Megaloceros: Beast of the Week

Today we shall be getting familiar with a prehistoric mammal that once called the plains of Ice Age Eurasia home.  Megaloceros giganteus, or as it's commonly referred to, the "Irish Elk", was the largest tallest deer to ever walk the earth, measuring almost seven feet tall at the shoulder.  It's antlers were by far the largest of any animal's, measuring twelve feet wide.  They lived not only in Ireland but across most of Europe and Asia during the Pleistocene alongside mammoths and early humans.  The oldest known Megaloceros specimen is 1.2 million years old, while the youngest is from 7,700 years ago.  The genus name translates to "Great Horn".  Despite its common name, Megaloceros' closest living relative is the Fallow Deer, and not elk.

Life reconstruction in watercolors of Megaloceros, by Christopher DiPiazza.  During the late fall, like all deer, Megaloceros would have shed a velvety layer off its antlers, revealing the horn-like antlers underneath.

There is much debate surrounding why the Irish Elk's antlers were so large.  The most likely answer is probably sex.  Like most species of animals with antlers, it is likely that only male Megaloceros had them. (Few exceptions exist, like Reindeer, where females also have antlers.)  Sexual selection probably pressured the males to evolve more impressive antlers over time to attract the females and to combat rival males.  Despite how antlers this large seem like they would have been more of a burden than an advantage in life, as long as they increased a male's chances of mating before he dies, they would continue to be passed on.  This is actually the case with most animal courtship displays.  They communicate that the individual sporting them is healthy enough to thrive despite the burden, and therefore is worthy of being chosen for mating by potential partners.

Megaloceros skeleton at the Natural History Museum in Dublin, Ireland.

Some scientists proposed that the huge antlers may have been the Megaloceros' ultimate demise, regardless of how sexy females found it.  This was proposed for a few reasons.  The first is that the antlers were believed to be so wide that towards the end of the ice age, when there were more trees and foliage growing, made it difficult for male Megaloceros to maneuver in their environments, who were more adapted to open tundra landscapes.  Another hypothesis is the antlers became too expensive to maintain.  Antlers, unlike horns which are part of an animal's skull, fall off and regrown every year.  In order to grow such large structures within a period of just a few months, the animal must consume more nutrient-rich food.  Modern deer have even been observed and documented killing and eating other animals (yup, predatory deer) to supplement their diets during this rapid growth stage.  Don't believe me?  Check out this video.  Also notice that the deer is in the process of growing a new set of antlers.  So imagine how much nutrient rich food, plant or animal, Megaloceros must have needed to get in order to maintain itself during this time.  Now imagine the dilemma it must have faced when its environment, including its food sources, suddenly changed at the end of the ice age.  

The last hypothesis about their extinction is common for most large animals that lived during the end of the Pleistocene; over hunting by humans.  We know that our ancestors probably hunted them sometimes thanks to Megaloceros appearing in cave paintings, but there is really no solid evidence that humans were the exact cause of their extinction.  

Painting of Megaloceros done by prehistoric humans found at the Cougnac Caves in France.  Note how there are individuals with and without antlers shown.  Also note the hump over the shoulders and the dark pattern at the top of the neck and on the body.  Photo found at Don's Maps.

Perhaps one the coolest things about Megaloceros, is since it lived alongside humans, we have hints of what it looked like when it was alive from those humans in the form of cave paintings! On a cave wall in France that could be as old as 22,000 years, what are almost certainly Megaloceros are heavily featured.  They were painted with and without antlers, supporting the idea that females did not have them, unlike reindeer.  Those people also depicted them with a prominent hump over the shoulders, which must have been full of fat, since the actual skeletons of these deer do not suggest such a feature.  Lastly, the paintings also include dark markings under the chin and also across the body in two stripes originating from the hump.  Some argue these are meant to represent the animal's actual coat pattern.  

References

Gould, Stephen J. (1974): Origin and Function of 'Bizarre' Structures - Antler Size and Skull Size in 'Irish Elk', Megaloceros giganteus. Evolution 28(2): 191-220. 

Lister, A. M. 1994. The evolution of the giant deer, Megaloceros giganteus (Blumenbach). Zoological Journal of the Linnean Society 112, 65-100.

Lister, A. M., Edwards, C. J., Nock, D. A. W., Bunce, M., van Pijlen, I. A., Bradley, D. G., Thomas, M. G. & Barnes, I. 2005. The phylogenetic position of the ‘giant deer’ Megaloceros giganteus. Nature 438, 850-853.

 Moen, R.A.; Pastor, J. & Cohen, Y. (1999): Antler growth and extinction of Irish Elk. Evolutionary Ecology Research 1: 235–249. 

Stuart, A.J.; Kosintsev, P.A.; Higham, T.F.G. & Lister, A.M. (2004): Pleistocene to Holocene extinction dynamics in giant deer and woolly mammoth. Nature 431(7009): 684-689.

Avisaurus: Beast of the Week

 This week we will be looking at a true bird that lived alongside some of the most famous dinosaurs.  Check out Avisaurus darwini!

