From Newsgroup: sci.bio.paleontology

Fossils of Some of the Last Dinosaurs in North America Have a Story to Tell
A trove of specimens from New Mexico may help settle a long-running
argument about the diversity of dinosaurs before their extinction.

A life reconstruction of Alamosaurus sanjuanensis, from the Naashoibito
Member in the San Juan Basin of northwestern New Mexico, at the time of
the dinosaursrCO extinction.Credit...Natalia Jagielska


Listen to this article -+ 4:47 min Learn more
Share full article


By Asher Elbein
Oct. 23, 2025
Scientists have found that a trove of fossils from New Mexico represents
a group of some of the last dinosaurs that lived in North America. The discovery has the potential to settle a long-running paleontological
argument about exactly how the dinosaurs went extinct.

When a six-mile-wide asteroid slammed into the Yucat|in Peninsula 66
million years ago, the age of the dinosaurs came to an apocalyptic end.
For decades, paleontologists have debated whether that extinction
arrived as a bolt from the blue, wiping out a flourishing dynasty; or as
a final stroke, clearing the decks at the late stages of the animalsrCO
long decline.

For much of the 20th century, people studying the end of the period when
the asteroid struck, the Cretaceous, have focused on the fossil-rich
badlands around Montana and Wyoming. The area produced species like Tyrannosaurus rex, Triceratops and the duck-billed Edmontosaurus.

The presence in those regions of just one massive predator and only a
few species of big herbivores represents a less biodiverse landscape
than those in previous eras, said Andrew Flynn, a paleontologist at New
Mexico State University and an author of a paper published Thursday in
the journal Science.

Advertisement

SKIP ADVERTISEMENT

But in New Mexico and Texas, paleontologists in the early 1900s
uncovered a number of dinosaurs unlike those farther north, Dr. Flynn said.

A particularly notable set of fossils was found in the Naashoibito
Member in the De-Na-Zin Wilderness Area, south of Farmington, N.M. It
included the 100-foot-long, 80-ton Alamosaurus, which was rCLamong the
largest sauropods that ever lived, anywhere, at any time,rCY said Steve Brusatte, a paleontologist at the University of Edinburgh and an author
of the paper.

Yet as exciting as the remains were, paleontologists werenrCOt sure how
old they were, Dr. Flynn said. Some suggested that they hailed from a
slightly earlier stage of the Cretaceous period, from which the dinosaur
fossil record is significantly better known.

Beginning in 2013, Dr. Flynn started investigating the age of the New
Mexican remains. He and his colleagues relied on an unusual ally: magnetism.

ImageResearchers working on a mound in a vast desert landscape on a
partly cloudy day.
Researchers collecting paleomagnetic samples from the late Cretaceous at
the Naashoibito Member in the San Juan Basin in New
Mexico.Credit...Steven L. Brusatte
Throughout history, Dr. Flynn said, the polarity of EarthrCOs magnetic
field has flipped about every half-million years. By measuring the
magnetic pole direction in the rocks from the formation rCo and comparing
them with the geochemical age of the crystals in the surrounding
sandstones rCo the team was able to get a direct fix on the formationrCOs age.

It is always wrong to suppose that the fossil record we have accurately
potrays the diversity of life in the Creatasous, or even before then or
even after.

The number of fossils that survive is thin at best.

Advertisement

SKIP ADVERTISEMENT

rCLThe rocks were deposited in the last 380,000 years of the Cretaceous period,rCY Dr. Flynn said, around the same time as the more famous
dinosaur ecosystems of the North. rCLThese are the very last dinosaurs
alive in New Mexico before the asteroid impact.rCY

Dr. Brusatte noted that the dinosaurs of the Southwest and the Badlands
in the North rCLare very different from each other.rCY While the New Mexican population shared animals like Tyrannosaurus and Triceratops, its most
common herbivores were crested hadrosaurs and giant long-necked
sauropods, both totally absent further north.

In these fossils, the researchers saw evidence of radically different
northern and southern dinosaur communities. They therefore argued that
North America still hosted a diverse population of dinosaurs.

The evidence, the team argues, suggests that the asteroid arrived as a
brutal shock to a thriving array of species.

EditorsrCO Picks

This Sheet-Pan Shrimp and Rice Is Extremely Nice

These A.I. Dreamers DonrCOt Fit the Stereotype

10 New Movies Our Critics Are Talking About This Week
SKIP ADVERTISEMENT

Advertisement

SKIP ADVERTISEMENT

rCLDinosaurs were still going strong up to the moment the asteroid hit,rCY
Dr. Brusatte said. rCLThere is no sign they were gradually wasting away to extinction as many paleontologists once believed. It really does seem
like the asteroid fell out of the sky one day and struck down dinosaurs
in their prime.rCY

Michael Benton, a paleontologist at the University of Bristol in England
who did not work on the paper, said the diversity of the New Mexican
dinosaurs shown in the new research does not mean declines werenrCOt
happening elsewhere in North America or other parts of the world.
Western North American dinosaurs seem to have dropped from 43 known
species earlier in the Cretaceous to 30 during the last six million
years of the period, Dr. Benton added, even if there were habitats rich
in different faunas rCLwhere climates were favorable.rCY

But Philip D. Mannion, a paleontologist at University College London who
was not involved in the study, called the analysis rCLrobust.rCY

If it hadnrCOt been for a sudden astronomical accident, he said, rCLthe Age
of Dinosaurs would almost certainly have continued for much longer and
might even still be the case today.rCY
--- Synchronet 3.21b-Linux NewsLink 1.2