December 2024 – Nix Illustration

Isisaurus

Isisaurus colberti was a sauropod dinosaur that lived in what is now India and Pakistan at the end of the Cretaceous Period, about 70-66 million years ago.

It was part of the titanosaur group of sauropods and had some unusual proportions* compared to its close relatives, with elongated forelimbs and a relatively short chunky neck. Since it’s only known from a partial skeleton its full size is unknown – estimates have been made as large as 18m long (~60′) but it was probably somewhat smaller, closer to around 11m in length (36′).

(*Measurement errors in the original paper resulted in some very weird proportions, but more recent and rigorous reconstructions have made Isisaurus not quite so cursed-looking.)

Like most other titanosaurs it probably lacked the thumb claws seen in other sauropods, and it may also have had some bony osteoderm armor studding its skin.

Coprolites that may represent Isisaurus’ poop show evidence of several different fungi that grow on tree leaves in humid tropical and subtropical climates, suggesting that this sauropod was a selective browser somewhat like modern giraffes.

Angelosaurus

Angelosaurus dolani was an early synapsid, part of the caseid family and closely related to the more well-known Cotylorhynchus.

Living in what is now Texas, USA during the mid-Permian, about 270 million years ago, it’s only known from partial skeletal remains but was probably around 3m long (~10′).

Like other large herbivorous caseids it would have had a tiny head with proportionally big nostrils, a short neck, a large barrel-shaped body accommodating a voluminous gut, a long tail, and strong sprawling limbs. But compared to its relatives Angelosaurus was particularly bulky, with shorter thicker heavily-muscled limbs and stubbier digits ending in broad hoof-like claws.

In closely related caseids the presence of teeth on the roof of the mouth and a well-developed hyoid apparatus suggests these animals had big tough tongues, which may have been used to mash mouthfuls of plant matter against the palatal teeth to partially break it up before swallowing.

Based on skin impressions from other early synapsids, Angelosaurus probably had crocodilian-like scutes on some parts of its body – likely on its underside and tail, and maybe also on the top of the head as indicated by the pitted bone texture of caseid skulls – but whether the rest of its skin was scaly or naked and glandular is currently unknown.

In recent years there have also been some proposals that large caseids may have had a semiaquatic hippo-like ecology, but this idea is controversial.

Rhacheosaurus

Metriorhynchids were a group of fully marine crocodyliforms known from the mid-Jurassic to the early Cretaceous of Europe and the Americas. They were the most aquatic-adapted of all known archosaurs, with streamlined bodies, smooth scaleless skin, small front flippers, larger hind flippers, and shark-like tail flukes. They may also have been endothermic, and might even have given live birth at sea rather than laying eggs.

Rhacheosaurus gracilis here was a metriorhynchid that lived in warm shallow waters around what is now Germany during the late Jurassic, about 150 million years ago. Around 1.5m long (~5′), its long narrow snout lined with delicate pointed teeth suggests it fed on small soft-bodied prey, a niche partitioning specialization that allowed it to coexist with several other metriorhynchid species in the same habitat.

Unlike most other marine reptiles metriorhynchids didn’t have particularly retracted nostrils, which may have had a limiting effect on their efficiency as sustained swimmers since higher-set nostrils make it much easier to breathe without having to lift the whole head above the surface. The lack of such an adaptation in this group may be due to their ancestors having a single nasal opening formed entirely within the premaxilla bones at the tip of the snout, uniquely limiting how far it could easily shift backwards – other marine reptiles had nostrils bound by the edges of multiple different bones, giving them much more flexibility to move the openings around.

(By the early Cretaceous a close relative of Rhacheosaurus did actually evolve nostrils bound by both the premaxilla and the maxilla, and appeared to have started more significant retraction, but unfortunately this only happened shortly before the group’s extinction.)

Metriorhynchids also had well-developed salt glands in front of their eyes, but the large sinuses that accommodated these glands may have made their skulls ill-suited to deep diving, being more susceptible to serious damage from pressure changes and restricting their swimming to near-surface waters only.

Preserved skin impressions in some metriorhynchid fossils show several unusual “irregularities”, including curl shapes, small bumps, and cratering. It’s unknown what exactly caused these marks, but they may represent scarring from external parasites such as lampreys and barnacles.

Anthracodromeus

Anthracodromeus longipes was an early reptile* that lived in what is now Ohio, USA, during the late Carbonifeorus about 307-305 million years ago.

(*or possibly a very reptile-like stem–amniote)

Around 20cm in total length (~8″), it had a rather lizard-like shape with a long body and a short tail. The digits on all four of its limbs were highly elongated with hooked claws, which appears to have been an adaptation for climbing.

It inhabited a coal forest dominated by lycopsid trees and seed ferns, and as one of the earliest known tetrapods to develop climbing behavior it would have had some ecological advantages over its relatives, being better able to escape from predators and access new food sources.

Tauraspis

Osteostracans were an ancient group of jawless fish, closely related to early jawed vertebrates, whose fossils are known from the mid-Silurian to the late Devonian of what is now North America, Europe, and Asia.

They were heavily armored, with bony head shields and rows of large scales covering their bodies. While their flattened shapes and upward-facing eyes have resulted in them traditionally being interpreted as mud-grubbing bottom-dwellers, their paddle-shaped pectoral fins, dorsal fins, and strong tails indicate they were also quite good swimmers – and their diverse hydrodynamic head shield shapes suggest they probably had a much wider range of ecologies than previously thought.

Although many osteostracans had large flaring spines on the sides of their heads, or long snout-like spikes at the front, Tauraspis rara here was unique in having two long front-facing horn-like projections.

Around 7.5cm long (~3″), it lived in brackish and freshwater environments in what is now northern Siberia during the early Devonian, about 410-407 million years ago. Like other osteostracans it had a small keyhole-shaped “nostril” opening, and large patches of sensory organs known as “cephalic fields” on the sides and top of its head shield.

The fields were covered with a mosaic of small bony plates, and their exact function is still a mystery – but they may have been involved in sensing vibrations in the water, or possibly even been electric organs.

Similarly, what Tauraspis used its unusual pair of “horns” for is also unknown.