Avisaurus was a bird (theropod dinosaur) that lived in what is now Montana, United States, during the latest Cretaceous period, about 66 million years ago.  Based on the bones that are known it would have been roughly the same size as a modern hawk. Possibly weighing about 2lbs (.9kg) and having a wingspan of about 4 feet (1.2 meters).  (Again these numbers are very rough estimates based on literally some foot bones and are therefore subject to change/variation.). The genus name translates to "Bird-Lizard" because it was initially thought to be a different kind of theropod, and not a bird.  The species name is in honor of the famous scientist, Charles Darwin.  When alive, Avisaurus, was likely a meat-eater, which we will discuss more below. 

Avisaurus darwini life reconstruction in watercolors by Christopher DiPiazza.  Parallels between Avisaurus' foot bones and the same bones of modern birds imply it may have been a hunter of larger prey, including smaller dinosaurs. In this case it is shown with a captured baby Acheroraptor.

Unfortunately fossil birds are usually only known from very fragmentary remains because they tend to have hollow bones, which often decompose before they fossilize. Avisaurus, which is only known from some foot bones, is no exception. That being said, scientists were still able to learn a lot of interesting information about this prehistoric bird based on the little material they did find.  The parts where the foot bones would have met the toe bones are similar to the foot bones of certain modern birds, like hawks, eagles, owls, and falcons.  All these birds have extremely flexible and powerful toes, tipped with hook-like talons for capturing prey.  Here it is important to note that Avisaurus was not directly related to modern birds, but evolving this trait common to modern birds of prey isn't outlandish.  This is due to the fact that most modern birds of prey aren't related to one another and evolved these predatory feet independently of one another.  That's right, owls, falcons, and hawks/eagles are all from completely separate families of birds and do not share a common ancestor with grasping talons. (falcons in particular are much closer to parrots than they are to other birds of prey.) So it isn't outlandish for an extinct lineage of bird from the Cretaceous to have independently evolved this trait as well.

Foot bones from Avisaurus darwini. Photo from paper by Clark et. al., referenced below. 

We know nothing else about what Avisaurus looked like other than its feet, but based on more completely-known fossil birds that were related to it we can make a few educated guesses.  It likely had clawed fingers under its wings.  (A trait still present in certain modern birds, like ostriches and chickens, but was more widespread in more ancient birds) It also may have had some teeth in its mouth, unlike the toothless beaks of all living birds.  

When alive, Avisaurus would have lived in a seasonally wet, almost swampy environment.  It almost certainly could fly, and based on its feet, was likely hunting smaller animals, like insects, reptiles, small mammals, other birds, and likely even small/baby dinosaurs and pterosaurs.  It would have shared its world with some of the most famous dinosaurs, like Tyrannosaurus, Triceratops, Edmontosaurus, Ankylosaurus, Pachycephalosaurus, to name just a few.  

References

Chiappe, Luis M. (1992) "Enantiornithine (Aves) Tarsometatarsi and the Avian Affinities of the Late Cretaceous Avisauridae" "Journal of Vertebrate Paleontology" September 3, 1992, Volume 12 no. 3 pp. 344-350

Clark, Alexander D.; Atterholt, Jessie; Scannella, John B.; Carroll, Nathan; O’Connor, Jingmai K. (2024-10-09). "New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus". PLOS ONE. 19 (10): e0310686.

Mononykus: Beast of the Week

 This week we'll be going over an interesting little dinosaur with unique arms.  Check out Mononykus olecranus! 

Mononykus was a small, bird-like, likely feathered dinosaur that lived in what is now Mongolia, during the late Cretaceous period, about 70 million years ago.  It only measured a little over 1 meter (between 3 and 4 feet) long from nose to tail and was likely a meat-eater, specializing in insects, when alive. The name, Mononykus, translates to "one claw" because it only had one large functional claw on each hand.  

Life reconstruction of Monoykus in watercolor by Christopher DiPiazza.

As the name implies, Mononykus possessed only one claw on each hand, which is perhaps its most distinctive trait.  The claws are proportionally large and curved, and are backed up by extremely stubby but powerful arms.  In fact, despite their length, Mononykus' ulnas (outer forearm bone) extend out past the upper arm bones, implying they had large muscle attachments there in life.  We see similar features in living animals like aardvarks, which use those kinds of muscles and large claws to dig into termite mounds.  Even though aardvarks and dinosaurs are not closely related at all, this may provide insight as to how Mononykus was using its unusual anatomy.  

Arm bones of a modern Aardvark(top) and Mononykus (bottom).  Note the extended ulna.

Other parts of Mononykus' anatomy imply it may have been digging for insects other than the arms and hand claws.  Its vertebra would have been interlocked in a way that stiffened the spine in life, making it more sturdy, another trait that would benefit digging.  The inside of the skull suggests Mononykus had superb hearing, comparable to that of modern owls, who are specially adapted for hunting prey they cannot see.  Again, this is a useful trait to have for an animal that is hunting for insects underground or inside of dead logs.  Other, more completely known, relatives of Mononykus, imply it likely had a narrow, toothless, beaklike mouth, and large, forward-facing eyes.  

Mononykus may have used its superb hearing to detect insects underground or inside logs, then would have needed to press its body against that surface in order to dig them out with its short arms.

