Repenomamus spp. vs Psittacosaurus lujiatunensis

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Repenomamus is a gobiconodontid mammal genus containing two species, Repenomamus robustus and Repenomamus giganticus. Both species are known from fossils found in China that date to the early Cretaceous period, about 125-123.2 million years ago. R. robustus is one of several Mesozoic mammals for which there is good evidence that it fed on vertebrates, including dinosaurs, though it is not possible to determine if it actively hunted live dinosaurs or scavenged dead ones. R. robustus was described by Li, Wang, Wang and Li in 2000, and R. giganticus was described by Hu, Meng, Wang and Li in 2005. Adults of R. robustus were the size of a Virginia opossum with an estimated mass of 4–6 kg (8.8–13 lb). Features of its shoulder and legs bones indicate a sprawling posture as in most of small to medium sized living therian mammals, with plantigrade feet. Unlike therian mammals, Repenomamus had proportionally longer bodies with shorter limbs. 
Weapon(s): Powerful jaws filled with teeth 
vs

Weapon(s): Claws, beak, horns

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This thread was moved to sympatric from hypothetical since these two animals actually lived with each other. For this reason I decided to keep it, and will post some thoughts on it later.

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First and foremost, here's a size comparison of P. lujiatunensis and R. giganticus (the larger of the two species of Repenomamus) (this-> is the artist).




Psittacosaurus holds a clear size advantage. While weight estimates vary, I have seen figures up to nearly 40 kg for this particular species. Looking at the size of Psittacosaurus in the size comparison above, weights of 37-38 kg don't seem too unlikely to me.

Zhao et al. (2013)

Femur length comparisons informed by histology suggest larger adult body size in P. mongoliensis, but inferred growth curves for the two species show a considerably higher mass for P. lujiatunensis than for P. mongoliensis (38 kg in the former versus 25 kg in the latter). There are several possible solutions to this paradox, including the fact that the mass estimates used to construct the growth curves were based on circumference rather than length measurements and could represent a difference in robustness between the two species, but distinguishing among the various possibilities will require an analysis directly comparing the two species in terms of both histology and skeletal proportions. We note, however, that both logistic growth curves extrapolate well beyond the available histological data, and we therefore prefer the direct histological and meristic evidence suggesting that P. mongoliensis grew to a larger final size than P. lujiatunensis. The greater inferred adult femur length of P. mongoliensis presumably indicates that the body mass of mature individuals was also greater in this species than in P. lujiatunensis, unless the two taxa were strikingly different in their proportions.

Erickson et al. (2009)

We were recently afforded the opportunity to study the age distribution for a large sample (N = 80) of the basal ceratopsian dinosaur Psittacosaurus lujiatunensis that is suitable for life table analysis. The animals derive from the Lujiatun Bed of the Lower Cretaceous Yixian Formation, Liaoning Province of China (He et al., 2006). They appear to have perished simultaneously in a volcanic mudflow (lahar; He et al., 2006) that trapped individuals irrespective of size (Fig. 2). Consequently, representatives throughout development were preserved, and presumably in proportions equivalent to the population's age structure. (The skewedness to the distribution is consistent with this assumption [see results].) These specimens provided the unique possibility to explore variation in life history patterns among nonavian dinosaurs by contrasting survivorship patterns for P. lujiatunensis with those in North American tyrannosaurs (Erickson et al., 2006), the only other nonavian dinosaurs for which life tables have been rigorously reconstructed. (Lehman [ 2007] studied the population biology of the ceratopsian Chasmosaurus mariscalaensis from skeletal material, and Lockley [ 1994] that of sauropods from trackways. Neither definitively established specimen longevity or met assumptions for life table construction.) Because these dinosaurs differ substantially with regard to phylogeny (Ornithischia versus Saurischia), maximum size [37 kg versus 5,600 kg (Tyrannosaurus; Erickson et al., 2004)], trophic ecology [herbivorous versus carnivorous] and presumably other life history related parameters (e.g., longevity, growth rates, developmental timing, neonate and clutch size, predation pressure, behavior, etc.), it seemed likely that this comparison would reveal differences in life history pattern among nonavian dinosaurs, should they exist. Explicit questions we sought to address with the P. lujiatunensis life table and growth curve include: (1) What was the age structure, growth pattern, and lifespan for this taxon? (2) Did it show Type B1 survivorship, like tyrannosaurs and most comparable-sized birds (avian dinosaurs) and mammals? (3) What were the reproductive dynamics (e.g., age at sexual maturity, reproductive lifespan and population size, generation time, etc.) for this taxon?

