NEW YORK (NYT NEWS SERVICE ).- Snakes, with their sleek bodies and kaleidoscopic diversity, have long entranced humans. But we know very little about the evolutionary past of these legless lizards because of a scarcity of fossils left by snake ancestors that shared the earth with dinosaurs.

That’s why recently excavated snake fossils from Argentina, described in a study published Wednesday in Science Advances, are such a big deal for serpent fans. The intricate fossils, mostly skulls, are nearly 100 million years old and belong to the extinct snake group Najash, which still retained hind legs. The fossils suggest that snakes lost their front legs much earlier than had previously been believed but also held onto their hind legs for millions of years. The find will also help to resolve mysteries over when snakes began their transition to their modern form.

Fernando Garberoglio, who led the research, discovered the most spectacular of these new skull specimens, called MPCA 500, in 2013 when he was an undergraduate student.

“That skull is now the most complete Mesozoic snake skull known and preserves key data on ancient snake anatomy,” said Garberoglio, who is pursuing a doctorate at the Fundación Azara at Universidad Maimónides in Buenos Aires.

The exceptional preservation of the fossils enabled Garberoglio and his colleagues to study long-standing mysteries about snake development, such as the sequence of events that led to their limbless bodies. The team examined the fossils using microcomputed tomography scanning, an imaging technique that allows minute details of fossils to be studied without damaging them.

Scientists have not found fossils of the snake family’s four-legged ancestors, although they are certain these tetrapod forebears existed. The new study suggests that those mysterious proto-snakes probably lost their forelimbs early in snake evolution, at least 170 million years ago. But the back legs stuck around for tens of millions of years.

That means that hind-legged snakes, such as the Najash group, did not represent a short-lived evolutionary phase. Instead, snakes retaining two of their legs were a successful body plan that sufficed for eons until most snakes transitioned into fully limbless slitherers during the latter half of the Cretaceous period.

“‘Snakeness’ is really old, and that’s probably why we don’t have any living representatives of four-legged snakes like we do all of the other lizards,” said Michael Caldwell, a vertebrate paleontologist at the University of Alberta and a co-author of the study.

“Snakes probably were one of the first lizard groups to start experimenting with limblessness, but what’s really intriguing is that they were also very clearly showing the characteristics of their skulls, which are their specialization.”

To that point, MPCA 500, which is a “near perfectly preserved three-dimensional skull,” according to the new study, could resolve a debate among scientists about a crucial skeletal feature in early snakes.

Modern snakes lack a jugal bone, which is analogous to a cheekbone. But it has been unclear whether ancient snakes possessed this cranial feature. MPCA 500 has a large jugal bone, clearing up the mystery. The discovery suggests that unlike the smaller mouths that are characteristic of modern, burrowing snakes, their primitive ancestors had bigger mouths with this cheek structure.

“The absence of the jugal in snakes has long been considered to be a defining ‘shared feature’ of all snakes, fossil and living,” Garberoglio said. “This new 3D specimen of Najash makes it clear that the jugal was present in ancient snakes and subsequently lost in modern snakes.”

The new discoveries also highlight the geological history of La Buitrera Paleontological Area in northern Patagonia, where the fossils were found. The area was once covered by sandy dunes, and ancient snakes and other small organisms that died along their slopes are often immaculately preserved as articulated 3D fossils.

The murky evolutionary history of snakes is still full of secrets. But their extraordinary variety in our modern world — from noodle-width threadsnakes to venomous vipers to behemoth pythons — is a testament to the success of their unique heads and serpentine bodies.

“It’s quite spectacular what they’ve been able to do as completely limbless animals,” Caldwell said. “And they’ve been doing it for a very long time.”