Arundelconodon: Beast of the Week

 This week we will be looking at a unique mammal that lived alongside some of the largest dinosaurs!  Check out Arundelconodon hottoni.  

Arundelconodon was a small mammal that lived in what is now Maryland, in the United States, during the early Cretaceous Period, about 115 million years ago, making it one of the oldest known mammals from the east coast of the US.  It is estimated to have measured about eight to ten inches (20-25cm) long from nose to tail, roughly the same size as a rat.  Based on the teeth it would have been a meat-eater when alive.  It's genus name translates to "Arundel Cone Tooth" after the county in Maryland the location of its fossils is name after.  The species name is in honor of paleontologist, Nicholas Hotton.  

Despite its size Arundelconodon may have attacked larger animals, like this Aquilops, a roughly housecat-sized dinosaur.

Mesozoic mammal fossils are generally extremely difficult to find because the animals they would come from tended to be small and therefore delicate and unlikely to fossilize well.  When they do preserve, it is usually in the form of their teeth, which are made of more durable material, called enamel, than bones.  Luckily teeth are also extremely diagnostic for mammals, so scientists can deduct at least the group of mammals a tooth belongs to most of the time even with no other parts available. Based on teeth and jaws, Arundelconodon was identified as a member of the family of mammals called the triconidontids.  This was a now extinct group of mammals that flourished during the Jurassic and Cretaceous periods that are identified by having three distinct cone-like points on their molars. (The name even translates to "three cone tooth")  Triconidontids would have branched off the mammal family tree relatively early in their history, being more ancient than the first placental mammals but more recent than monotremes.  Because of this it is thought that they birthed live young instead of laying eggs, but little else is currently known about their reproduction, like if they kept their young in a pouch, like marsupials.  Like all mammals, however, they would have produced milk for their young.

Jaw and teeth of Arundelconodon.  This entire fossil is about 1cm long. Image from the Prince George County website.

The teeth of Arundelconodon were notably long compared to the jaw.  The three prongs on the molars were also arranged in a single line, and would have self-sharpened against the teeth of its upper jaw as it opened and closet its mouth.  It is because of this that we are almost certain it was a meat-eater in life.  It may have specialized in hunting small prey, like insects, but it also may even had been able to take down larger prey similar to how weasels and shrews can take large prey today.  It is interesting to imagine Arundelconodon possibly attacking small or baby dinosaurs!

Like most mammals of the Mesozoic, it is likely Arundelconodon was nocturnal or crepuscular (active at dawn and dusk) to avoid predation from predatory dinosaurs, many of which were likely diurnal.  During the day it may have spent time hiding and sleeping underground or maybe even in trees.  It would have needed to look out for predators like the meat-eating dinosaur, Deinonychus and the multitude of different crocodilians that are known from the same fossil site.  The extremely large meat-eater, Acrocanthosaurus, also probably would have posed a threat to Arundelconodon when it was a juvenile. (As an adult it may not have noticed it.) Other dinosaurs it would have coexisted with were the small ceratopsian, Aquilops, the sauropod, Astrodon, and the extremely large armored dinosaur, Priconodon.   

References

Celik, Mélina A.; Phillips, Matthew J. (8 July 2020). "Conflict Resolution for Mesozoic Mammals: Reconciling Phylogenetic Incongruence Among Anatomical Regions". Frontiers in Genetics. 11: 0651.

Cifelli, Richard L.; Lipka, Thomas R.; Schaff, Charles R.; Rowe, Timothy B. (1999). "First Early Cretaceous Mammal from the Eastern Seaboard of the United States". Journal of Vertebrate Paleontology. 19 (2): 199–203.

Luo, Zhe-Xi; Kielan-Jaworowska, Zofia; Cifelli, Richard L. (2002). "In quest for a phylogeny of Mesozoic mammals". Acta Palaeontologica Polonica. 47 (1): 1–78.