As stated before, Mononykus had extremely short arms, which seems to be the only trait inconsistent with animals that dig for their food.  The arms are so short, in fact, that the only way for Mononykus to have even reached a surface to dig is if it's torso was pressed up against it.  I can imagine Mononykus listening to the outside of a dead log, and then rapidly positioning itself to a diffing position with its body pressed against the log, vigorously tearing at the bark with its claws, and then hopping back up to a standing position to listen again or root out the now exposed insects with its beak-like snout.  

Mononykus skeletal mount on display at the American Museum of Natural History in New York.

Mononykus also had extremely long and slender legs.  This implies it was a fast runner when alive, which would have been a useful trait for avoiding predators, but it also has another function that may also be related to finding food.  Long legs means Mononykus could have more easily walked long distances without getting tired.  Since we know Mononykus lived in a desert, it would have needed to regularly travel across large expanses of land in order to find areas where insects actually were, like oasises, or sparsely occurring logs in an otherwise barren environment.  

When alive, Mononykus would have shared its environment with other dinosaurs, like Prenocephale, Gallimimus, Deinocheirus, Therizinosaurus, and the large tyrannosaur, Tarbosaurus, to name a few.  It likely avoided confrontation with the vast majority of these other animals by possibly being nocturnal.  

References

Choiniere, Jonah N.; Neenan, James M.; Schmitz, Lars; Ford, David P.; Chapelle, Kimberley E. J.; Balanoff, Amy M.; Sipla, Justin S.; Georgi, Justin A.; Walsh, Stig A.; Norell, Mark A.; Xu, Xing; Clark, James M.; Benson, Roger B. J. (2021-05-07). "Evolution of vision and hearing modalities in theropod dinosaurs". Science. 372 (6542): 610–613.

Perle, A.; Norell, M. A.; Chiappe, L. M.; Clark, J. M. (1993). "Flightless bird from the Cretaceous of Mongolia". Nature. 362: 623−626.

Perle, A.; Chiappe, L. M.; Rinchen, B.; Clark, J. M.; Norell, M. A. (1994). "Skeletal morphology of Mononykus olecranus (Theropoda, Avialae) from the late Cretaceous of Mongolia". American Museum Novitates (3105): 1−29.

Spectrovenator: Beast of the Week

 This week we will be showcasing a dinosaur with a particularly spooky name.  Check out Spectrovenator ragei!

Spectrovenator was a meat-eating dinosaur that lived in what is now Brazil during the early Cretaceous period, about 125 million years ago.  From snout to tail it only measured about 2.5 meters (8 feet) long.  It's genus name translates to "Ghost hunter" in reference to how it was found during the month of October (Halloween!) plus the fact that it was found serendipitously while paleontologists were excavating the bones of a larger dinosaur, called Tapuisaurus. Almost as if the Spectrovenator's bones were hiding...like a ghost.

Life reconstruction of Spectrovenator in watercolors by Christopher DiPiazza

Spectrovenator belonged to a group of theropod dinosaurs called ceratosaurs, known first appearing on the fossil record during the early Jurassic period, almost 200 million years ago, and persisted all the way to the end of the Cretaceous period, 66 million years ago.  Ceratosaurs were particularly diverse group of theropods, evolving a wide variety of shapes and sizes during the long time they existed on earth.  Spectrovenator was specifically part of the abelisaurid family of ceratosaurs, known for having extremely short snouts and tiny arms.  Their arms were so small, in fact, that they lacked the ability to use their fingers, which were reduced to mere stubs in most cases.  Later abelisaurids, like Carnotaurus, would lose the ability to bend their elbows, having small stumps for arms that could only swivel on one joint at the torso.  Spectrovenator was an earlier form of the abelisaurid group, appearing to be a somewhat transitional form between earlier Jurassic forms and later Late Cretaceous forms.  For instance, its snout was relatively short compared to most theropods, but not as short as the almost bulldog-like face of its later relatives, like Carnotaurus.

Spectrovenator had long, curved teeth that were serrated, like steak-knives on the anterior edge, ideal for slicing through meat.  Its lower jaw was relatively slender, implying it had a weaker bite force than its later relatives, possibly specializing in snatching smaller prey or slashing, rather than crushing or overpowering its prey.  The top of its skull, over the snout and eye sockets, possessed a rough texture, implying there could have been bony keratin there in life, possibly some sort of ornamentation for display.  

Skull of Spectrovenator from the paper by Zaher et. al. referenced below.

Spectrovenator also had relatively long and slender legs, implying it would have been a fast runner when alive.  This would have been helpful for a small predator for running down prey, and possibly escaping even larger meat-eaters it shared its environment with.  

References

Hendrickx, Christophe; Mateus, Octávio (2014). "Abelisauridae (Dinosauria: Theropoda) from the Late Jurassic of Portugal and dentition-based phylogeny as a contribution for the identification of isolated theropod teeth". Zootaxa. 3759: 1–74.

Senter, P. (2010). "Vestigial skeletal structures in dinosaurs". Journal of Zoology. 280 (1): 60–71.

