Despite the excellent body fossil record for ceratopsians, little is known about marginocephalian reproduction, although some speculations about their pre-mating behaviors have gained considerable attention. Similar to some ornithopods, especially hadrosaurs (Chapter 11) and thyreophorans (Chapter 12), marginocephalians are subject to hypotheses concerning the probable sexual advertisements they carried as part of their bodies. Ceratopsids in particular are regarded as the divas of dinosaurs because of their broad and ornate head shields, bedecked with horns, bosses, and knobs. These head shields may have provided protection against the occasionally encountered predators, but more likely they served an everyday use as easy visual-recognition cues within their respective species. Included in this recognition process must have been not only potential mate identification, but also intraspecies rivalry. Intraspecies competition expressed through combat has been long hypothesized for marginocephalians on the basis of similar behavior in mammalian males that have prominent headgear and aggressively compete for females in a herd (such as bighorn sheep, moose, and elephants). Furthermore, given the thick, bony skulls of pachycephalosaurs and cer-atopsians, and the prominent nasal and antorbital horns of some ceratopsians, these features certainly could have aided in such competition.
Pachycephalosaurs are perhaps the dinosaurs most often used as examples for intraspecies competition. Both popular and serious scientific publications alike show them butting heads with one another. Although this inferred behavior prompts their comparison to bighorn sheep, pachycephalosaurs weighed considerably more, as much as two tonnes, a fact that should be kept in mind during discussion of their behavior. Evidence in favor of head-butting includes:
1 thick skulls;
2 robust occipital condyles at the back of the skull, thus cushioning the skull in its articulation with the first cervical vertebra; and
3 tightly interlocking dorsal vertebrae.
The physics behind combat scenarios might help to illuminate what types of forces would have been involved in head butting.
The latter trait would have reinforced the back posterior to the neck and skull, preventing lateral movement of the vertebrae (which encased the spinal cord) during any forceful impact. However, other observations that would supplement these data, such as obvious dents in pachycephalosaur skulls or other signs of trauma, are still forthcoming or not clearly defined.
Using a hypothetical example of two male Pachycephalosaurus, each weighing about 1.5 tonnes (1500 kg), and the familiar formula for calculating force (F = ma: see Eqn 4.8
in Chapter 4), the force generated by a single pachycephalosaurid running (and accelerating) at 5 m/s2 would be considerable:
The impact of two objects of the same mass and at the same acceleration causes additive force, which would have generated 75,000 N of force in this example. Using the formula for stress (o = F/A: see Eqn 4.9 in Chapter 4) and assuming that the skulls impacted on a point-to-point contact in a small area, such as 100 cm2 (10 x 10 cm), the total stress generated would have been
Step 1. o = 15,000 N/0.1 m2 Step 2. = 1.5 x 105 N/m2
This is a frightening amount of stress, corresponding to about 15 N/cm2. For example, compare this figure to that derived for "foot stress" in Eqn 14.1, Chapter 14.
However, these are idealized calculations based on many assumptions. The speeds could have varied, the masses could have differed, the contacts may not have been applied at directly opposing directions and to such small areas, deceleration from the impact has been discounted, and so on. Calculating the kinetic energy of a running pachycephalosaur would derive a more realistic estimate. Kinetic energy is calculated by the following formula:
where Ke is kinetic energy, m is mass, and v is velocity in meters per second. For one of the pachycephalosaurids in our example, the kinetic energy would have been:
Step 1. Ke = 0.5(1500 kg)(5 m/s)2 Step 2. = 0.5 x 7500 kg x m/s2 Step 3. = 18,750 N
For one pachycephalosaurid to completely stop the other, the force must be absorbed over a given distance that varies with the material doing the absorbing. This relation is expressed by the formula:
where Ae is the energy absorbed and d is deceleration distance. To bring the pachy-cephalosaurid to a screeching halt, the calculated force needed is expressed by:
where Fa is the absorbed force. The safety airbag in an automobile can illustrate this principle: it absorbs a driver or passenger, which results in the person traveling a greater distance into the airbag than if he or she hit the steering wheel or windshield. This means that less force is transmitted to the person by impacting the airbag.
In our pachycephalosaurid example, assuming deceleration distances of 0.0015 meter (15 mm) for bone (after all, bone does not compress very easily) and 0.1 meter i
(10 cm) for flesh (think about the skull sinking into the side of the rival pachy-cephalosaurid instead of contacting the other skull), the absorbed forces would be:
Example 1 (bone)
Step 1. Fa = (1500 kg)(5 m/s)2/2(0.0015 m) Step 2. = 7500/0.003 Step 3. = 1.25 x 107N
Example 2 (flesh)
Step 1. Fa = (1500 kg)(5 m/s)2/2(0.1 m) Step 2. = 37,500/0.2 Step 3. = 1.87 x 105N
Consequently, an impact into flesh would have generated two orders of magnitude less force than a head-to-head collision. In other words, a head-to-flesh collision would have survival advantages for both animals, not just the one doing the ramming, and especially if ramming was a frequently expressed behavior. Of course, the deceleration distance would not have been only for the head but would have been passed down the entire length of the body, increasing the distance and correspondingly decreasing the force.
