A good-sized (muskrat) Cretaceous mammal fossil has been found in
Colorado. It lived in a swampy environment, probably not inhabited by
dinosaurs. The fossil consists of a jaw with some teeth.

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0310948

Abstract

Heleocola piceanus, a new, relatively large metatherian from Upper
Cretaceous (‘Edmontonian’) strata of the Williams Fork Formation in
northwestern Colorado is described, based on a recently discovered jaw
fragment (MWC 9744), in addition to three isolated teeth initially
referred by other studies to Aquiladelphis incus and Glasbius piceanus.
Although sharing several morphologic characters with the Lancian genus
Glasbius, H. piceanus lower molars are considerably larger than those of
Glasbius and differ from the latter in lacking a buccal cingulid,
possessing carnassiform notches on the cristid obliqua and entocristid,
and bearing an entoconulid on m3. To examine the relationship of
Heleocola piceanus to other metatherians, H. piceanus was scored into a
previously existing taxon-character matrix. Our phylogenetic analysis
recovers H. piceanus as the sister taxon to Glasbius, which is
consistent with our morphologic comparisons. H. piceanus represents the
oldest member of the Glasbiidae. A regression equation for predicting
body mass of dentally conservative metatherians that utilizes the length
of m1 estimates the mass of H. piceanus at 855–1170 g, which is
comparable in mass to today’s muskrat (Ondatra zibethicus) and large
relative to other Late Cretaceous pediomyoids. Based upon its molar
morphology, specifically the low inflated cusps, low height differential
between the trigonid and talonid, and near-bunodont morphology, H.
piceanus is interpreted as an omnivore with a plant-dominated diet.

On 10/24/24 8:31 AM, erik simpson wrote:
> A good-sized (muskrat) Cretaceous mammal fossil has been found in
> Colorado.  It lived in a swampy environment, probably not inhabited by
> dinosaurs. The fossil consists of a jaw with some teeth.
>
> https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0310948
>
> Abstract
>
> Heleocola piceanus, a new, relatively large metatherian from Upper
> Cretaceous (‘Edmontonian’) strata of the Williams Fork Formation in
> northwestern Colorado is described, based on a recently discovered jaw
> fragment (MWC 9744), in addition to three isolated teeth initially
> referred by other studies to Aquiladelphis incus and Glasbius piceanus.
> Although sharing several morphologic characters with the Lancian genus
> Glasbius, H. piceanus lower molars are considerably larger than those of
> Glasbius and differ from the latter in lacking a buccal cingulid,
> possessing carnassiform notches on the cristid obliqua and entocristid,
> and bearing an entoconulid on m3. To examine the relationship of
> Heleocola piceanus to other metatherians, H. piceanus was scored into a
> previously existing taxon-character matrix. Our phylogenetic analysis
> recovers H. piceanus as the sister taxon to Glasbius, which is
> consistent with our morphologic comparisons. H. piceanus represents the
> oldest member of the Glasbiidae. A regression equation for predicting
> body mass of dentally conservative metatherians that utilizes the length
> of m1 estimates the mass of H. piceanus at 855–1170 g, which is
> comparable in mass to today’s muskrat (Ondatra zibethicus) and large
> relative to other Late Cretaceous pediomyoids. Based upon its molar
> morphology, specifically the low inflated cusps, low height differential
> between the trigonid and talonid, and near-bunodont morphology, H.
> piceanus is interpreted as an omnivore with a plant-dominated diet.

Of course I turn immediately to the phylogenetic analysis. Do you have
any idea why there were no crown-group taxa in the data set?

On 10/24/24 7:49 PM, John Harshman wrote:
> On 10/24/24 8:31 AM, erik simpson wrote:
>> A good-sized (muskrat) Cretaceous mammal fossil has been found in
>> Colorado.  It lived in a swampy environment, probably not inhabited by
>> dinosaurs. The fossil consists of a jaw with some teeth.
>>
>> https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0310948
>>
>> Abstract
>>
>> Heleocola piceanus, a new, relatively large metatherian from Upper
>> Cretaceous (‘Edmontonian’) strata of the Williams Fork Formation in
>> northwestern Colorado is described, based on a recently discovered jaw
>> fragment (MWC 9744), in addition to three isolated teeth initially
>> referred by other studies to Aquiladelphis incus and Glasbius
>> piceanus. Although sharing several morphologic characters with the
>> Lancian genus Glasbius, H. piceanus lower molars are considerably
>> larger than those of Glasbius and differ from the latter in lacking a
>> buccal cingulid, possessing carnassiform notches on the cristid
>> obliqua and entocristid, and bearing an entoconulid on m3. To examine
>> the relationship of Heleocola piceanus to other metatherians, H.
>> piceanus was scored into a previously existing taxon-character matrix.
>> Our phylogenetic analysis recovers H. piceanus as the sister taxon to
>> Glasbius, which is consistent with our morphologic comparisons. H.
>> piceanus represents the oldest member of the Glasbiidae. A regression
>> equation for predicting body mass of dentally conservative
>> metatherians that utilizes the length of m1 estimates the mass of H.
>> piceanus at 855–1170 g, which is comparable in mass to today’s muskrat
>> (Ondatra zibethicus) and large relative to other Late Cretaceous
>> pediomyoids. Based upon its molar morphology, specifically the low
>> inflated cusps, low height differential between the trigonid and
>> talonid, and near-bunodont morphology, H. piceanus is interpreted as
>> an omnivore with a plant-dominated diet.
>
> Of course I turn immediately to the phylogenetic analysis. Do you have
> any idea why there were no crown-group taxa in the data set?
I hadn't conisidered that. I imagine it's because of the limited
characters in fossil they've got. Still, at least one crown taxon would
have been a good idea.