Zaher, H.; Pol, D.; Navarro, B.A.; Delcourt, R.; Carvalho, A.B. (October 2020). "An Early Cretaceous theropod dinosaur from Brazil sheds light on the cranial evolution of the Abelisauridae" (PDF). Comptes Rendus Palevol. 19 (6): 101–115.

Meniscoessus: Beast of the Week

 This week we'll be checking out a beast that shows us that mammals were actually thriving, not merely surviving, during the Mesozoic.  Enter Meniscoessus robustus! 

Meniscoessus was a roughly house cat-sized mammal that lived during the late Cretaceous Period, between 70 and 66 million years ago, in what is now North America, specifically Alberta in Canada, and Wyoming, Montana, and South Dakota in the United States.  From nose to rump it measured about two feet (60 cm) and was likely an omnivore when alive.  The genus name translates to "Small Crescent Tooth" in reference to its rodent-like front teeth.  There are actually several species of Meniscoessus that lived during the later Cretaceous period, but today I'll be focusing on Meniscoessus robustus, which is known from many fossil specimens.

Meniscoessus life reconstruction in watercolors by Christopher DiPiazza.

Meniscoessus was a mammal that belonged to a broader group of mammals called the multituberculates.  Multituberculates are a completely extinct group of mammals, despite superficially resembling rodents in many ways.  In fact, this group of mammals was separate from the three major groups of mammals alive today, the monotremes (egg-laying mammals), the monotremes (give birth to tiny underdeveloped young that finish growing in the mother's pouch), and placentals (young develops for a relatively longer period of time inside the uterus, and lack bones in the pelvis allowing for birth of larger babies).  Scientists have speculated that multituberculates likely did not lay eggs, and were more likely similar to placentals and monotremes in how they gave birth, although some research suggests they evolved this trait independently, instead of sharing a common ancestor that gave live birth.  Multituberculates actually survived past the extinction caused by the meteorite that wiped out most of the dinosaurs, not going extinct until the late Eocene, about 35 million years ago.  

Meniscuessus robustus skull on display at the Smithsonian National Museum of Natural History's temporary "Last American Dinosaurs" exhibit.  Photo credit: Eden, Janine and Jim

Meniscoessus had teeth in the front of its mouth that resemble the incisors (front teeth) of modern rodents.  Unlike rodents, who's front teeth are continuously growing, the front teeth of Meniscuessus, however, underwent the same replacement process as most mammal teeth.  They likely used these teeth like rodents do today, to chew through tough plant material, possibly even gnawing the bones of dead animals to access calcium as well.  The back teeth had multiple cusps and the fourth back tooth on the lower jaw was notably larger than the others, a trait common to the multituberculate group.  Yet another unique trait of this kind of mammal is how their jaws would have moved front to back, rather than side to side, as they chewed.  It is possible Meniscuessus was an opportunistic omnivore, not hesitating to eat insects, eggs, and possibly smaller vertebrates.  

When alive, Meniscoessus robustus would have shared its habitat with Tyrannosaurus, Acheroraptor, Pectinodon, Anzu, Triceratops, and Edmontosaurus, to name a few.  It likely would have been preyed upon by the carnivorous dinosaurs, and may have been nocturnal or crepuscular (most active during dawn and dusk) to best avoid crossing paths as many dinosaurs as possible, sleeping and hiding either underground or trees during the day.  In turn, however, Meniscoessus may have eaten dinosaur eggs as a source of protein when it could.  

Meniscuessus, and its family in general, are important in reminding us that although mammals were generally much smaller during the Mesozoic, they were by no means struggling in an evolutionary sense, but rather diversifying and thriving in their own ways.

References

Cifelli, Richard; Eberle, Jaelyn; Lofgren, Donald; Lillegraven, J.; Clemens, William (2004). "Mammalian Biochronology of the Latest Cretaceous in North America"

Cope, E.D. (1882). "Mammalia in the Laramie Formation". American Naturalist. 16 (10): 830–831.

Mao, Fangyuan; Li, Zhiyu; Wang, Zhili; Zhang, Chi; Rich, Thomas; Vickers-Rich, Patricia; Meng, Jin (2024-04-03). "Jurassic shuotheriids show earliest dental diversification of mammaliaforms". Nature. 

Osborn, Henry Fairfield (1891). "A Review of the "Discovery of Cretaceous Mammalia"". The American Naturalist. 25 (295): 595–611.

Weil, Anne (June 1997). "Introduction to Multituberculates: The 'Lost Tribe' of Mammals"

Ouranosaurus: Beast of the Week

 This week we will be checking out a beautiful and unique plant-eating dinosaur.  Get ready for Ouranosaurus nigeriensis!

Ouranosaurus was an ornithopod dinosaur that lived in what is now central Africa, specifically Niger and Camaroon, during the early Cretaceous period, between 121 and 113 million years ago.  It was a plant-eater when alive and adults would have measured about 25 feet (7.6 meters) long from beak to tail.  The genus name, Ouranosaurus, translates to "Courageous Lizard" from a combination of Arabic and Greek languages.  The species name, nigerensis, is in reference to the country, Niger, where its bones were first discovered. 

