Category: not paleoart

Almost-Living Fossils Month #12 – The Other Nautiluses

Nautiloids are represented today by just two living genera (Nautilus and Allonautilus), but they have a lengthy evolutionary history going back almost 500 million years.

The peak of their diversity was during the first half of the Paleozoic, with many different shapes of shells from coiled to straight, then they began to decline when their relatives the ammonites and coleoids appeared and began to compete for similar ecological niches. Although a few groups of nautiloids survived through the end-Permian mass extinction, most of them had disappeared by the end of the Triassic, leaving just one major remaining lineage known as the Nautilina (or Nautilaceae).

During the mid-to-late Jurassic (~165 mya) two new groups split away from the ancestors of the modern nautiluses – the cymatoceratids and the hercoglossids.

Cymatoceratids such as Cymatoceras sakalavum here had shells with a ribbed texture. Living during the Early Cretaceous, about 112-109 million years ago, this particular species is known from Japan and Madagascar and could reach a shell diameter of over 15cm (6″).

Hercoglossids, meanwhile, were much more smooth in appearance, but both groups also had more complex undulating sutures between their internal chambers than modern nautiluses do.

These nautiluses made it through the end-Cretaceous mass extinction and had a brief period of renewed success, filling the ecological roles left vacant by the extinct ammonites. But by the end of the Oligocene (~23 mya) both the cymatoceratids and hercoglossids vanished, possibly unable to deal with cooling oceans and the evolution of new predators.

Some of the hercoglossids’ Cenozoic descendants, the aturiids, managed to last a little longer into the Early Pliocene (~5 mya) before another period of cooling seems to have finished them off. Past that point, all that was left of the once-massive nautiloid lineage were their cousins the nautilids, who gave rise to today’s few living representatives.

(It’s also worth noting that the classification of the cymatoceratids seems to be in flux right now. Some paleontologists currently don’t consider Cymatoceras itself to actually be part of the group, instead being a nautilid much closer related to modern nautiluses. If this is the case then the cymatoceratids may not have actually survived past the Late Cretaceous – but the Cymatoceras genus alone still counts as an “almost-living” fossil since its various species ranged from the Late Jurassic to the Late Oligocene.)

Almost-Living Fossils Month #09 – Horned Sharks

All modern species of sharks and rays are part of a single lineage of cartilaginous fish known as neoselachians, and the closest evolutionary “cousins” to all of them were the hybodontiformes.

First appearing way back during the Devonian, about 400 million years ago, these early sharks were widespread around the world and incredibly successful as a group, living in both marine and freshwater environments.

Although due to their cartilaginous skeletons hybodontiformes are mostly known from fossilized teeth, there are still some complete specimens known that show us their overall body shape. They had two dorsal fins, each with a long spine in front, and an asymmetrically-shaped tail. Some of them also had small horn-like spines on their heads – this seems to be a sexually dimorphic trait, since the ones with “horns” also have claspers which show they were males – and they generally had powerful jaws with teeth specialized for crushing.

They were probably fairly slow swimmers most of the time, but would have still been capable of occasional bursts of higher speed, and various species were adapted to a wide range of food sources. Some had wider flatter teeth for cracking open hard-shelled seafloor invertebrates, and others were more opportunistic hunters that would have crunched on pretty much anything they could fit in their mouths.

Hybodontiformes were the dominant type of shark around the world before the end-Permian “Great Dying” mass extinction (~252 mya), and then went on to recover and flourish once again up until the mid-Jurassic.

Hybodus hauffianus was one of the Early Jurassic species, living around 183 million years ago in Europe. About 2m long (6′6″), it had two different types of teeth in its mouth – sharper ones in the front and flatter ones in the back – suggesting it was a generalist predator eating whatever it could catch. We do know its diet at least included the fast-swimming squid-like belemnites, since some fossils preserve clusters of their internal hard skeletons in Hybodus’ stomach region.

Towards the end of the Jurassic neoselachians began to diversify and take over most of the marine shark ecological niches, and the hybodontiformes became increasingly restricted to freshwater. During the Cretaceous they continued to do fairly well in those environments, but most of them still disappeared around the time of the end-Cretaceous extinction (~66 mya). Since most other sharks weren’t actually particularly affected by the extinction event, it’s not clear whether the hybodontiformes were more vulnerable for some reason or whether it was the ongoing competition from neoselachians that drove the majority of them extinct at that time.

Still, a few of them did seem to make it through to the Cenozoic, although they were absent from the fossil record until the Miocene. Freshwater deposits in Sri Lanka have evidence of a late-surviving member of the group living perhaps as recently as 5 million years ago – so they would have only gone completely extinct sometime after that, and we probably missed seeing them alive by only a few million years at most.