The point being made here is that when two massive animals run toward one another and their body parts directly impact, they could have less easily absorbed the force on bone than flesh. This sort of force applied to bone should have left marks, no matter how thick or spongy the bone. It also quite likely would have exceeded the structural limits of the dinosaurs' spinal columns for absorbing the impact behind the skull. Based on the realities represented by these calculations and the previously mentioned information, the likelihood that pachycephalosaurs actually rammed into one another head-to-head is doubtful. Furthermore, the selection pressures caused by simultaneously inflicted paralysis or death surely would have caused this behavior to quickly disappear. After all, dead or paralyzed animals cannot pass their genes on to a successive generation. Thus, rather than stating that pachycephalosaurs did not use their heads for defensive or pre-mating purposes at all, a good compromise hypothesis is that if any ramming happened it was directed to softer areas.
Similarly intriguing features in ceratopsid skulls are apparent healed wounds which offer evidence for intraspecific competition in ceratopsians. These wounds are visible as scars or defects that seem to have been applied to the skull while the animal was alive. One example in a Triceratops skull is a hole that passes through the jugal and has a diameter similar to that for the distal end of a Triceratops horn. Other ceratopsid genera reported with similar skull defects are Diceratops, Pentaceratops, and Torosaurus. The coincidence of such scars found in the skulls of ceratopsids that have substantial horns is currently considered as a reasonable basis for a cause-and-effect hypothesis. One statement that can be made about these possible trace fossils is that they are related to intraspecies combat, although what may have prompted the fighting in individual cases is still uncertain. In extant species, fighting may be triggered by competition for mates, establishing territory, asserting dominance, or illnesses, such as rabies, that cause paranoia or other aggressive behavior. The styles of intraspecific combat behavior also would have varied with the skull morphology of the ceratopsids. For example, large-frilled chasmosaurines, such as Chasmosaurus, may have only had to turn their heads, giving a rival a full frontal view for intimidation (see Fig. 13.9A). In contrast, the short-frilled centrosaurines, such as Centrosaurus or Styracosaurus, did not have such obvious tools of intimidation, so they may well have locked horns more often than their large-frilled relatives. Regardless, actual evidence supporting these generalized behaviors is still scanty and subject to further critical review.
As far as actual reproductive behavior is concerned, little is known about marginocephalians. Presumed ceratopsian eggs and nests, attributed to Proto-ceratops, were discovered in Late Cretaceous rocks of Mongolia in the 1920s by the American Museum of Natural History expeditions to that region (Chapter 4). However, later analyses revealed that at least some of the eggs belonged to the theropod Oviraptor (Chapter 9), thus rendering all other similarly identified "pro-toceratopsian" eggs and nests as suspect. Since these corrections were made, no undoubted ceratopsian nest or embryonic remains within an egg have been identified. However, 15 hatchlings of Protoceratops, found together in a deposit in Mongolia, is persuasive evidence favoring the proximity of a nest. Still, no ceratopsian eggs have been definitely linked with hatchlings. No eggs, embryos, or hatchlings of pachycephalosaurs have been recognized, although a few skulls of juvenile Stegoceras have been described. These limited data mean that the reproductive habits of marginocephalians as a clade are poorly understood.
Nevertheless, embryonic and other juvenile remains have been identified for Bagace-ratops, Breviceratops, Psittacosaurus, and Protoceratops, all coming from Cretaceous strata in Mongolia and China. Furthermore, a recent and spectacular find of one adult and 34 juvenile Psittacosaurus, in Lower Cretaceous rocks of China, provides the most convincing evidence of parental care in ceratopsians. The 35 dinosaur skeletons were all complete and concentrated in a bowl-shaped depression with an area of 0.25 m2. The juveniles were fairly grown (21 cm long) and the same size, strongly suggesting that they were from the same generation. This situation argues for a parent Psittacosaurus in close association with a large brood of its young at the time of their burial, the latter of which must have been nearly instantaneous to preserve them so well (Chapter 7).
Growth series have been described for a few ceratopsians, most notably Psittacosaurus and Protoceratops. Many specimens represent these Cretaceous ceratopsians from Mongolia and China (more than 100 just for Psittacosaurus) and most of the specimens are complete. As a result of this teeming abundance, growth series have been interpreted on size analyses of these ceratopsians, which reflect changes in ontogeny. Psittacosaurus was rather small as far as dinosaurs are concerned, reaching about 2 meters long as an adult; Protoceratops was not much larger, with some individuals reaching 2.5 meters long. Measured skull lengths of Protoceratops have a range of 5 to 50 cm, reflecting a minimal tenfold increase from juvenile to adult. In this growth series, the smaller forms tend to look alike, but a two-part split into large-frilled and small-frilled forms is apparent in skulls longer than about 25 cm. This divergence in forms within the growth series for this species is considered to be among the best-documented evidence for sexual dimorphism in dinosaurs. Presently, the large-frilled adult specimens are designated as "males" and the small-frilled ones as "females." Of course, the opposite may be true in these sex determinations, but more compelling evidence from the fossil record, such as a mother Protoceratops brooding its egg clutch, is still forthcoming. Sexual dimorphism has also been proposed for a few ceratopsids (Centrosaurus, Chasmosaurus, and Triceratops). The data sets for these genera are good, but are not as robust as for Protoceratops.
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