Ouranosaurus life reconstruction in watercolor by Christopher DiPiazza

Ouranosaurus was an ornithopod dinosaur closely related to Iguanodon and Mantellisaurus.  Like them, it would have been able to walk on all fours or its hind legs.  Its arms were proportionally long and ended with five digits on each hand.  The first digit, like those of Iguanodon, consisted of a thumb-spike, but it was proportionally smaller.  Its second, third, and fourth fingers were likely fused in life and would have supported its weight as it walked on all fours.  Its fifth finger was relatively long, but not flexible.  

Mounted skeleton of Ouranosaurus on display at the Natural History Museum of Venice in Italy. Photo from Felippo Bertozzo's 2017 paper (referenced below).

Ouranosaurus had a long and laterally flattened snout, with its nostril holes facing upwards, a trait similar to some more distantly related hadrosaurs ("duck-billed" dinosaurs).  Ouranosaurus had a broad beak at the front of its mouth, and small closely-spaced teeth, forming what is referred to as a dental battery, in the back of its jaws, for processing plants.  The bones that make up the top of Ouranosaurus' snout were actually unfused to the rest of the skull, suggesting flexibility in life, possibly an adaptation for feeding a certain way.  

Image of Ouranosaurus' skull from Taquet's 1976 paper (listed below) Red arrows pointing to two rounded knobs on the top of its head where the nasal bones begin.  Some think there were small horns or crests there in life while others point out the surface texture does not suggest any such characteristic.

The most notable feature of Ouranosaurus is definitely its extremely tall neural arches, the sections of its vertebra that extend upwards from its back.  This is a feature that has independently evolved in several kinds of dinosaurs and other animals across history.  The most famous example of this feature is probably the theropod dinosaur, Spinosaurus, but many other prehistoric animals as well as many modern reptiles have evolved this trait in varying degrees.  In life, this structure would have formed a meaty sail-like structure on Ouranosaurus', back, drastically expanding the dinosaur's profile.  Experts have suggested such a bold adaptation could have been for display within the species or possibly to help control the Ouranosaurus' body temperature by expanding its surface area to either absorb warmth from the sun or expel excess heat if it became too hot.  

Clockwise from tom left, Sailfin Chameleon (Trioceros montium), Green Basilisk (Basilicus plumifrons), and Sailfin water dragon (Hydrosaurus pustulatus), are all modern lizards that independently of one another evolved tall spines on their vertebra, forming fleshy sails on their backs.

The environment Ouranosaurus lived in would have been a seasonally wet floodplain and it would have coexisted with a number of other dinosaurs, including fellow herbivores, Lurdusaurus and Nigersaurus, as well as the large spinosaur, Suchomimus.  

References

Bertozzo, F.; Dalla Vechia, F.M.; Fabbri, M. (2017). "The Venice specimen of Ouranosaurus nigeriensis (Dinosauria, Ornithopoda)". PeerJ. 5: e3403.

McDonald, A.T.; Kirkland, J.I.; DeBlieux, D.D.; Madsen, S.K.; Cavin, J.; Milner, A.R.C.; Panzarin, L. (2010). Farke, Andrew Allen (ed.). "New Basal Iguanodontians from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs". PLOS ONE. 5 (11): e14075.

Taquet, P. (1976). "Géologie et Paléontologie du Gisement de Gadoufaoua (Aptien du Niger)" (PDF). Cahiers de Paléontologie. Éditions du Centre National de la Recherche Scientifique, Paris: 1–191.

Silesaurus: Beast of the Week

 This week we will be checking out an interesting and a mysterious creature, Silesaurus opolensis!

Silesaurus was a reptile, closely related to dinosaurs, that lived in what is now Poland during the Triassic period, between 237 and 227 million years ago.  Footprints that could have been from Silesaurus, or at the least something very closely related to it, have also been found on the east coast of the United States, including Maryland.  When alive it would have been an omnivore, eating plants and also small animals, like insects.  Adult Silesaurus measured about 7.5 feet (2.3 meters) from beak to tail.  The genus name, translates to "Selesia Reptile" In reference to the region in Poland in which its bones were discovered. 

Life reconstruction of Silesaurus in watercolors by Christopher DiPiazza.

Silesaurus had a narrow snout with a beaklike mouth.  Its teeth were somewhat leaf-shaped and lacked prominent serrations.  Microwear studies on Silesaurus teeth suggest it ate lots of plants but fossilized dung associated with Silesaurus was also found to have beetle shells in it, showing it was more likely an omnivore, eating insects and maybe other small animals on occasion in addition to a mostly plant diet.

Silesaurus skeletal mount on display at Jurapark Science Park, in Poland.

Silesaurus would have walked on all fours most of the time but may have been able to rear up on its hind legs on occasion if it needed.  All four of its limbs were long and slender, making it look like a reptilian greyhound of sorts. (in my opinion at least) It had five very short fingers and claws on each of its hands and its feet each had three toes.  Thanks to tracks we know its thumb and pinky fingers were held off the ground when it walked. The neck was relatively slender and would have been held in a gentle S shape most of the time, a feature similar to dinosaurs. 

