Petrolacosaurus kansensis was discovered in a smoothed layer of shale in eastern Kansas. The discovery included a skull with two temporal fenestrae (part of the defining characteristics of diapsids), a large orbit and teeth. It also included over 60 caudal vertebrae, 7 cervical vertebrae, the pectoral girdle, radius, ulna, fibula, and articulated tibia and astragalus. This articulation tells us a lot about the morphology and development of the reptilian astragalus and ankle joint, which is why many papers have been written on this specific aspect of the fossil remains. Manual and pedal phalanges were also discovered, allowing researchers to know the arrangement of the digits; this is actually quite important evolutionarily as digit order and retention has changed over time (some animals have presented with certain digits lost and sometimes knowing which digits were retained or lost can inform evolutionary relationships). These characteristics have helped to inform the position on the reptilian and diapsid family tree that Petrolacosaurus currently maintains. Further finds of this animal and others will help to refine that positioning further of course, but at this moment we have a good idea of where the animal is phylogenetically and what its position means for the origins of specific regions and parts of the reptilian body plan, making this an extremely important animal.
18 April 2017
|From Reisz 1977|
17 April 2017
There are many animals known only from the screen for many people. Petrolacosaurus is a prominent member of this group of animals as many only know it from its appearance in the Walking with Monsters series. Despite its dinosaur-like name, Petrolacosaurus was a small reptile and, as we can see from the first clip featuring it, at the mercy of many other animals like the enormous amphibians, arthropods, and insects it was contemporaneous with. The second clip shows that not all of these interactions went poorly for Petrolacosaurus though. The small predatory reptile was the beneficiary of tragic natural events like forest fires (as shown) and was capable of running down prey smaller than itself.
16 April 2017
One of Dr. John Merck's online lectures for one of his geology classes summarizes some aspects of diapsida and specifically references the characters of Petrolacosaurus that have caused it to be assigned to the base of the diapsid family tree. Other sites have information on Petrolacosaurus as well. These include Prehistoric Wildlife which has a short paragraph describing what is known about the small reptile. A more extensive entry and probably the most comprehensive and easy to read entry outside of the Wikipedia article or a scholarly journal entry is that found on the Walking with Monsters version of the wiki sites.
15 April 2017
We have discussed the dawn of the dinosaurs, multiple times depending on the taxa discussed in any given week, the dawn of mammals, birds, amphibians, and a number of other introductory taxa. Some of these have been composed of disarticulated nearly complete skeletons and some have included single limbs or even single bones. One of the most complete purported "first" taxa is one of the earliest diapsids, a small reptile with two holes in its lateral skull wall (from which we gather the meaning of the word diapsid). Discovered in 1932, the fossil of Petrolacosaurus kansanensis (Lane 1945) comes from the Pennsylvanian age of the Carboniferous (approximately 302 MA) and includes the skull, pectoral girdle, elements of the hind and fore limbs, and a large portion, if not the entirety, of the axial or vertebral skeleton.
14 April 2017
Murals and large scale paintings of ocean scenes often include Crinoids. Despite their lower population now, comparison to the Permian, these odd animals have always been prevalent members of the ocean.This level of involvement in the ecosystem makes the inclusion of such creatures in artwork almost essential, especially in panoramas of more ancient subject matter. Some good images that include Crinoids can be found at this link (some may be erased in the future, FYI). Crinoids have been the subject of professional as well as amateur art for a very long time. They have been main subjects as often as they have been background animals actually. Line drawings of Crinoids have populated the notes of scientists since before natural history was even considered a legitimate profession (scientific history is one of my favorite side hobbies and I promise that the history of natural history is very interesting). The best Crinoid-centric image I have found this evening is presented below. This image is older, as we can tell by its artist, Heinrich Harder, but is one of the best Crinoid centered images that can be found online. The image shows a variety of Crinoids, or as Harder called them "seelilien", swaying gently in a current and anchored into the sandy bottom of the ocean. The colors have faded over the years, but imagining the brilliance of the reds, yellows, and purples of the Crinoids one can really see the beauty of these strange animals from Harder's perspective.
13 April 2017
Crinoids possess a fairly simple appearance from an external view. A large stalk or stem, a calyx, and a feathery appendage are used for stability and locomotion, feeding and reproducing, and directing food into the mouth respectively. We can think of Crinoids as upside down starfish with the stalk growing out of what would be the dorsal surface of the starfish and a sucker or root-like tendrils anchoring the animal when it does not want to float on the currents. Some of these root structures are comprised almost entirely of the cirri that originate in the stalk. Their tough fibrous nature allows certain Crinoids to use the cirri as small and mostly inefficient legs as well, moving the Crinoid slowly from area to area. It is this stalk that is so often discovered by amateur fossil hunters in roadcuts in places like Missouri and Kansas; in case you find the shear number of fossils hard to believe consider these links please: (Missouri state fossil and Crinoid Stonehenge Model). The calyx is also frequently found, complete with the feathery arms, but is still not as regularly discovered as the stalk by amateurs. The calyx has, like other echinoderms, mouth, reproductive organs, and anus in close proximity to one another; if this sounds confusing, consult this graphic to the right. The feathery arms of the animal are called Pinnules and are ciliated, or covered in small hair-like structures called cilia that are capable of moving gathered food items toward the oral cavity of the animal. Crinoids add even more amazingness to their life history in that they are pentaradial echinoderms. Echinoderms are unique in that they begin life as bilaterally symmetric ("mirrored") larvae and during their life cycle grow to be pentaradial, or having five main segments arranged at 72º intervals around the mouth. Consider the ontogenetic image below to better visualize this change.