Ancestrally, all animals fed in water - a dense, viscous medium. In some cases, the methods they employed transferred onto land easily. Others could only occur underwater.
Suspension feeding: The filtration of particulate matter or very small prey from the water. This is the topic of an upcoming lecture.
Appendicular feeding: Ancestrally arthropods and their kin captured prey with their limbs and stuffed it into their mouths. Although the tagmosis of arthropod groups and the homology of these limbs differs, all retain this basic pattern in that their paired mouthparts are modified limbs.
In a variation of this, nektonic cephalopods grapple onto their prey with their arms (lined with hooks or suckers) then dismember it with their mouths.
- Gnathobase feeding in horseshoe crabs and trilobites
- Use of dedicated limbs such as the chelicerae of chelicerates.
The feeding apparatus of the ancestral gnathostome relied on the scissor-like action of the upper and lower jaws to bite and hold prey. However, swimming up to a prey item in the water and taking it into the mouth is difficult because the streamlining of currents around the predator's head tends to sweep prey away from its mouth. Ways around this problem include:
- Suction feeding: Most nektonic engulf prey by sucking both it and the water around it into their mouth and pharynx, then expelling the water and swallowing or processing the prey. This process requires that:
- the aperture of the mouth be restricted to enable incurrent water to reach a higher velocity. (Remember the significance of aperture restriction in recoil locomotion.) Of course, the prey must still fit in.
- it be possible rapidly to increase the volume of the pharynx, to suck the water in.
NOTE: Many suction feeders have teeth. These are to hold the prey in place after it has been sucked in. In some, the jaws hold prey while tongue or phayrngeal teeth disassemble it.
- In ray-finned fish, the pharynx is expanded by the rapid abduction of the gill covers and opening of the jaws. The shape of the mouth is modified by the movement of two pairs of hinged bones.
- In other aquatic gnathostomes, including lungfish and aquatic tetrapods, the pharynx is usually expanded by the rapid depression of the hyoid skeleton, which supports the throat and tongue.
- Use of a long rostrum: Prey can be attacked using a long snout that is swept rapidly back and forth. Variations include:
- Elongate jaws lined with uniform teeth, in which prey is grasped. (E.G.: gar.)
- A long snout with which prey is disabled before being picked up in the jaws. (E.G.: sailfish.)
Biting: In some circumstances, creatures get away with using the ancestral scissor-like bite.
- Carnivory: If your prey is big enough to have sufficient inertia that it will not just float away when you try to bite it, and you have sufficiently sharp teeth and powerful jaw muscles, then taking bites out of your prey without engulfing it is an option. (E.G.: cookie-cutter shark. See also the wounds it inflicts.)
- Herbivory: If your "prey" is vegetation anchored to the substrate, yo don't have to worry about it floating away. Marine herbivores typically are adapted for cropping algae close to the substrate, and tend to have teeth that form sharp blades and are frequently replaced. (E.G. parrotfish.)
- Durophagy: The ability to process hard-shelled prey items. These typically are easy enough to get into the mouth, but require significant bite forces to crack. In such cases teeth are bulbous or flat, in order that bite forces not be focused at the tips, causing fracture. (E.G. stingray or lungfish. Lungfish video)
Engulfing of whole prey: In water, this is the most effective method, but in air, suction feeding is impossible. Nevertheless, the first land vertebrates inherited the whole ingestion of small prey from their aquatic ancestors. After the evolution of initial adjustments such as the neck, many vertebrates persisted in simple engulfing of small prey. E.G.: salamander.
Some groups developed special techniques for engulfing whole prey including:
Two major groups evolved the ability to bite pieces off of their food,
- Tongue projection in frogs, some salamanders, and some lizards (most notably chameleons.) In all cases, this is accomplished by the transformation of the hyoids skeleton and muscles that support and power the tongue.