Hand (left) and foot (right) tracks from what may have been Silesaurus (or a very close relative) that were found in Maryland, United States, on display at the Natural History Society of Maryland in Baltimore.

Silesaurus is an interesting animal because its exact placement on the tree of life is still highly debated.  That by itself wouldn't be that unusual (loads of fossil animals we don't know the exact natural history of) but what makes Silesaurus' identity especially noteworthy is because the discourse is over whether or not it is a dinosaur. (dinosaurs are popular) The original placement of Silesaurus was that it was an extremely close relative of dinosaurs without quite being a true dinosaur, itself.  It shares a number of dinosaur-like characteristics, like having its limbs positioned directly under it's body, resulting in a fully erect posture while standing and walking.  It also has several diagnostic dinosaur characteristics in its leg and hip bones.  Because of these extreme similarities to dinosaurs, some experts propose that the silesaurid family should be reassigned as true dinosaurs, specifically early ornithiscian dinosaurs, the broad branch of dinosaurs which includes famous taxa like Triceratops, Stegosaurus, and Iguanodon.  This is partially due to the fact that there is very little, if any, ornithiscian dinosaur fossils known from the Triassic period. (unless they're the silesaurids). Other experts, however, are still unconvinced, since Silesaurids still don't possess some other key dinosaur characteristics, like a specialized ridge on the anterior of the humerus for muscle attachments, called the deltopectoral crest, as well hip sockets that form an open window through the pelvis. 

Images of the humerus of Silesaurus (left) from Piechowski's 2020 paper alongside the humerus of early ornithiscian dinosaurs, Heterodontosaurus (middle) and Abrictosaurus (right) from Galton's 2014 paper. Note the Silesaurus bone is lacking the deltopectoral crest at the top of the bone compared to the other two. 

Regardless if Silesaurus was a true dinosaur or not, it very well may have sported feathers of some kind when alive.  This is because both dinosaurs and pterosaurs are believed to have shared a common ancestor with some kind of feathers.  Since silesaurids were at least more closely related to true dinosaurs than to pterosaurs, they likely would have had some kind of feathers as well.  This method of predicting features in fossil groups based on known features in surrounding groups is known as phylogenetic bracketing.  

References

Dzik J (2003). "A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland" (PDF). Journal of Vertebrate Paleontology. 23 (3): 556–574.

Galton, P.M. (2014). "Notes on the postcranial anatomy of the heterodontosaurid dinosaur Heterodontosaurus tucki, a basal ornithischian from the Lower Jurassic of South Africa" (PDF). Revue de Paléobiologie, Genève. 1. 33: 97–141.

Nesbitt, Sterling J. (2011). "The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades". Bulletin of the American Museum of Natural History. 352: 1–292.

Piechowski, Rafał; Tałanda, Mateusz (2020). "The locomotor musculature and posture of the early dinosauriform Silesaurus opolensis provides a new look into the evolution of Dinosauromorpha". Journal of Anatomy. 236 (6): 1044–1100. 

Acrocanthosaurus: Beast of the Week

This week we are checking Acrocanthosaurus atokensis!  Acrocanthosaurus was a meat-eating dinosaur that lived in what is now the United States, including Oklahoma, Texas, Wyoming, and even as far east as Maryland, during the Early Cretaceous Period, between 125 and 100 million years ago.  Acrocanthosaurus measured about thirty eight feet (11.5 meters) long from snout to tail, and was likely the top predator in its environment.  The name Acrocanthosaurus, translates to "High Spine Lizard" and is in reference to this dinosaur's particularly tall neural arches, which gave it a distinctive ridge down its back.

Watercolor reconstruction of Acrocanthosaurus by Christopher DiPiazza.

Acrocanthosaurus' spine was interesting in that we haven't found another theropod dinosaur with anything similar.  It is often compared to the unrelated, Spinosaurus' sail-like structure, but Acrocanthosaurus' runs all the way from the back of the skull to about midway down the tail, rather than just the back like in Spinosaurus.  Acrocanthosaurus also has a close relative from Spain, Concavenator, which has a triangular-shaped hump over its hips, but again, this is concentrated to one area of the spine. The purpose of this ridge-like look is a mystery.  Some believe it would have formed a hump rather than a ridge, like what you might see on a modern bison's skeleton.  (Which also have extended neural arches, but mostly over the shoulders.)   As of now the safe answer could always be that Acrocanthosaurus possessed this ridge down its spine for display purposes to other members of its species.  Maybe they were larger in males than in females and played a role in mate selection?  Perhaps it was significantly shorter in juveniles?  There is still a lot we don't know!

Bison skeleton.  Note the tall neural arches above the shoulders.  The living animal has a hump there.

The skull of Acrocanthosaurus is long, low, and narrow in the front, but the back of the lower jaw become increasingly deep, suggesting there was a lot of muscle there in life, allowing this dinosaur to bite down hard.  The teeth of Acrocanthosaurus were slightly curved and serrated on both the front and back edges.  They were also thicker than the very blade-like teeth of its later relatives, like Giganotosaurus and Cacharodontosaurus, suggesting Acrocanthosaurus was more of a generalist hunter.  This is also further supported by the fact that Acrocanthosaurus' range was so widespread across what is now the United States, which would have consisted of multiple different kinds of habitats, and therefore likely different kinds of prey.

