The Tilly-bat


Dr. Glenn Jepsen

The Tilly-bat by G. Jepsen, recited by Brigid Christison

A curious beast is the Tillybat
It surely seems odd and quite silly that
With a brain shape so batty,
We’d find glenoids so catty!
You see why we call it a dilly, Pat?
“The midbrain is hilly, —
and further”, says Tilly,
“Look here quick and see
Those colliculi!
It had to squeak, not mew, —
it never walked, it flew!
Jep, don’t be so placid,
It’s not a miacid!”

Glenn “Jep” Jepsen - 1957

Guest art by Greer Stothers (

The “Tillybat” in the poem is the unknown Paleocene mammal whose braincase was preserved as an endocast. Tilly Edinger thought it belonged to a bat, and Glenn “Jep” Jepsen thought it belonged to a miacid (a group that appeared before cats and other carnivores). Another palaeontologist Bryan “Pat” Patterson was invited to mediate but couldn’t be convinced one way or the other. The specimen is still classified as a miacid today.

The natural history of this poem

Most people are familiar with bats, but not everyone has heard of a “miacid” before. Miacids were members of a now-extinct group of carnivorous mammals (Miacidae) that lived from approximately 62 to 34 million years ago. Miacids were closely related to modern carnivorous mammals (Carnivora), and they looked a bit like a cross between modern mongooses and housecats.

Tilly and “Jep” (Glenn Jepsen) tried to figure out the Tilly-Bat’s identity based on two main features mentioned in the poem. The first mentioned are the glenoids. The glenoids (or glenoid fossae) are the main “sockets” in the skull where the mandible (lower jaw) fits into. The shape of the glenoids can vary greatly between different animals based on how their jaw muscles and bones must move and be used for feeding. The glenoids in miacids and bats might then be expected to look different due to differences in their diet, as miacids likely ate more meat and bats commonly eat small insects.

The second feature mentioned used to identify the Tilly-Bat is the midbrain, and specifically the colliculi (plural of colliculus). The midbrain connects the forebrain, which is responsible for sensory information, and the hindbrain, which is responsible for motor information. The colliculi are the paired structures in the midbrain responsible for taking sights and sounds and transforming them into information for the motor system. The auditory colliculi are unique in bats because bats use echolocation to “see,” giving their midbrains a “hilly” appearance like the Tilly-bat has.

Dr. Tilly Edinger

Dr. Tilly Edinger – portrait by Katrin Emery

Tilly Edinger was born to a wealthy Jewish family in Frankfurt in 1897, and grew up with a keen interest in science. In 1921 she earned a doctoral degree in geology, zoology, and psychology from Frankfurt University. She spent the 1920s reinventing comparative fossil brain anatomy, developing the field of paleoneurology, doing science education, and unpaid curatorial work at the Senckenberg Museum. Unpaid because it was illegal to pay Jewish citizens.

When the Nazi regime began restricting the lives of Jewish citizens in the 1930s, Tilly was forced to keep a low profile at the Museum and stay out of public light. She was eventually forced to leave all of her friends and family in Germany, and with the support of her overseas colleagues she was able to obtain a temporary visa in London in 1939 and a Research Associate position at the Harvard university Museum of Comparative Zoology (MCZ) in 1940. With the resources available to her at the MCZ, Tilly’s research on fossil braincases thrived, and in 1963 she became the first woman president of the Society of Vertebrate Paleontology.

Dr. Glenn Jepsen

Glenn Jepsen, YPMN Archives

Glenn Jepsen’s entire academic career was spent at Princeton University, conducting field work and research on behalf of their Geological Museum. On one of his expeditions he became close friends with William Sinclair, whom he would eventually succeed as Curator of Vertebrate Paleontology. Glenn believed that the sole purpose of natural history museums like the one at Yale was to educate their students and preserve the past.

Glenn dedicated his research to the Paleocene (66-56 million years ago) fauna of Polecat Bench, Wyoming, and particularly to the fossil mammals there. He chose to study the mammals from this area because of how important they are in the history of mammal evolution. The Paleocene was the first time that mammals could evolve without dinosaurs around, so they diversified into a lot of the mammal groups we would recognise today, along with many strange animals that have since gone extinct.