Tuesday 7/12/21 - Where are all the medium sized Theropods?
"The Horrors of Thieving", Allosaurus, Ornitholestes; FredtheDinosaurman (deviantart)
Ecosystems these days are made up of animals and plants of varying sizes and niches. Niche partitioning is a term used to describe how different roles in an ecosystem are taken up by different species. Carnivores for example, can be suited to a variety of roles, even within the same biological family.
Think of North American canines for example. Red Foxes are a smaller predator, hunting rodents, small birds etc. Then there's Coyotes who hunt those smaller prey too, but also hunt larger animals like beavers, and porcupines, and even pack hunt for animals like deer. Then there's Wolves, which may opportunistically scavenge, but are specialised in larger prey as a pack, like bison. Just like prey, predators can inhabit a small size, medium size, or large size niche. But this divide was long-thought to be seemingly absent in dinosaurs.
Red Fox (smallest), Coyote (middle), Grey Wolf (largest), National Parks Service
Ok so when you think of the size of a meat-eating dinosaur, you either think of a large apex predator like Tyrannosaurus or a smaller agile hunter like a Velociraptor. Late Mesozoic Theropods are consistently either huge, or man sized, and palaeontologists were puzzled as to why there were so few species in the middle. There were small, medium, and large prey animals in the age of reptiles, so why was there no medium sized theropods to fill in that niche?
Recently, there has been a revision on how we understand the growth and diet of predatory dinosaurs that may help to understand a solution to this question. Modern pack hunting animals share their kills with their family groups. An African Wild Dog puppy is not large or powerful enough to kill an Antelope, but a large portion of their diet will be Antelope because their parents will share the kill.
Deinonychus catch, Raph Herrera Lomotan
Studies of coprolites (fossil poo) and fossilised stomach remains in young dinosaurs have revealed it may have worked out differently. Young Deinonychus have been found to have different nutrients in their diets to adults, and palaeontolgists think this is because they hunted different animals (Frederick et al 2020). Adult Deinonychus had the power and skill to kill dinosaurs larger than themselves, but juveniles could not, so would've pursued smaller reptiles, mammals, or even invertebrates. Adults didn't share their kills with their young, so they had to find their own food.
This ontogenetic (relating to aging and development) variation in niches has also been theorised in larger theropods, to the point where some smaller to medium-sized dinosaur species have been identified as potential juveniles of larger species. Nanotyrannus was a species of smaller Tyrannosaur that has a very different body shape compared to the largest of the group. It had been described as about as tall as an adult human, with long slender legs, and had an extensive covering of feathers. But new studies have discussed the possibilities of Nanotyrannus being the juvenile form of Tyrannosaurus rex itself.
Nanotyrannus stalking Anatotitan, Doug Henderson
If this was the case, as many palaeontologists are on board with, it means that young Tyrannosauruses were specialised for a very different lifestyle than their adult counterparts. The juveniles were pursuit predators, running after smaller prey items, whereas the adults would take on larger dinosaurs, and did not have nearly enough leg strength to run for longer distances.
Juvenile/Adult Tyrannosaurus, Gabriel Ugueto
Scipionyx, the first dinosaur species discovered from Italy, has also theorised to be the juvenile of a larger species. It was initially classified as part of the group Compsagnathidae, but palaeontologists have begun to question whether the group itself is natural, or if scientists have just grouped together unrelated species of the same body shape and size. Given the shape and plasticity of its bones, the known specimen of Scipionyx is thought to be a hatchling, but a hatchling of what? Italian Paleaontologist Andre Cau theorised (Cau 2021 (its in Italian)) that Scipionyx was a juvenile Carcharadontosaurid, and further, that most of the Compsagnathid group consists of juveniles of large bodied theropods. The "Compsagnathid" body shape could in fact just be the body shape of juvenile Tetanurans (the clade including most theropods).
Scipionyx, Lukas Panzarin
This type of ontogenetic niche partitioning exists today in some reptile groups, particularly in Komodo Dragons, where the juveniles are aboreal insectivores, and the adults hunt mammals many times larger than themselves.
Juvenile Komodo Dragon, Mark Ralston
Dinosaurs were not mammals, and in fact are unlike many other groups alive today, and understanding their biology and behaviour requires looking into many different vertebrate groups. So, to answer the question posed by this post's title? Where are all the medium sized theropods? Its likely they're the bigger ones going through a phase.
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