miércoles, 27 de julio de 2016

How dinosaurs hopped across an ocean

Ancient land bridges may explain how animals navigated breakup of continents

BRIDGE THE GAP  Land crossings may have allowed dinosaurs (such as Supersaurus, shown in this artist’s illustration), 
lizards and early mammals to migrate across the Atlantic Ocean around 150 million years ago, one researcher proposes.

Two land bridges may have allowed dinosaurs to saunter between Europe and North America around 150 million years ago.

The bridges would explain how dinosaurs, mammals and other animals were able to hop from one continent to the other after the Atlantic Ocean formed during the breakup of the Pangaea supercontinent. Some species of Stegosaurus, for instance, appear in the fossil record on both sides of the Atlantic.

Leonidas Brikiatis, an independent biogeographer in Palaio Faliro, Greece, proposes that two strips of land bridged North America and Europe during the late Jurassic and early Cretaceous periods. One bridge spanned from eastern Canada to the Iberian Peninsula, where Spain is today, and lasted from around 154 million to 151 million years ago. The other linked North America and Scandinavia from around 131 million to 129 million years ago, Brikiatis reports in the August Earth-Science Reviews.

The routes allowed dinosaurs to “foil plate tectonics’ plan to break up the world,” says Paul Sereno, a vertebrate paleontologist at the University of Chicago who was not involved in the study, which reviewed recent studies of vertebrates in the fossil record appearing on opposite sides of the Atlantic. “A continent can’t contain a dinosaur; they’ll escape. This work highlights two of the routes they took.”

A LINK BETWEEN WORLDS A land route may have connected eastern Canada 
and the Iberian Peninsula around 150 million years ago. That bridge would have 
allowed animals to hop between Europe and North America
Dinosaurs, including species of Supersaurus and Allosaurus,probably made the transatlantic trek alongside turtles, lizards and early mammals. While the Atlantic Ocean was narrower back then, it was probably too wide to swim across. Brikiatis used the dates of the relocations to establish a potential window of time when the bridges existed and considered potential crossings that might have existed at the time. The best contenders are patches of relatively shallow water called ocean shelves. Tectonic activity could have lifted these shelves above sea level, creating narrow strips of land around 80 to 160 kilometers across, Brikiatis says. Over time, the bridges may have sunk back below the sea.

Those land routes would have been somewhat similar to other ocean crossings, such as the Bering land bridge humans traversed around 23,000 years ago between Asia and North America (SN: 8/22/15, p. 6) and the modern Isthmus of Panama that links North and South America (SN: 5/2/15, p. 10).

The ancient bridge connecting North America and Scandinavia may have coexisted with another land route that connected Europe and what would later become Russia, allowing migrations across much of the world, Brikiatis proposes.

While the routes proposed in the work are plausible, the dates might be off, says Octávio Mateus, a paleontologist at the Universidade Nova de Lisboa in Caparica, Portugal. Species may have migrated earlier than evidenced in the fossil record, he says. “Just because you find them then doesn’t mean they came then. They could have come millions of years before, but just didn’t leave fossils.”

The bridges may also have been more like stepping stones than an unbroken migration highway, says vertebrate paleontologist Anne Schulp of the Naturalis Biodiversity Center in Leiden, the Netherlands. “A narrow body of water is not impenetrable,” he says. “You don’t need a full bridge.”

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