Read Dinosaurs Without Bones Online
Authors: Anthony J. Martin
So with these search images in mind and an active lust for such trace fossils, paleontologists should have all of the right tools for ensuring their explorations all have happy endings. Here’s to getting lucky.
Dinosaur Eggs, Over Easy
Dinosaur mating surely happened many times during their 165-million-year history, and more than a few of those couplings successfully resulted in fertilized eggs getting laid by dinosaur mothers. So are dinosaur eggs—which are normally preserved as fossil eggshells and filled with sediment—considered trace fossils or body fossils? One would think these and eggshell material in general would qualify as trace fossils, because they seem like indirect evidence that doesn’t involve any obvious body parts, except for rare instances when they also hold embryonic dinosaur bones.
Nonetheless, eggs and eggshells are actually body fossils. The reason why is because eggshells served as extra body parts for developing embryos (similar to how skin functioned in an adult dinosaur), protecting softer parts inside while also allowing an embryo to breathe through pores in the eggshell. Thinking more ecumenically, though, I like to consider dinosaur eggs as traces of the mothers that developed them. By extension, then, every dinosaur body is a trace fossil of its mother, and ultimately of both parents mating. But such thoughts might be a little too metaphysical, so let’s just go back to thinking about eggs as body fossils.
Despite their lowly status as body fossils, dinosaur eggs provide valuable clues about dinosaur reproduction, brooding, and their surrounding environments, which all supplement information
provided by the dinosaur fossils that really matter—namely, their trace fossils. In fact, the mere pattern shown by a number of eggs laid by a dinosaur mother in one egg-laying episode (known as a clutch) constitutes a trace fossil in itself, regardless of whether a nest structure is associated with the clutch or not. Or if a dinosaur parent actively pressed an egg against the side of a nest and thus made an external mold of that egg, that would count as a trace fossil of the parent, too.
Vertebrates that lay eggs today, such as reptiles, birds, and monotremes (egg-laying mammals, such as the platypus and echidnas), are all classified as
amniotes
. This means they share a common ancestor that evolved an
amnion
(a sac containing a developing embryo), which was enclosed by an egg. This novel trait likely developed in response to widespread arid conditions, which naturally selected amphibian ancestors capable of putting watery conditions inside an egg, regardless of whatever climate might have been outside of them. This new reproductive strategy evolved in vertebrates by about 320 to 330
mya
, or about 100 million years before dinosaurs existed.
Later in their evolutionary history, amniotes evolved myriad reproductive strategies, including live birth which cut out the “middle egg” (so to speak) by developing an embryo inside the mother until it’s ready to emerge, instead of trusting this to all take place in an egg. We currently have no reasonable evidence that dinosaurs gave birth to live young, but this trait (
viviparity
) showed up in ichthyosaurs—marine reptiles unrelated to dinosaurs—by the Early Jurassic, around 190 million years ago. Live birth in ichthyosaurs made sense evolutionarily, because it kept ichthyosaur mothers in the seas instead of having to lumber up on land to dig nests and lay eggs. Although that strategy has worked very well for sea turtles for the past 100 million years, it apparently was selected out of ichthyosaurs early on in their evolutionary history. Viviparity also occurs in some modern species of lizards and snakes, including sea snakes; the latter shows how other marine reptiles came up with the same reproductive mode.
The oldest known dinosaur eggs come from Early Jurassic rocks, or about the same time ichthyosaurs were using live birth. Dinosaurs, however, were very likely laying eggs in the Late Triassic Period, too; but these may have been soft, leathery eggs that were not as easily preserved as the Jurassic ones. The Early Jurassic fossil eggs, which were preserved because of calcite in their shells, give us a minimum time for when dinosaurs had developed mineraized eggs. Basically, this made eggshells that went from “squish” to “crunch” when another dinosaur stepped on them. Hence, the eggs we are so familiar with in our everyday lives, thanks to those most famous of modern dinosaurs (chickens), got their origins relatively early in the evolutionary history of dinosaurs.