Repenomamus giganticus weighed some 12-14 kg (Prothero, 2017). This would be congruent with the claim that adult Psittacosaurus would be twice the size of Repenomamus (Lomax & Nicholls, 2021).

Apart from size, I also think Psittacosaurus had a number of physical defenses to its disposal. First, the jugal horns were pretty large and look really formidable, based on the now well-known Psittacosaurus sp. specimen that exquisitely preserves soft tissue (SI info of Vinther et al. (2016)). It could try striking/stabbing the Repenomamus with these jugal horns.



Second, the reconstruction above also seems to suggest the pedal unguals (foot claws) were more pointed and claw-like than the hoof-like nails of more derived quadrupedal ceratopsians. Lo and behold, the pedal unguals of P. sibiricus are described as claw-like (Averianov et al., 2010). So while they weren't specialized weapons, their claw-like, pointed shape could still be damaging when used to kick like a modern ground bird (it's worth noting that in ground-dwelling birds, the claws don't need to be actual specialized weapons to be used for kicking; everything from emus to cassowaries will kick, and even emus, with plain feet, can potentially cause serious injury with their foot claws).

The pedal phalangeal formula is 2-3-4-5-0. The preungual phalanges are dorsoventrally flattened and gradually decrease in size distally. The ligament pits are weak and dorsally exposed. The unguals are claw-like, long, narrow and slightly dorsoventrally flattened. The claw grooves are well developed, in unguals of the second and fourth digits they are asymmetrical: the groove is distinctly longer on the side adjacent to the third digit.

Third, the forelimbs of Psittacosaurus may have had some use in grabbing and carrying objects (like vegetation) so long as they were underneath its belly, which Repenomamus conveniently is (although, at least based on this analysis that does not account for articular cartilage, it probably could not bring things to its mouth) (Senter, 2007). Available manual unguals (finger claws) are also described as pointed rather than hoof-like (Zhao et al., 2013). So I think it's likely that Psittacosaurus could literally grab a Repenomamus trying to attack it from underneath.

The limited reach of the forelimb of Psittacosaurus reveals that (1) it could have scratched only its belly, posterior flank or knee with its hands, (2) it could have swatted or acquired an object only if that object were close to the sides or beneath the belly, (3) it could not have passed an object any further forward than the glenoid (e.g. the animal could not reach its own mouth with its hands). Sereno (1990) suggested that Psittacosaurus used its hands to acquire vegetation. Given the limits of forelimb mobility in Psittacosaurus, it could have done so only with vegetation that was very close to its sides or belly. It could not have used the forelimbs to pass such vegetation to the mouth, but it could have carried the vegetation, for example to deposit elsewhere for a later meal or for nesting material.

Fourth, Psittacosaurus had a powerfully built skull that could bite hard. The beak had a rounded tip instead of a hooked and pointed one, limiting the kind of damage it could do, but it could probably still be damaging based on sheer force. When the upper and lower beak were aligned, the beak would have a shearing and cropping function (primarily for vegetation, but maybe it could cut through the skin and flesh of a mammal less than half its weight?). When the lower beak was retracted such that it was inside the upper beak, it would take on a crushing function (so maybe it could crack the bones of a much smaller animal if it tried) (Sereno et al., 2007).

Upper and lower beaks in psittacosaurs have a semicircular contour of nearly equal size, judging from their supporting bones, the predentary and rostral (figure 4). Neither beak tapers to a recurved median tip as occurs in all other ceratopsians and in most birds including psittaciforms, where the form of the bill is reflected by the underlying bone (figure 7c). The psittacosaur upper and lower beak, which extends onto the premaxilla and dentary from the rostral and predentary, respectively (figure 8b), appears to have had a cropping function when aligned. When the lower beak was retracted and directed inside the upper beak, on the other hand, it appears to have had a shelling or nutcracking function (figure 4c,d).

