The lifestyle of spinosaurid dinosaurs has been a topic of lively debate ever since the unveiling of important new skeletal parts for Spinosaurus aegyptiacus in 2014 and 2020. Disparate lifestyles for this species have been proposed in the literature; some have argued that it was semiaquatic to varying degrees, hunting fish from the margins of water bodies, or perhaps while wading or swimming on the surface; others suggest that it was a fully aquatic underwater pursuit predator.
Doing what they do better than any animal that ever evolved, a pair of Spinosaurus aegyptiacus wade into nearshore waters for prey as pterosaurs soar overhead some 95 million years ago on the northern coast of Africa. Image credit: Daniel Navarro.
Paleontologists generally agree that Spinosaurus aegyptiacus was a fish-eater, but exactly how these dinosaurs caught their prey is the subject of lively debate, with some researchers suggesting that they hunted on the shore, some that they waded or swam in the shallows, and others that they were aquatic pursuit predators.
One recent study provided support for the latter hypothesis using a fairly new statistical method called phylogenetic flexible discriminant analysis (pFDA) to analyze density and proportions of Spinosaurus bones.
In a new study, University of Chicago’s Professor Paul Sereno and his colleagues critically assess the methods of that prior research and identify significant flaws.
“Spinosaurus, and its close relatives, are fascinating because of their unusual anatomical features, the scarcity of specimens, and the fact that scientists had not found bones from parts of its body until very recently,” they said.
“Unlike other meat eating dinosaurs, there are strong clues that it lived near water and ate fish or other aquatic fare.”
“This has fueled a lot of controversy about how Spinosaurus lived — was it a fast swimming predator that chased fish like a sea lion? Or was it an ambush predator at the water’s edge, grasping with its clawed hands like a giant version of a brown bear chasing salmon, or plunging its head into water like a 7-ton heron from hell?”
The authors began by asking new questions about bone density, such as how to digitize thin sections, where to slice through the thigh and rib bones, and whether to include bones from more than one individual.
Some modern aquatic mammals like manatees have swollen, dense bones to help them stay underwater, like a scuba diver’s weight belt.
Large land animals, like elephants and dinosaurs, also have dense bones to support their increased body mass.
Most modern birds and many dinosaurs including Spinosaurus have the opposite condition, with air sacs attached to lungs or inside bones that act like a life vest preventing submergence.
Assessing the aquatic abilities of an extinct species like Spinosaurus needs to take account of all these factors.
Given the complexity of understanding the meaning of bone density, the paleontologists reevaluated how the statistical technique used in the previous study was applied to support the claim that Spinosaurus was a deep diver.
The pFDA approach is like machine-learning: it trains a classification algorithm on a group of species whose lifestyle is well understood.
In principle, researchers could then use the algorithm to estimate the likelihood that a poorly understood species such as Spinosaurus falls into one behavioral group or another.
“In practice, however, there are problems that must be overcome,” said Intellectual Ventures researcher Nathan Myhrvold.
“Unfortunately, that technique does not work properly unless you have lots of data, you make apples-to-apples comparisons, and you check that the data meet certain statistical prerequisites.”
“None of those requirements were met in that earlier study, so the results just didn’t hold up under reexamination.”
The new paper should help paleontologists understand the pitfalls of pFDA and other kinds of broad statistical analyses and how to avoid them.
The researchers show that it is important to use consistent and objective criteria to decide which species to include and exclude, as well as how to classify their behavior.
The findings also demonstrate the importance of taking measurement errors and individual variations into account when assessing bone density.
“We think Spinosaurus, one of the largest predatory animals ever to have evolved, needed extra bone strength to support its weight on its relatively short hind limbs,” Professor Sereno said.
“Spinosaurus was able to wade into waterways more than six feet deep without floating, where it could ambush fish of any size with its claws and jaws — but all while keeping its toes firmly anchored in the mud.”
The study was published in the journal PLoS ONE.
_____
N.P. Myhrvold et al. 2024. Diving dinosaurs? Caveats on the use of bone compactness and pFDA for inferring lifestyle. PLoS ONE 19 (3): e0298957; doi: 10.1371/journal.pone.0298957