Despite the fact that dinosaur eggs are technically considered body fossils, paleontologists were still motivated enough to devise a classification scheme that helped distinguish different types of eggs. Paleontologists, using a system parallel to a biological classification, came up with
ootaxonomy
, in which the “oo” prefix refers to eggs. These categories, based on shell microstructure, arrangements of the pores, and overall forms, were given linguistically daunting names such as Spheroolithidae, Ovaloolithidae, Megaloolithidae, Dendroolithidae, Faveoloothidae, and at least a half dozen others. Despite such names causing computer spell-checkers to run and hide, they are essential for the small and dedicated group of dinosaur-egg paleontologists to better communicate with one another.
Unfortunately, these egg categories do not always exactly match clades of dinosaurs. Although we can be reasonably sure that theropods, sauropods, ornithopods, and ceratopsians produced certain types of eggs or egg clutches, initial identifications are occasionally tested by the discovery of embryonic bones in an egg. Moreover, as of this writing, no one has yet identified an undoubted stegosaur, ankylosaur, nodosaur, or pachycephalosaur egg, which make for surprisingly big holes in our knowledge of the life cycles for these dinosaurs.
Given that eggs are body fossils, is there any way to elevate their importance further by somehow making trace fossils out of them,
too? The answer is yes. At least two species of dinosaurs,
Citipati osmolskae
and
Troodon formosus
, provide evidence of post-laying movement of eggs in a nest, an action that must have been done by one or both dinosaur parents. This sort of arrangement constitutes a trace fossil of that parent’s behavior like how a flower arrangement reflects the handiwork of a florist.
In the case of
Troodon
nests, distribution patterns and orientations of the eggs provided marvelous clues about female
Troodon
reproductive anatomy, as well as what she did with the eggs after they exited her body. Paleontologists who studied
Troodon
egg clutches were surprised to notice that not only were eggs paired but also aligned vertically. Based on their statistically significant pairing, paleontologists surmised that this pattern must have been caused by the mother laying eggs two at a time. Secondly, the eggs were longer than they were wide (elliptical), which meant they naturally should have rolled onto their sides once deposited onto the ground surface. As a result, their vertical orientation means they were righted after laying.
To best accomplish this feat, the mother or father
Troodon
likely would have used their hands to turn the eggs upright, then buried the lower end of each egg so that they wouldn’t just roll onto their sides again. No one knows whether
Troodon
mothers did this with each pair of eggs—two at a time—or whether they waited until the entire clutch was laid before turning them all sunny-side up. In an evolutionary sense, the turning-two-at-a-time scenario seems more likely than the let’s-wait-until-they’re-all-out one. But sometimes evolution works in mysterious ways, going counter to the expectations of us geologically short-lived primates.
Did dinosaurs ever engage in cuckoo-like behavior, laying their eggs in with the eggs of other species in a nest, and letting some other species’ parents do all of the work of raising the young? Thus far, we only know of one instance in which an egg assemblage contained embryonic bones of more than one species of dinosaur, which was found in Late Cretaceous rocks of Mongolia. The bones showed that one dinosaur was probably
Oviraptor
(closely related to
Citipati
) and the other was
Byronosaurus
, a dinosaur more closely
related to
Troodon
. One would think a discerning mother
Oviraptor
would have been able to tell whether she was brooding and feeding a
Byronosaurus
child. But such is the insidious nature of nest parasitism in modern birds: most bird parents don’t know they have a changeling in their midst until it is too late. However, this one instance of possible parasitism is hard to test further, as eggs also could have easily been transported and deposited together in the same place by currents. One way to test this hypothesis, though, would be through trace fossils, such as two distinctive sets of hatchling tracks in a nest. Have hatchling tracks ever been found in a nest? Not exactly, but they have been found very close to nest sites, as will be explained later.
