Thursday 22 December 2022

Microraptor zhaoianus: The stomach contents of a small Dromaeosaurid Dinosaur, and the implications for this on its diet and ecology.

Determining the diets of extinct Animals is remarkably problematic, and can generally only be accomplished where clearly identifiable feeding traces or stomach contents can be found. Even where Animals thought likely to be predatory are found in close proximity to those thought likely to be their prey, it is impossible to state with any confidence that this reflects the behaviour of the living Animals, and is not a postmortem association. Stomach contents are only very rarely preserved, and then only at sites with exceptional fossil preservation, meaning that most known examples come from a very small number of locations. Furthermore, the stomach contents of an Animal can usually only tell us what it has consumed, and provides little information about its wider behaviour. We cannot, for example assume that an Animal directly hunted another species based upon its stomach contents, as almost all carnivorous Animals will consume carrion given the opportunity.

Theropod Dinosaurs are generally assumed to have hunted prey smaller than themselves. This is true of most modern Mammals, Birds and Crocodilians. Other than Mustelids, which regularly hunt prey larger than themselves, predatory Mammals generally hunt large prey only when in packs, so that the total mass of the predators outstrips that of the prey, even if the individuals do not. Large Theropods are likely to have mostly hunted other Dinosaurs, as these would have represented most of the large prey available in their environments. Small Theropods, on the other hand, are likely to have had a much wider range of suitably sized prey, including even smaller Dinosaurs, Lizards, small Amphibians, small Mammals, Arthropods, and Molluscs, and are likely to have fed on all of these. 

Very few Dinosaurs have more than one specimen known with preserved stomach contents, giving us a very limited conception of their diet. However, the small Early Cretaceous Dromaeosaurid Microraptor has four known individuals with preserved stomach contents, providing the opportunity for an analysis of its diet not really possible for any other Dinosaur. Individuals of Microraptor have previously been shown to have consumed a Bird, a Lizard, and a Fish.

In a paper published in the Journal of Vertebrate Paleontology on 20 December 2022, David Hone of the School of Biological and Behavioural Sciences at Queen Mary University of London, Alexander Decechhi of the Division of Natural Sciences at Mount Marty College, Corwin Sullivan of the Department of Biological Sciences at the University of Alberta, and the Philip J. Currie Dinosaur Museum, Xu Xing of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and Hans Larsson of the Redpath Museum at McGill University, describe the stomach contents of the fourth specimen of Microraptor in which this is known, the holotype of Microraptor zhaoianus, and discuss the implications of this for our understanding of the diet of the species.

The specimen, IVPP V 12330, is a compression fossil broken into a number of fragments, with most of the part and counterpart present, showing the incomplete but articulated skeleton of the small Dinosaur. The abdominal and thoracic cavities show no signs of having been ruptured, and, visible within the rib-cage of the Animal, is the articulated pes (foot) of a small Mammal. This visibly overlies the ribs of the left side of the thoracic cavity, and is in turn overlain by the ribs of the right side. The visible part of the foot includes ll tarsals and metatarsals and most phalanges,including unguals of digits I and probably III. A number of long bones are also present, beneath and beside the pes, which hint that more of the Mammal might be present but obscured.

Holotype specimen of Microraptor zhaoianus (IVPP V12330) with Mammal foot gut contents. (A) Entire specimen. Box inset indicates the location of (B) and (C). (B) Close-up view of Mammal foot. (C) Illustration of visible bones: dark gray elements are Microtaptor ribs, yellow bones are the articulated Mammalian foot and light gray are unidentified bones. Note the juxtaposition of the foot over the inside of the left ribs and the overlap of the right ribs over the foot, particularly over digits II and III. Abbreviations: ast, astragalus; cal, calcaneum; mtI, metatarsal 1; nav, navicular; lr, left rib; rr, right rib. Scale bar in (A) equals 100 mm and in (B) and (C) equals 5 mm. Hone et al. (2022).
The phalanges of the foot are slender, resembling those of the Early Cretaceous Mammals Eomaia and Sinodelphys, both of which come from the same deposits as Microraptor. However, they are neither as elongate nor as curved as seen in these species, suggesting that this is another, unknown, Mammal, and that it was a primarily ground dwelling species (Eomaia and Sinodelphys are both thought to have been arboreal in nature).

