Island: An area of land southwest of Bynum in the state of Montana, USA that during the late Cretaceous period was an island in the center of a lake.
Egg Mountain: A locality name for a site in the 'Willow Creek Anticline' (a range of small hills southwest of Bynum in the state of western Montana, USA). Within the 'Two Medicine Formation'. 'Egg Mountain' was discovered in 1979 by Princeton University undergraduate Fran Tannenbaum, while on an expedition under the direction of Dr John Horner.
Hadrosaur: A duck-billed, plant-eating dinosaur similar in body form to that of the iguanodontids (large and heavily built). However the skull was much flatter in profile, and the diamond-shaped teeth were arranged in batteries, with as many as 700 being visible at one time. The tail too was laterally flattened, but retained the bony tendon support. Typical length: 13m.
Hypsilophodontid: A medium sized (Typical length: 2.5m), light bodied, land-dwelling, biped dinosaur. They show their primitiveness in having teeth situated in the front of the upper jaws, and by the extreme length of the backward facing pubic rod. The cheek teeth were of a shearing type, arranged in a single row and replaced in batches of three. The feet were four toed each ending in a pointed hoof. It is supposed to have fed upon the fern and cycad-like plants that dominated the Cretaceous.
Late Cretaceous: The period of geological time between 65 and 88 million years ago.
Oviraptor: A meat-eating, biped dinosaur of the group Theropoda found in the late Cretaceous of Mongolia. These dinosaurs left bird-like tracks, made by three clawed toes, when walking. A further toe, which did not reach the ground, pointed backwards. Oviraptor was an unusual Theropod as it was toothless. Oviraptor was badly named (explained in "The Mongolian nests" section).
Protoceratops: A small, stocky quadruped characterized by a horny beak, a small nose horn, leaf-like cheek teeth set in a single line, and a much reduced neck frill containing large perforations. It was thought to live in open areas, where it fed upon plant stems that it cut with its powerful beak and then sheared into smaller pieces by its cheek teeth. Typical length: 1.8m.
Pterosaurs: The only flying reptile. With a light delicate body and skull this animal had extended first fingers that moved out from the body to support the wing membrane making it capable of flight. It is thought that some of these animals would have chased its prey on the wing, in a similar manner to that of certain bird. They vary widely in size from 30cm species (thought to have preyed on insects) to those having a wing span of over 15 feet (thought to have preyed on large fish in the same manner as 'Frigate Birds').
Sauropod: Not only the largest dinosaurs but also the largest known land animals. Some have been estimated to have weighed 30 to 80 tonnes. Typically in these plant-eaters, the neck and tail were extremely long, and the body relatively short and bulky. They had stout; columnar legs with short toed feet rather link elephants. The skull was very small, only about 60cm long in Diplodocus (which was the largest sauropod at about 27m in length), and the brain cavity unusually small. Fossil track ways have shown that they moved in herds but there is still argument as to weather their main habitat was swamps or dry land.
Troodon: See Hadrosaur.
Tyrannosaurs: Perhaps the largest terrestrial carnivore of all time. These animals were fierce predators, with a large head and powerful deep jaws housing large dagger like teeth. They had shortened forelimbs bearing only two fingers (a typical feature of a carnivorous dinosaur). Typical length: 14m.
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