Another type of interpretable hatchling trace fossil would be an exit hole (“hatching window”) in an eggshell, made by hatchlings as they emerged from their temporary confinement. Sea turtles, for example, have a temporary extension of their beaks when born, which is applied like a can opener from the inside of the egg to open it. Sea-turtle researchers can thus pick up an empty sea turtle egg from a previous year and instantly tell whether its former occupant successfully hatched, died in the egg, or an egg predator got to it before hatching. Similar parts have been described from Late Cretaceous sauropod embryos in Argentina, so these baby dinosaurs could have made such traces in eggs. Have hatchling trace fossils been identified in dinosaur eggs? Yes, which is amazing when one considers how much more fragile an egg would have become once abandoned. Indeed, telling the difference between fractures caused by a hatchling coming out of its egg versus those inflicted afterwards would be very challenging. Nonetheless, trace fossils of “hatching windows” were interpreted in 2002, evident in Cretaceous eggs from Mongolia and China. More such traces could be reasonably proposed for fossil eggs with localized and consistently sized hatchling-appropriate holes in them, especially if more than one is seen in the same clutch of eggs within a nest structure.
Other trace fossils that might be preserved in eggs would be those from the aforementioned egg predation, where another
animal—whether a dinosaur, crocodilian, lizard, snake, mammal, or insect (depending on when in geologic time this might have happened)—nibbled, gnawed, bit into, or chomped an egg. Yet these trace fossils have not been discovered yet either, despite many decades of our demonizing mammals for eating too many dinosaur eggs toward the end of the Mesozoic Era.
We also currently do not know whether dinosaurs laid unfertilized eggs—like chickens or some other birds—but we presume that if their eggs are found in a nest structure, these were probably fertilized. The extra effort required to build a nest represents an investment of time, resources, and energy and would have exposed parent dinosaurs to predators by keeping them in the same place for a while, thus making them predictable. Instincts that would have impelled parent dinosaurs to make nests with every laying of an unfertilized egg clutch would have been quickly selected out of those lineages.
An adage I often tell my students when discussing extinction is: “If you want to make a species go extinct, stop it from reproducing.” The huge success of dinosaurs throughout nearly every land environment within much of the Mesozoic Era attests to how they reproduced just fine, probably aided through the vast majority of them making nests for their egg clutches at the right times and in the right places.
Starting a Dinosaur Family: Building Nests
Given that at least some, if not most, dinosaur mating successfully resulted in fertilization, and a mother dinosaur started internally producing eggs soon afterwards, the next step for either her or both parents should have been nest building. Although this supposition might be taken for granted, what is surprising to many people is that actual trace fossils of dinosaur nests are only known for a few dinosaurs, and the details of how these were made are still a bit murky. The known nest builders are:
Other dinosaurs that we are sure made nests, even though we do not have direct evidence of a nest structure, are the Late Cretaceous theropods
Oviraptor
and
Citipati
from Mongolia. In a famous case of dinosaur-parenting misattribution, paleontologist Henry Fairfield Osborn, of the American Museum of Natural History, found an
Oviraptor
skeleton on top of an egg clutch during an expedition to Mongolia in 1923. Nonetheless, he figured the eggs were those of the small ceratopsian
Protoceratops
, whose bones were abundant in the area, too. In Osborn’s scenario,
Oviraptor
died and was buried while raiding the nest of another dinosaur, not while taking care of its own eggs. This idea was disproved much later, and through a closely related dinosaur,
Citipati
. In one of many spectacular finds from American Museum of Natural History expeditions to Mongolia in the 1990s, a nearly complete specimen of
Citipati osmolskae
—missing only its head—was found in a sitting position above a clutch of long oval eggs. Even better, the clutch was oriented radially. This suggests that the mother or father
Citipati
arranged the eggs after laying, with such a pattern constituting a trace fossil in itself. When viewed from above, the skeleton and eggs paint a striking portrait. The dinosaur’s arms are held wide in a semi-circle
around the eggs, evoking images of a protective mother or father that died in a last vain effort to keep their unborn children from harm. Paleontologists have also documented direct associations of dinosaur parents and eggs—some of which have embryonic bones linking them with their parents—with another specimen of
Citipati
and several examples of
Oviraptor
.