Acrocanthosaurus skull from the North Carolina Museum of Natural Sciences.

The front limbs of Acrocanthosaurus were short, but powerful, and had a decent range of motion to them.  Each hand possessed three hooked claws.  It is difficult to say exactly how Acrocanthosaurus would have utilized these arms since they were so short, but they may have played a part when it was going after large prey, like a sauropod, at close quarters.

Acrocanthosaurus skeletal mount on display at the North Carolina Museum of Natural Sciences.

When alive, Acrocanthosaurus, would have coexisted with many other dinosaurs, including the much smaller predator, Deinonychus, the ornithopod, Tenontosaurus, and a few different kinds of large sauropods, like Sauroposeidon and Astrodon on the east coast.  In fact, there is a dinosaur trackway in Texas consisting of large theropod prints that are believed to have been made by Acrocanthosaurus along with those of a large Sauropod.  These tracks show, since at times the theropod prints overlap the sauropod ones, that those of the theropod were made later, and could have possibly been following the larger herbivore.  At one point some scientists believe the theropod even attacked the sauropod, since the meat-eater tracks skip for a while where they finally intersect. (did it latch onto the side of its intended prey?)  This hypothesis can't be fully proven, however.  Even though no bones from either dinosaur were found with this trackway, it is suggested the predator was Acrocanthosaurus because the size, location, and age of the tracks.

Photograph of the tracks believed to have been from Acrocanthosaurus possibly stalking a sauropod in the field in Texas.  You can now see these tracks in person on display at the American Museum of Natural History in New York City.

That is all for this week!  As always please comment below or on our facebook page!

References

Currie, Philip J.; Carpenter, Kenneth. (2000). "A new specimen of Acrocanthosaurus atokensis (Theropoda, Dinosauria) from the Lower Cretaceous Antlers Formation (Lower Cretaceous, Aptian) of Oklahoma, USA". Geodiversitas 22 (2): 207–246.

Lockley, Martin G. (1991). Tracking Dinosaurs: A New Look at an Ancient World. Cambridge: Cambridge University Press. p. 252pp.

Senter, Phil; Robins, James H. (2005). "Range of motion in the forelimb of the theropod dinosaur Acrocanthosaurus atokensis, and implications for predatory behaviour". Journal of Zoology 266 (3): 307–318.

Stovall, J. Willis; Langston, Wann. (1950). "Acrocanthosaurus atokensis, a new genus and species of Lower Cretaceous Theropoda from Oklahoma". American Midland Naturalist (American Midland Naturalist, Vol. 43, No. 3) 43 (3): 696–728.

Thomas, David A.; Farlow, James O. (1997). "Tracking a dinosaur attack". Scientific American 266 (6): 48–53.

Rhamphorhynchus: Beast of the Week

Rhamphorhynchus was a pterosaur that lived in what is now Germany during the late Jurassic period, between 150 and 148 million years ago.  The adults measured about four feet (1.2 meters) long from beak to tail and had wingspans of about six feet (1.8 meters) wide.  When alive Rhamphorhynchus would have eaten meat, mostly in the form of fish and other marine life.  The genus name translates to "Beak Snout" from Greek.  Rhamphorhynchus muensteri is by far the most well known species of Rhamphorhynchus, known from many individual specimens, however, there is also a species known from a single wing fossil found in what is now Dorset, England, called Rhamphorhynchus etchesi.

Rhamphorhynchus muensteri life reconstruction in watercolors by Christopher DiPiazza.

Rhamphorhynchus belonged to the group of pterosaurs that existed from the Triassic to the early Cretaceous periods, characterized by their proportionally smaller heads short legs, and long narrow tails, called rhamphorhynchoids. (named after Rhamphorhynchus, itself.)  In addition to their wings, which form from flexible skin attached from the tip of their fourth finger (which was extremely long and sturdy) to about the ankle, pterosaurs from this group also sported a membrane between their legs, which likely aided in flight.  Pterosaurs from this group also sported teeth, and Rhamphorhynchus was no exception.  The teeth of Rhamphorhynchus, were particularly long, slightly curved, and interlocked when the jaws were closed.  The tip of its jaws also curved into each other and were beaklike.  This seems to be an adaptation for grasping fish, cephalopods, and other slippery marine prey.  

Rhamphorhynchus skeleton on display at the Royal Ontario Museum that showcases preserved soft tissue, like wing membranes and the tail vane.

At the tip of its tail, Rhamphorhynchus sported a fan-like structure, called a tail vane, which may have helped it steer during flight.  Since Rhamphorhynchus is known from many individuals of different stages of maturity when they died with soft tissue preserved, we know that the shape of this structure changed as the animal aged.  The youngest individuals sported narrow, oval-shaped vane, slightly older individuals had vanes that were more pointed and diamond-like, while in the most mature individuals the tip of the vane flared out into an almost sideways triangle shape.  Because of this change it is possible the vane may have had a role in some sort of communication, like sexual display within the species, as well.  Originally, scientists thought Rhamphorynchus consisted of several more species than the two listed at the top of this post, grouping them based on size and tail vane shape.  More recently, however, it was determined they more likely mostly the same species at different developmental stages of life. 

