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, so they have not direct modern relatives.  In fact, this group of mammals was separate for 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 that allows 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 had live birth.  Multituberculates actually survived past the extinction caused by the meteorite that wiped out most of the dinosaurs, not going extinct until the lat 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"