Time and Place: 231.4 million years ago, in the Carnian of the Late Triassic
Pisanosaurus is known from the Cancha de Bochas Member of the Ischigualasto Formation in San Juan Argentina.
Physical Description: If a Silesaurid - as is currently thought - Pisanosaurus would have been a small, slender, and quadrupedal animal, only about 1 meter in length. It had an open hip socket, like dinosaurs, which may point to a very interesting phylogenetic position (see the Other section below). It had very elongated bones in its hands, and its upper hips were weirdly wide as well. Beyond that, we don’t know much about with Pisanosaurus may have looked like. It seems logical to suppose it would have had a small head with a little beak in the front of the mouth - as both Silesaurids and early Ornithischians (the other hypothesis for the type of creature Pisanosaurus was) have such structures for snipping off plant material. Given its small size, Pisanosaurus - like all other early members of the group of reptiles that would later include birds (Avemetatarsalia) - would have been covered with fluff all over its body. If Pisanosaurus was an early Ornithischian and not a Silesaurid, it would have been bipedal, with short forelimbs not used in locomotion.
Diet: Either way, Pisanosaurus would have been an herbivore, eating low-lying vegetation in its densely forested home.
Behavior: As a small, lithe herbivore, Pisanosaurus would have been very skittish - running at the slightest sign of danger, making sure to avoid the many large predators it shared a home with. It probably wouldn’t have been very social - given it wasn’t very abundant! - but it may have foraged in large groups of mixed herbivores, sticking together to rely on each other in the event of danger. This reliance would have allowed bulkier herbivores to notice and react to danger quicker (since the small lithe ones like Pisanosaurus would have already been running away) - and the smaller ones would have had large, bulky roadblocks to stop the approach of predators. As an early dinosaur, it most likely partook in some sort of care of its young, though of course, we do not know what.
(As an Ornithischian) by Michael B. H., CC BY-SA 3.0
Ecosystem: The Ischigualasto Environment is one of the more famous ecosystems of the Late Triassic - due to it being a hotbed of early dinosaur discoveries, including some of the earliest potential members of the group. In fact, it is such an important environment that today the rock formation is considered a UNESCO World Heritage Site. It was an extensive series of rivers channeling through a large floodplain, erring towards the wetter side of the scale - mud was the name of the game, and there was a wide variety of plant material present, including a dense conifer forest, ferns, and horsetails. It did experience seasonal rainfall, with possible times of extremely heavy precipitation. Occasionally, everything would be buried in volcanic ash - leading to the beautiful preservation of the environment.
Pisanosaurus wasn’t the only Silesaurid here - there was also Ignotosaurus, the slender and small Silesaurid. There were also famous early dinosaurs such as Eoraptor - the small, bipedal early Saurischian(?), Herrerasaurus and Sanjuansaurus, the large taxonomically-confusing predators, and Chromogisaurus, one of the earliest known “prosauropods.” Of course, this being the Triassic, dinosaurs were only a small part of the ecosystem. The Ischigualasto - like most places of the time period - was absolutely lousy with other archosauriformes! Non-Dinosaurian Archosauriformes were extensively diverse at this time, making up a large chunk of the “large charismatic land animal” roles. There was Aetosauroides, an Aetosaur (sort of like a cross between an ankylosaur, an ant-eater, and a crocodile), Proterochampsa and Pseudochampsa - crocodilian-like creatures that were actually equally closely related to crocodiles and dinosaurs - both frequented the rivers of the environment. Saurosuchus, a large and bulky stem-croc, would have been a huge pain in the rear for herbivores like Pisanosaurus. Sillosuchus was a weird stem-croc, bipedal and strangely dinosaur like - with even, potentially, a beak - but utterly scaly, and bulky in stature! There was also Venaticosuchus, an Ornithosuchid (one of the most basal groups of stem-crocs), and Trialestes - a fast moving stem-croc, and one of the earliest Crocodylomorphs (the group of crocodilians and their closest relatives).
(Without Feathers) by Nobu Tamura, CC BY-SA 4.0
It being the Triassic, this wasn’t an environment free of non-reptiles! Temnospondyls - large carnivorous amphibians - were crawling about; as were a variety of Synapsids. Small, carnivorous dog-shaped cynodonts like Chiniquodon would have directly competed with the local dinosaurs; they even grew to be quite large and fast, like in Diegocanis and Ecteninion. They also came in large, bulky herbivorous forms, like Exaeretodon. Dicynodonts were present too, with their strange pig-like appearance: Ischigualastia was a common synapsid on the floodplains of Pisanosaurus’ home. In short, Pisanosaurus was surrounded with a cast of characters showcasing some - but certainly not all - of the weirdness that the Triassic had to offer.
Other: What Is Pisanosaurus? Back in the day, Pisanosaurus was a famous creature for being one of the earliest “Ornithischian” dinosaurs - one of two major groups of dinosaurs, famous for including such later iconic members as Stegosaurus and Triceratops. In fact, for the longest time, Pisanosaurus was… the only Ornithischian known from the Triassic. This is odd, to say the least - there are dozens of Triassic dinosaurs known, they’re just all from the other group, the Saurischians (containing such iconic later forms as Apatosaurus, Tyrannosaurus, and… all birds). So, for the longest time, Pisanosaurus stood as a focal point of dinosaur research - an important piece of the puzzle of the origin of this elusive, but important group.
By Nix
Except it isn’t an Ornithischian. Lately, studies have shown time and time again that Pisanosaurus actually more closely resembles the Silesaurids - a group of almost dinosaurs that were quadrupedal, active herbivores living around the world at the time of the Triassic, before going extinct at the end-Triassic extinction. This would make its appearance much different than what a “basal Ornithischian” would suggest - and, of course, the fact that Pisanosaurus is known from only a single fragmented skeleton does not make solving this problem much easier. Weirdly enough, there are some hypotheses which suggest that Silesaurids are… the earliest Ornithischians, (as per Pisanosaurus having an open hip-socket), representing a weird side-branch of the group from the Triassic. As this hypothesis gains traction, it may become increasingly true that Pisanosaurus was a Silesaurid - it was just also an early Ornithischian. Only time will tell in the solving of this mystery - for now, we must wait for more evidence.
~ By Meig Dickson
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