The first digit is 8.1 mm in length excluding the ungual (claw), or 9 mm in length if the ungual is included. This is roughly equivalent to those of Sinodelphys, Yanoconodon, and Eomaia, and may therefore indicate an Animal of similar size, although if this Mammal did have a different ecological role, then the possible size bracket can be expanded somewhat. Nevertheless, Hone et al. estimate a size in the range 13-43 g (the range that includes most modern Mice, Voles, and Shrews).

Numerous examples of Microraptor have been recovered from the beds which make up the Jehol Biota, and many studies have been done on this Dinosaur's ecology. Nevertheless, considerable controversy remains about the lifestyle of Microraptor, with different researchers having suggested that it might be nocturnal, diurnal, capable of gliding flight, capable of powered flight, arboreal in habit, and terrestrial in habit. What is generally accepted is that Microraptor was capable of some-sort of areal locomotion, and probably able to climb reasonably well (although there is considerable doubt as to whether it was capable of living in arboreal environments with small-diameter branches, an environment which the only non-Avian Theropods thought likely to have colonised are the Scansoriopterygians. 

Over 300 specimens of Microraptor are known, which have been divided into three separate species, of different sizes and with slightly different anatomical traits, although it is still possible that this represents different growth stages and variability within a single species. Whatever the taxonomic implications, this physical variation probably represents a degree of ecological difference between the morphotypes, which may indicate differences in diet. The four specimens of Microraptor for which stomach contents are known belong to two different species, Microraptor zhaoianus, of which a previous specimen was shown to have consumed a Lizard, while Hone et al.'s study shows consumption of a small, terrestrial, Mammal, and Microraptor gui, in which two specimens have been shown to have consumed a Fish, and a small Bird, respectively. This may, therefore, represent two different species with slightly different dietary habits, but is also likely just to represent a degree of flexibility in feeding habits.

Most modern predators take food items considerably smaller than themselves, and this is unlikely to have been different in the Early Cretaceous. With the exception of Mustelids, modern Mammals of 21 kg or less typically target prey no more than 40% of their own size. This rule appears to be applicable to Microraptor, with all four specimens with stomach contents having consumed Animals significantly smaller themselves. The Bird known from the stomach contents of a specimen of Microraptor gui had an ulna length of 10.5 mm, while the ulna of the Dinosaur that had eaten it was 80 mm long. The Lizard eaten by a specimen of Microraptor zhaoianus had femur 13.4 mm long, while its consumer's was 75 mm in length. The Fish appears to have been of a similar proportion to the Dinosaur which ate it, based upon rib size, while Hone et al. estimate that the small Mammal from their study was about 10% of the size of the Microraptor.

Any consideration of the diet of an Animal should also take into account its jaw morphology. Typically, Vertebrates with shorter, more robust jaws tend to consume larger prey, which needs to be subdued and processed before consumption, whereas those with longer jaws tend to consume smaller prey, with an emphasis on shovelling down as many prey items as quickly as possible. 

This principle has previously been applied to the Eudromaeosauria, with the group found to split conveniently into three clusters, species with short deep jaws, such as Deinonychus and Atrociraptor, species with intermediate-lengthed jaws, such as AchillobatorSaurornitholestes, Bambiraptor, and Acheroraptor, and long-snouted forms such as Linheraptor, Tsaagan, and Velociraptor. Interestingly, all of the species known to have short jaws are from North America, as are most of those with intermediate jaws, while most of the long-jawed species are Asian, although, with the jaws of many species unknown or known only from fragmentary remains, it is unclear how significant this is.