Lastly, Psittacosaurus had stupidly thick, tough skin for an animal of its size. It is 10-15 mm thick (Lingham-Soliar, 2008).

The entire cross section lies adjacent to surface integumental structures and extends from a height of approximately 10–15 mm (figure 2a,b: areas 1 and 2, respectively) and width of approximately 35–40 mm.

To put this into perspective, a freaking elephant has skin 10-15 mm thick on the sides and back (Shadwick et al., 1992).

According to Harkness & Harkness (1965), elephant skin thickness varies from about 10 mm to 15 mm on the sides and back. This is not substantially different from the thickness predicted by the scaling model of figure 2, based on a body mass of 5000—6000 kg (Owen-Smith 1988).

The fact that Psittacosaurus had skin comparable in thickness to that of an elephant, an animal literally two orders of magnitude larger than itself (and often touted for having tough, thick skin, although elephant skin is actually about as thick as expected for an animal of its size) is utterly astounding. Furthermore, it contained numerous layers of collagen fibers arranged in a way that made the skin very tough. This is thought to be an adaptation against predators, similar to the nurse shark's highly thickened and reinforced skin (Lingham-Soliar, 2008).


Now, what about Repenomamus? Gobiconodontids were carnivorous Mesozoic mammals, and the robust jaws and teeth of taxa like Gobiconodon and Repenomamus could be used to deliver a powerful bite, perhaps even allowed for some degree of bone crushing. The procumbent incisors were designed for piercing or grasping. Gobiconodon was described as having a "bearlike build" (link->), so these mammals were also powerfully built predators too.

Where known, the skull of eutriconodontans is large relative to the body, and the skeleton is robustly built (figure 7.1). Specializations in various lineages include great foreshortening of the jaw and increased depth of the symphysis in Klamelia (Chow and Rich, 1984a), increased posterior jaw depth in Triconodontidae and Gobiconodontidae; robust, procumbent incisors designed for piercing (Gobiconodontidae, Jenkins and Schaff, 1988) or grasping (Cifelli and Madsen, 1998); and remarkable molar interlocking patterns (Patterson, 1951; Cifelli et al., 1998: figure 2).

The larger eutriconodontans were probably carnivorous and therefore occupied the highest trophic levels, among mammals, in their respective communities. Speculating further, we concur with the suggestion of Jenkins and Schaff (1988) that large-bodied taxa such as Gobiconodon (and Repenomamus) may well have relied, in part, on scavenging: their enormously robust jaws and dentition presumably conferred some bonecrushing ability (e.g., Werdelin, 1989) and may therefore have provided access to nutrients that were largely unavailable to other Mesozoic mammals.

Overall, I think an adult Psittacosaurus, with its numerous potential (co-opted) weapons, thick skin, and >2-fold greater body mass would be able to best a Repenomamus most of the time. In fact, it appears Psittacosaurus' growth patterns suggest that upon reaching a certain age/size, predation pressure was greatly diminished (Erickson et al., 2009), consistent with the idea that an adult Psittacosaurus would probably be out of reach for Repenomamus for the most part (although, the part that says that there were no predators that could indisputably prey on adults is by now outdated; Yutyrannus could easily prey upon any Psittacosaurus).