- Cranial kinesis: One means of increasing the size of the prey you can ingest is to increase the mobility of the bones of the skull relative to one another. This allows several advantages:
E.G.: all squamates have this ability to some extent. In snakes it is extremely developed, with extreme mobility of the bones to which the jaw attaches, intramandibular joints in the jaw, and disarticulation of the joint between the two jaw halves. Additionally, rows of palatal teeth can move independently of the jaws.
Snakes are famous for their ability to ingest very large prey items.
- Widening the range of possible food items
- Improving digestive efficiency
Biting - Archosaur-style: Although archosaurs have evolved a wide range of feeding adaptations, their last common ancestor was something like Erythrosuchus, a predator with a uniform row of blade-like serrated teeth, with which it could bite off pieces of its prey, rather than swallowing it whole.
Biting - Synapsid-style:
Synapsids also developed the ability to bite off hunks. In their case, this was accomplished by teeth that were differentiated along the tooth row. Initially, there were large canines for immobilizing prey, and pre- and post-canine teeth for nipping and shearing. These ultimately evolved into the mammalian array of incisors, canines, premolars, and molars.
Ecological specializations: Both synapsids and archosaurs have experimented with a wide range of feeding ecologies, including herbivory and durophagy. Adaptations to herbivory are remarkably similar, despite the phylogenetic differences of the animals. Key adaptations include:
These adaptations are visible in both synapsids:
- Displacement of the jaw joint dorsally to the tooth row. This represents the ironic fact that although they don't need to snap quickly, herbivores often need to bite harder than carnivores. Thus, their jaw joints optimize strength by increasing mechanical advantage - the efficiency of force transfer in a lever system. Strictly,
Mechanical advantage = Force out/force in
and is proportional to
- Development of a gap between the anterior nipping teeth (or similar structure) and posterior grinding teeth.
Also evident is the evolution of grinding teeth with self-sharpening surfaces.
In some herbivorous dinosaurs, food was ground with batteries of teeth with little or no enamel on one side. These would self-sharpen with use and fall out and be replaced with new teeth when worn.
In many herbivorous mammals (synapsids), food is ground with teeth containing complexly infolded layers of enamel. With wear, these form complex ridges on the tooth's occlusal surface. Often, these are continuously growing.
Browsing vs grazing: Cenozoic mammals diversified during the same interval in which grasses became common. The processing of grass imposes special demands, mostly because they incorporate large quantities of phytoliths - silicate nodules - in their tissues, making them very abrasive. Mammalian herbivores that became grass specialists evolved special adaptations as grazers, including:
- Hypsodonty: A tendency to evolve deep-rooted, high-crowned, continuously growing teeth.
- Broad snouts suitable for cropping vegetation off the ground. (Compare grazing white rhino with browsing black rhino.)
In addition to this comparative evidence, we have direct evidence of diet from patterns of microscopic enamel wear. major types include:
- Macrofractures: major breaks due to contact with hard tissues
- Microwear: microscopic damage
- Fine polish: surface delicately polished, making prisms more apparent. Typical of browsers.
- Microscratches: 1 micron wide scratches that tend to obliterate prisms. Typical of grazers.
- Microfailures: 1-10 micron wide scratches & fractures. Typical of carnivores.
Case studies: What might be the feeding specializations of:
Tylosaurus (a mosasaur):
Propleopus (a kangaroo):
An enigma given that:
- As in other kangaroos, there is a large gap between front teeth and premolars and molars.
- Teeth are relatively low crowned (not hypsidont)
- The first premolar is large, serrated, and has microfailures.
- Close relative of living rat-kangeroos (3 kg.) but Propleopus was closer to 70 kg.
Finally, examination of the victim can reveal the ecology of predators. Consider the fate of Blue Babe, a Pleiostocene steppe bison who met a violent end on the mammoth steppe of Alaska.
Puncture marks in his hide reveal that he was killed by big cats. But by whom? Suspects include:
Evidence takes the form of:
- Fragment of a carnassial tooth (meat-shearing molar of a mammalian hyper-carnivore)
- Shape of puncture made by canines. Which cat do they conform to? schematic
Answer: Blue Babe was done in by lions.