While many specimens of Microraptor are known, it has proven hard to fit it into this matrix, as in the majority of specimens both maxillae are absent, unexposed, damaged, or difficult to fully demarcate from adjacent bones. However, two specimens, BMNHC PH881 and IVPP V 13475, both identified as Microraptor sp., have maxillae which appear to fit into the intermediate group, possible suggesting prey fairly easy to seize, but requiring some force to subdue. Since Microraptor is itself a small Animal, probably massing under 1 kg, Mouse-sized Mammals and similar small Vertebrates would seem to fit into this category, while most Insects would be too small.

The Mammal consumed by Microraptor IVPP V 12330 is thought to have been ground dwelling, and about the right size for the presumed prey of Microraptor, which is itself also thought to have been ground dwelling. This would seem, at first sight, to be fairly good evidence for Microraptor having predated this Mammal. However, Hone et al. caution against making such an assumption, observing that almost all carnivorous Animals will consume carrion if it is available, rather than hunting, and that, therefore, an Animal having consumed another Animal cannot be taken as direct evidence of a predator-prey relationship.

Hone et al. also not that, while Dromaeosaurs are generally assumed to have been capable of swallowing quite large items, all of the known stomach contents of Microraptor have been quite small, in this case the foot only of a Mammal, which is a low-nutrition part of the body, and generally among the last parts consumed. This makes it quite conceivable that the majority of the Mammal might have been consumed by another Animal, with the Microraptor, subsequently consuming an overlooked portion of the carcass. 

It has previously been suggested that Microraptor might have been capable of actively hunting small Birds in the treetops, based upon the presence of a partial wing and both feet of an Enantiornithine Bird within the body cavity of another specimen. However, these are again low nutrition parts of the body, likely to have been left by another Animal eating the majority of the Bird. What remains of the wing, and the feet, are themselves articulated, appears to suggest that Microraptor was incapable of any further processing of its food; either tearing these pieces of a carcass then swallowing them, or finding them left behind after the majority of the Bird has been eaten by something else. Whichever is the case, the specimen does not seem to be, in itself, sufficient evidence to claim Microraptor was able to hunt small Birds. Even if the Bird could be assumed to have been killed and then consumed by the Dinosaur, this does not indicate that the predation event took place above the ground. Even the most arboreal of modern Birds spends a considerable amount of time on the ground, and many modern predators, such as Hyenas, Cheetahs, and Foxes, hunt them there.

The presence of a Fish in within the body cavity of Microraptor specimen has also been taken as evidence of direct predation on Fish, within an aquatic environment, although in that case scavenging was not ruled out. 

Hone et al. note that the suggestion that Microraptor was capable of hunting both in the treetops and in the water as highly dubious, noting that modern Animals which can do this tend to be very specialist predators, something that there is no evidence for in the case of Microraptor. They caution that such stories are attractive, but are not supported by the evidence, and that the presence of a wide range of consumed Animals within the stomach contents of a species is probably more indicative of it being a generalist scavenger that a specialist hunter.

The range of Animals now known to have been consumed by Microraptor is wider than for any other known non-Avian Theropod, although this is clearly related to the fact that more specimens with stomach contents are known than for any other Dinosaur. However, the assumption that Microraptor, whilst probably capable of hunting, would consume a wide range of Animals opportunistically, is probably also true for most Theropods. Furthermore, most Theropods are thought likely to have changed the Animals they consumed quite significantly over their long growth-cycles, and it is likely that members of the same species in different regions would have consumed different food items, and that individuals would have consumed different items at different times of year, all of which would have favoured a more generalist approach to feeding. Insects and other Invertebrates are likely to have made up a proportion of the diet of the smallest Theropods. These have never been found in any stomach contents, but this is probably reflective of their low preservation potential rather than their not being consumed.

Specimen IVPP V 12330, the holotype of Microraptor zhaoianus, provides direct evidence of the direct consumption of a Mammal by Microraptor, only the second time this has been demonstrated for a non-Avian Dinosaur. However, this does not provide evidence of a direct predation event, but rather adds to the mounting evidence that Microraptor was a generalist carnivore, largely eating small Vertebrates, but not necessarily hunting them.

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