Everyone eats, everyone consumes. Everything consumes. Regardless of whether you’re a bacterium or a tree or a giant tyrannosaur, organisms consume parts of other things to produce energy to live. Sometimes, those “things” are other organisms. When organisms are sufficiently developed, when they are diverse in form and habitat, they also want to survive.
The arms race of eaters and eatens is an old, old one, one that predates teeth, skulls, bones, backbones, even brains and nerve clusters. It predates organs. It goes back to single-celled organisms. The arms race of survival means that those seeking to eat have to find ways to get around defenses of those who do not want to be eaten; those that can’t be “gotten around” will survive, which forces their potential consumers to seek other methods of eating them. This goes back and forth for over 400 million years.
But sometimes, to survive it means organisms have to look at particular foods. There’s a lot of them, but they can increase their chances of survival by picking at a relatively under-consumed resource. Say, there’s a particular animal that is fairly toxic, but you can survive the toxin whereas your neighbors tend not to. That toxic organism can more readily be exploited by something, and so it does. Toxin-immune or -tolerant species are one specialization, as are heat-tolerant or cold-tolerant organisms. The absolute masters of tolerance are tardigrades, known to survive in below freezing, beyond boiling, and even utter vacuum and the intolerant radiation of space. But that’s survival. That’s one end of the arms race. Armored animals, toxic animals, fast animals; these things find ways to escape being consumed.
What happens to their consumers?
Diet in organisms varies widely, as do adaptations for feeding in these groups. When you specialize on a food, you bear features of that diet on your body. Humans well adapted to exploiting a high-vegetation, low-meat diet show considerably more wear on their molars than those who do the opposite, as a lot of oral processing is required to take these foods. Grit is often consumed when eating plants, and that grit helps wear teeth down. A high-meat diet means high muscle mass of the jaw adductors – you’re sawing through fibers constantly, building up bone stress that reacts by building up strong bone and muscle. Native peoples of the Arctic regions who primarily hunt for food, eating seal, whale, and little vegetation, show considerably stronger jaws than moderate diets of meat and vegetation. This is one way to tell, at least in humans, about diet.
Living animals that specialize on types of foods tend to develop features of their body that indicate this diet. Names for these diets are evocative, ending in –phagy and –vory, both coming from words meaning “to consume.”
What comes after is a series of posts that will tackle these –vorys, these –phagys, and seek to determine if we can certainly say anything about fossil animals as we can through observations of living ones. We have just the specimens left to us, but that’s enough.
We start with defining “diet.”
“Diet” for fossil animals can be defined using four criteria: Habitat, Ecology, Association, and Morphology (HEAM). HEAM in extant organisms is mostly about all the first stuff, little of the second stuff. We find, for example, that in some specialist consumers such as many egg-eating snakes that specializations for a type of food doesn’t actually restrict access to other foods. Ultimately, all animals are opportunists; they will eat what they can when they need to. The hypothesis here is that we can use the latter two factors (association and morphology) to infer the former two (habitat, ecology).
This is about where the animal exists: What foods are available, seasonality, and tendency of acquiring a given food item. Animals that feed on deciduous plants cannot consume them for certain times of the year, so deciduousness in plants means habitat changes for an animal; it also means resource depletion that occurs forces migration. If the environment is particularly fecund, and the resource needed doesn’t tend to go away, then the habitat will support this organism continuously. Ultimately, this is the hardest to infer, the best to know. It’s our baseline, our clearest understanding of how associations of organisms work. Animals present in the region must eat within it; some may be transitory, just flying through, but others are permanent residents. Burrows mean dwellers of burrows; nests mean builders of nests, and not just as they also mean that hatchlings must be fed. The egg mounds of the Two Medicine Formation and of Ukhaa Tolgod tell us about habitat, as does the structure of the formations themselves: braided river systems, sand dunes, paleosols of islands of earth or oases, etc. Where there is water, there is something that drinks, or lives in it. Trees means scurriers; bushes mean shelter.
The trophic web is an intricate association of foods: a series of eatens culminating in a fewer eaters. Many trophic webs terminate at their peak at a single ultra-consumer, and it in turn is the consumer of most things underneath it. The archpredator. Sometimes, other, smaller, predators are food for the archpredator. This is especially true in pelagic ecosystems, less so in terrestrial ones. As the saying goes: “There’s always a bigger fish.” Once a lion reaches a certain size, it will tend to be the eater of eaters, and never be eaten. Tend. Sometimes hyenas eat lions. A gap in the trophic web means there should be something there: A lot of small-bodied, seemingly similar-sized predatory or herbivorous reptiles implies the presence of a larger, singular predator that eats them, whereas a archpredator and a lot of plants implies something in between, an eater of plants and an eaten of predators. Examining these gaps means we can infer the presence of the missing element.
If you are found near a piece of food, chances are you are or were going to eat it. If it’s in your body – well, there’s no question. What does that say, though? You ate something. It’s pretty certain you did. But is it all you’d ever eat? Can I conclude from this that this is the sum total of your potential food resource consumption? Association of two organisms implies interaction, and the closer or most nested of these interactions implies method of interaction. Association, as much as anything else, implies behavior, so a fish in the gut of a pterosaur, or a bird in the gut of a dromaeosaur, means the one was eaten by the other.
If you eat something, you must have a method to do so. Egg-eating snakes have methods of opening those eggs: a series of spikes inside their throat, or a pair of special teeth, that slice open the shell. Lizards like gila monsters and tegus bite open the shells. If your food is elusive, you must be able to find it, so you have morphology to get it: kiwis have specialized sense organs in their nose that help them find food under forest-floor litter; ducks have similar for plying murky waters; yet still, crocs use them to sense when food enters their water and the method by which they swim. Sharks smell blood, and sense disturbed behavior: something is flailing … something is dying. So morphology hints at process, the final act of consumption, how an animal puts food into its body. Ultimately, though, morphology is best inferred from examining living animals. Back-checking living animals that do “a thing” to eat allows us to hypothesize something we cannot observe eating how it ate. A fish with broad, relatively short-toothed jaws probably swallowed prey, so if we find another animal with broad, relatively short-toothed jaws, we wouldn’t be so wrong in implying it swallowed prey. Not all morphological inferences need be drawn from the front end of the animal. The size of one’s rear end relates to food, too. Herbivores consume plants; for many, this means fermentation. If the size of one’s gut is small but the front end implies plants, then food is passed relatively quickly; generally, this means leaner vegetarians, more generalist feeders, who can crop from some tough but mostly softer foods and will pass this food fairly fast, meaning lots of feeding continuously. But large guts means fermentation, and that means you can harvest tougher, woodier foods and spend time processing that food without constantly consuming more food. Such animals will have extra organs in their guts for processing this food: caeca in the intestines, diverticulae of the esophagus called crops, or even extra chambers in the stomach. Sometimes you’re so efficient you have two of these (I don’t think any animal has all three).
No single one of these factors allows us to determine the diet of an organism; it is the knowing of all of them that does. We understand the environment in which the animal lives, its place in the trophic web, the organisms it regularly encounters, and the means by which it can procure food – all these relate directly into its diet. Our inference of the diet of an animal grows stronger the more of these things we employ, rather than our knowledge of merely one or two. These things, HEAM, equal “diet.”
We will start with the beginning of my discussion on one type of -vore, the piscivore (alternately, the ichthyophage).