Rhamphorhynchus fossil with imprints of the wing membranes and tail vane cast on display at the American Museum of Natural History in New York.

Since Rhamphorhynchus is known from many specimens of various sizes, paleontologists were able to study and predict how it would have grown as it aged when alive.  Thanks to a study in 2012, examining various specimens of different stages when they died, it was determined that like many modern reptiles, Rhamphorhynchus would have been able to survive on its own soon after hatching, meaning it was likely able to fly right away, and would have grown rapidly early in life.  This growth spurt seemed to slow down at about three years of age, which also seems to be when they were fully mature.  

The environment that Rhamphorynchus lived in would have been a series of islands with lagoons in a relatively shallow sea.  It likely would have hunted over the water, snatching small fish and other marine animals off the surface of the water while flying.  It would have coexisted with fellow pterosaur, Pterodactylus, and the dinosaurs Archaeopteryx and Compsognathus.  

References

Bennett, S. C. (1995). "A statistical study of Rhamphorhynchus from the Solnhofen Limestone of Germany: Year-classes of a single large species". Journal of Paleontology. 69 (3): 569–580.

Prondvai, E.; Stein, K.; Ősi, A.; Sander, M. P. (2012). Soares, Daphne (ed.). "Life history of Rhamphorhynchus inferred from bone histology and the diversity of pterosaurian growth strategies". PLOS ONE. 7 (2): e31392.

O'Sullivan, Michael; Martill, David M. (June 2015). "Evidence for the presence of Rhamphorhynchus (Pterosauria: Rhamphorhynchinae) in the Kimmeridge Clay of the UK" Proceedings of the Geologists' Association. 126 (3): 390–401.

Mei: Beast of the Week

 This week we will look out a dinosaur that helped solidify the connection between extinct dinosaurs and modern birds.  Let's check out Mei long!

Mei was a theropod dinosaur that lived in what is now China, during the Early Cretaceous period, about 125 million years ago.  It only measured about two feet (60cm) long as an adult, about the size of a modern duck, and would have eaten meat and possibly some plant material when alive.  The genus and species name, Mei long, translates from Chinese to "sleeping dragon" in reference to how this amazing fossil was discovered.  

Life reconstruction of Mei in watercolors by Christopher DiPiazza.

Mei's biggest claim to fame is how it was unearthed in a sleeping position, similar to how modern birds sleep with their head curled back and tucked under one wing and legs folded under the rest of the body.  Not only is this interesting purely because we know what kind of posture a dinosaur had millions of years ago, but it also strongly implies this dinosaur had feathers when alive, since the pose of the head tucked under the arm is so often done by modern birds with the use of wing feathers to shield the eyes.  The minerals between and over the bones of Mei imply the dinosaur was buried alive extremely quickly by ash from an erupting volcano, resulting in a beautifully preserved complete dinosaur skeleton for paleontologists to learn from.  

Cast of the juvenile specimen of Mei on display at the American Museum of Natural History in the seasonal "Dinosaurs Among Us" exhibit in 2014.

Mei was a member of the troodontid family of theropods.  Troodontids were birdlike, typically smaller dinosaurs that appear to have specialized in hunting small animals and possibly some plants when alive.  Troodontids typically had proportionally large eyes, narrow snouts, short arms, and long slender legs tipped with sharp claws, including a retractable second toe claw, which could have been an adaptation for pinning prey.  

Mei is known from two specimens, one young juvenile and one adult.  The adult retains what are generally considered juvenile characteristics, like proportionally short snout and large eyes.  Both specimens also exhibit relatively large nares (nostril holes in the skull) which is unusual for troodontids.  

Photograph of the second discovered specimen of Mei from the 2012 paper by Gao et al., which was an adult when it died. 

The environment that Mei lived in during the early Cretaceous appears to have been heavily forested with rivers and lakes throughout with nearby active volcanoes.  Since Mei was so small, it may have relied on hiding in the underbrush, or possibly even climbing trees to avoid predation from larger predators that shared its environment, like the gliding dinosaur, Changyuraptor, or even the large tyrannoysauroid, Yutyrannus.  Mei also would have crossed paths with dinosaurs like Beipiaosaurus and Tianyulong to name a few more.

References

Gao C, Morschhauser EM, Varricchio DJ, Liu J, Zhao B (2012) A Second Soundly Sleeping Dragon: New Anatomical Details of the Chinese Troodontid Mei long with Implications for Phylogeny and Taphonomy. PLoS ONE 7(9): e45203. https://doi.org/10.1371/journal.pone.0045203

Junchang Lü; Li Xu; Yongqing Liu; Xingliao Zhang; Songhai Jia & Qiang Ji (2010). "A new troodontid (Theropoda: Troodontidae) from the Late Cretaceous of central China, and the radiation of Asian troodontids" (PDF). Acta Palaeontologica Polonica. 55 (3): 381–388.

Xing Xu & Mark A. Norell (2004). "A new troodontid dinosaur from China with avian-like sleeping posture" (PDF). Nature. 431 (7010): 838–841.