when the jackboot needs to jump

An interesting paper (Vermeij, 2015) on the "empty phenotypic space", i.e. the forms and adaptations that we do not see in the living world, possibly relevant to the convergence vs. contingency debate.

Some examples:

Wheels: some curled-up arthropods can roll around, and bacterial flagella and some parts of weevil legs rotate on their axis, but macroscopic wheels with a free axle do not exist, probably because smooth surfaces on which they'd be useful are rare and it would be difficult to grow them through embryonal development.

Animal-provided pollination and dispersal do not exist in water, with the possible exception of one species of fish-pollinated seagrass (which is a descendant of terrestrial plants). Presumably water is already good enough at carrying gametes and propagules that buying the services of an animal is a useless expense.

Mineral reef-building does not occur on land nor, more surprisingly, in freshwater. The reason for the latter is not clear, since there are enough mineral ions in freshwater to build shells. Boring of rock, shells, and wood in freshwater is also extremely rare though common in the sea.

Gelatinous plankton like salps or jellyfish (with few exceptions of the latter) is also not found in freshwater, probably because they can't survive dispersal between separate water bodies.

Endothermy ("warm blood") is generally not found in small aquatic animal, probably because water leeches away heat much faster than water, so aquatic endotherms (tunas, sharks, seals, whales) need to be bulky. On land, however, endothermy is found among tiny vertebrates and even insects.

There is no passive air-floating plankton, since air is not dense enough to support living tissue or dissolved organic matter by buoyancy. For that reason filter-feeding is also rare outside of water, while carnivorous plants are not found in the ocean (the water already carries enough nutrient). Aquatic plants do not produce wood as buoyancy is enough to keep them upright.

Large terrestrial animals do not specialize as scavengers (all mammals famous for scavenging also hunt actively); large carcasses are too spread out. All specialist scavengers on land are either very small, or flying.

Herbivory is rare among active fliers, because plant matter has a low energy density and takes a long time to digest. Herbivorous birds and insects are poor fliers or flightless, and the best fliers, like geese, are the ones that can take shelter in water.

Many more examples are only excluded from specific groups (e.g. live-bearing, despite being very common in reptiles, never appeared in birds, probably because the bird egg-shell is too mineralized to be retained in the womb as transition toward full live-bearing).

Even though the author calls them "forbidden phenotypes", only some of them are actually impossible (because they cannot evolve in the first place, or because they cost more energy than they're worth), and others simply never happened to evolve. At the end of the paper there is a list of phenotypes that would have been "forbidden" in the aftermath of the Cambrian Explosion and Ordovician diversification, but which appeared later, and they include

cutins, suberins, lignins, flavonoids, alkaloids, vascular systems, roots, leaves, rigid frameworks of stems and branches, nutrition complemented by animal matter, and basal growth in land plants; nitrogen-fixing symbiosis on land; animal-mediated dispersal/pollination; silk-producing, sound-emitting, flying, eusocial, terrestrial herbivorous, wood-boring, terrestrial shell-bearing and endothermic animals; embryos nourished within the body of an animal or plant parent; mineralized phytoplankton; and rock-excavating marine herbivores. [...] photosymbiotic and chemosymbiotic molluscs, the bivalved condition in gastropods, terrestrial life in gastropods and vertebrates, complex septa within the phragmocone of externally shelled cephalopods, internalization and loss of the shell in cephalopods, cementation to the substratum with a glue of calcium carbonate and organic matrix in several animal groups (gastropods, brachiopods, bivalves and barnacles), spines on shells of several groups (brachiopods, bivalves and brachiopods), mineralized tubes in polychaete annelids, mobility in bryozoans and pelmatozoan echinoderms, jaws and teeth in vertebrates, and vascular systems in brown and red algae. A vast diversity of potent venoms also lay in the future as part of the defensive and aggressive arsenal of many gastropods, cephalopods, aculeate Hymenoptera, vertebrates and land plants.

Deadlock was glad he’d cultivated a reputation both for being loyal and for not giving a damn. It was easier to school your expression to boredom than attentiveness, and he’d get a pass for looking bored during a Megatron speech.