A second notable result of our analysis is that these animals appear to have reached threshold sizes somewhere between 3 and 4 years old (Figs. 4 and 5), as these animals were entering the transition to the exponential stage of growth (sensu Sussman, 1964) when they would have explosively increased in body mass (Fig. 6). In extant birds and mammals with Type B1 survivorship, this threshold typically corresponds to diminished predation pressure (even in large predators like the cheetah, Acinonyx; Kelly et al., 1998) owing to being more mobile, experienced, and/or large enough to deter or evade predators. Deaths of these somewhat larger, older individuals occur at much lower rates and are primarily from larger predators and/or other causes such as accidents or disease. Such deaths appear to have been a rarity in the Lujiatun P. lujiatunensis population, despite individuals being initially still quite small (less than 5-kg). Known predator-scavengers in the Yixian Formation include small theropods such as the dromaeosaurids Microraptor and Sinornithosaurus and the troodontids Mei and Sinovenator (Norell and Xu, 2005), moderate-sized mammals such as Repenomamus giganteus and R. robustus (Hu et al., 2005), and larger tyrannosauroids like Dilong and compsognathids like Huaxiagnathus (Norell and Xu, 2005), some of which could plausibly have preyed upon P. lujiatunensis at younger growth stages. Large macropredators that would incontrovertibly be capable of preying upon the largest known P. lujiatunensis individuals are so far not documented from the Yixian Formation, although this may reflect taphonomic biases toward preservation of small animals rather than true absence of such forms. Either way the survivorship curve suggests that during the exponential stage of development predation must have been relatively rare. It is perhaps worth noting that approximately 3% of the biomass was continually available in the first three size classes (Fig. 7), and it is from these groups that supposed crêche aggregations have also been found (Meng et al., 2004; Zhao et al., 2007; Table 1-LPM R 00142). It is plausible that predators fixated on these more available and vulnerable prey whose clumped nature may have facilitated such behavior. The discovery of a specimen of the triconodont mammal Repenomamus with a neonate Psittacosaurus individual among its stomach contents (Hu et al., 2005) and small theropod stomach contents showing even smaller prey items such as lizards and mammals (Currie and Chen, 2001) are consistent with this hypothesis.

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^Very informative post!

This is a Psittacosaurus lujiatunensis skeleton:


I actually think the Psittacosaurus could deliver powerful kicks. Its ilium looks well-developed, comparable to a similar sized terror bird like Andalgalornis, not to mention its claws would have been even larger and sharper in life due to the keratin sheathes. Maybe it could hold the Repenomamus in place with its forelimbs as well. Biting is certainly a viable strategy, though I consider the horns to be kind of awkward to use as offensive weapons.

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I mean all it needs to do is swing its head sideways to bring those horns to bear.

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Feb 28, 2023 3:19:21 GMT 5 Infinity Blade said:

I mean all it needs to do is swing its head sideways to bring those horns to bear.

Between its weaponry, extremely thick skin, and size advantage I now think this is a mismatch in favor of the Psittacosaurus. It would probably win with some damage from the Repenomamus’ jaws.

[Supercommunist]

At first this thread was of no interest to me but that psittacosaurus skin info is crazy. If that thing were alive today it would probably have feats of durability that would put the honey badger to shame.

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YO.


An extraordinary fossil captures the struggle for existence during the Mesozoic

Dinosaurs and mammals have coexisted for the last ~ 230 million years. Both groups arose during the Late Triassic and diversified throughout the Mesozoic and into the Cenozoic (the latter in the form of birds). Although they undoubtedly interacted in many ways, direct fossil evidence for their interaction is rare. Here we report a new fossil find from the Lujiatun Member of the Lower Cretaceous Yixian Formation of China, showing a gobiconodontid mammal and psittacosaurid dinosaur locked in mortal combat. We entertain various hypothesized explanations for this association, but the balance of the evidence suggests that it represents a predation attempt on the part of the smaller mammal, suddenly interrupted by, and preserved within, a lahar-type volcanic debris flow. Mesozoic mammals are usually depicted as having lived in the shadows of their larger dinosaurian contemporaries, but this new fossil convincingly demonstrates that mammals could pose a threat even to near fully-grown dinosaurs. The Yixian Formation—and the Chinese fossil Jehol Biota more broadly—have played a particularly important role in revealing the diversity of small-bodied dinosaurs and other fauna. We anticipate that the volcanically derived obrution deposits specific to the Lujiatun Member will likewise continue to yield evidence for biotic interactions otherwise unknown from the rest of the fossil record.

www.nature.com/articles/s41598-023-37545-8

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After a few days of thinking about this fossil, hearing points I haven't thought of or the authors don't address, I have become increasingly more convinced of the likelihood of scavenging in this recent fossil. If it was a predation event, I think the Psittacosaurus was in poor condition, as implied by the paper (and as one of the authors told me via email).

[Supercommunist]

Some people have also pointed out it's weird that its claws seemed to have been driven all the way through it's muscle. If the psittacosaurus was rotting that would explain how it's claws were able to penetrate so deeply.