Thread by @Paleoartologist

Okay, since Twitter hasn't had enough Spinosaurus stuff over the past few weeks, Ima spend #FossilFriday diving into whether or not #Spinosaurus had lips, from a completely non-osteological perspective!

First, why have lips? Lips serve two big functions in tetrapods: first, as protection against tooth desiccation (retaining moisture), and also for hydrodynamics in whales and dolphins, by smoothing out their profile. (2nd image: Florian Graner)

Let’s look at the proposed analogues for Spinosaurs. Crocodilians are a frequent comparison: freshwater piscivorous reptiles with similar snout and tooth shape, and no lips. Moray eels, however, also hunt fish, have crazy dentition, but also have lips! (Image 1: Hal Beral, Getty)

Number one, crocodiles are not pursuit predators, and are not streamlined. Eels and cetaceans (lipped aquatic predators) are streamlined. Spinosaurus follows crocodilians for this (slow + that sail is SUPER hydrodynamically inefficient), so it probably didn’t need lips for this

Number two, Crocodiles don’t need lips to keep their teeth from drying out, because they’re constantly immersed in water. Spinosaurus shows irrefutable evidence of a semi-aquatic lifestyle, from isotope analysis, anatomy, tooth shape, etc, so it doesn’t need lips for this either

Modern crocodilians don’t have lips, for several probable reasons. These include reduced drag for snap feeding (vs cetacean ram feeding), and the ability to quickly drain the mouth of water when swallowing prey (eels get around this by expelling water through their gills)

Spinosaurs did not have gills, and based on their flexible necks & long snouts, were likely snap feeders (neck + feeding link in Hocking et al 2017). They fit the crocodilian model very well here, which suggests pressures to reduce oral soft tissue (image: Ibrahim et al 2020)

Spinosaurs were incredibly aberrant and derived animals, so it’s possible they just said f-that to the crocodilian model, but it seems that there was not only no need for lips, but also pressures to lose them. For that reason, I'm on team no-lip Spino.

So yeah, tweet storm done. If I’ve missed anything (I invariably do), lmk. Figured I’d throw my 2 cents in the ring. Any images that were not credited to others are mine.

Image^ from Ibrahim et al 2020

Differential feeding strategies of aquatic mammals described in depth here, a link is made between snap feeding and neck flexibility: royalsocietypublishing.org/doi/pdf/10.109…

WOW, I didn't even realize this! River dolphins are unique among cetaceans as being slow-moving and having a lateral snap-feeding method similar to crocodilians (& presumably spinos), and have LOST THEIR LIPS!

https://twitter.com/JuliotheArtist/status/1258853290898608130?s=20