The “history of intercontinental expeditions” behind the fossilized sandhopper

Recently, researchers from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Paleovertebrate Paleontology) named and published a python fossil from the Linxia Basin of Gansu Province – Linxia sand worm. Interestingly, it is far from the living red sand cockroach distributed in Gansu today, but is a close relative of the Kenyan sand cockroach in East Africa. The fossils of sand worms reveal a little-known “intercontinental diffusion history” more than 10 million years ago, and the relevant research was published in the international journal Paleogeography Paleoclimate Paleoecology.

Ecological restoration map of Linxia sand worm and its arid habitat. Painted by Zheng Qiuyang

The sand cockroach from 8 million years ago was rediscovered

The sand cockroach is a short, stubby, slow-moving ground-dwelling snake. They prefer to bury their bodies in the sand, exposing only their eyes and nostrils to escape heat and predators, and wait for opportunities to prey on passing lizards, rodents and small birds.

The existing sand cockroach is mainly distributed in the arid areas of Asia and Africa, while there is only one red sand cockroach in China, which mostly inhabits the deserts, Gobi and other arid environments of Inner Mongolia, Xinjiang, Ningxia and Gansu. The distribution area of red sand cockroach in Gansu includes Baiyin, Wuwei, Zhangye, Jiuquan and other places, and the nearest distribution point is only hundreds of kilometers away from the Linxia Basin.

“The Linxia Basin is located at the intersection of the alpine region of the Qinghai-Tibet Plateau, the eastern monsoon region and the arid area of the northwest, and belongs to a typical continental monsoon climate, which is not suitable for the survival of sand worms in terms of natural environment and climatic conditions.” Dr. Shi Jingyi, the first author and corresponding author of the paper, said that the sandhopper is a typical drought-loving snake, and the discovery of sand cockroach and a series of xerodic fossils in the upper Miocene strata of the Linxia Basin proves that millions of years ago, there may have been an arid desert or grassland environment with insufficient water.

However, the fossils of Linxia sand worms are not easy to study. The fossil was scattered in the surrounding rock at the bottom of a bamboo rat’s model specimen, and the sharp-eyed Shi Jingxian accidentally glanced more when passing by the fossil repair room, before the sand worm, which had been “petrified” for nearly 8 million years, was seen again.

“After years of rain, snow, wind and frost, the fossil has become fragmented, and the 2-centimeter-long maxilla alone has been broken into three pieces, and it took two weeks of sorting and repairing to complete its stitching. The shape of the maxilla of snakes, the trend of changes in the number and size of teeth, and the position and shape of the forehead process and the outer pterygoid process are all very important classification and identification basis. Shi told China Science News.

Fragments of sand worm fossils removed from surrounding rock. Photo courtesy of Shi Jing

Evidence-wide phylogenetic studies

The discovery and restoration of specimens is only the beginning of research. Shi Jingyun said that for the study of snake fossils of the Cenozoic era, the most challenging part is to compare the fossils with the living species related to them, and even incorporate the morphological characteristics of the fossils into the living matrix for phylogenetic analysis. “This requires researchers to observe and compare a large number of skeletal specimens of living species as a basis for their work.”

Therefore, the research team spent a full three years, first combining the existing specimens on hand, the morphological data provided by the literature, and the skeletal specimens of foreign museums to comprehensively compare and study the skeletal morphology of the existing species of the entire sand cockroach family; On this basis, based on morphological data, combined with the results of previous research results, a matrix of a total of 19 species, 98 morphological and ecological characteristics of the genus and related genera was recoded, and combined with 2 mitochondria and 6 nuclear genes, the whole evidence phylogeny study was carried out with fossil age as correction information.

The fossil was revealed to be an extinct new species and named it Linxia sand worm. The Linxia sand cockroach and the red sand cockroach living in China are not “close relatives”, but form a sister group with the Kenyan sand cockroach. Two oviparous living species, the Sahara sandhopper distributed in West Africa and the Arabian sandhopper distributed in Western Asia, also constitute sister groups. Several species of the genus Sand Worm, which are also distributed in Africa, do not constitute sister groups. “This means that the spread of sandhoppers between Africa and Eurasia is not a simple stand-alone proliferation event.” Shi Jing said.

After three “intercontinental expeditions”

So, when and from where did the Linxia sand cockroach move into the Linxia Basin?

After ancestral geographical reconstruction, the researchers speculate that the Old World sarcarp family may have originated in Africa and undergone three migrations across Africa and Eurasia since the late Miocene.

Shi Jingyun introduced that the branches of Sahara and Arabian sand worms are located in relative basal positions, and the two diverged about 13 million years ago. That is, the first spread of the family may have been in the early late Miocene.

The second spread was the divergence of the Late Miocene Linxia sand cockroach from the branch of Kenya sand worm in East Africa, about 8 million years ago. He explained that the Linxia sand cockroach may have spread from eastern Africa along West Asia through the arid area of the northern Qinghai-Tibet Plateau to the northeast corner of the Qinghai-Tibet Plateau, that is, the Linxia area of Gansu Province during a large-scale drought in the Old World, and thrived here. Later, due to the continuous uplift of the Qinghai-Tibet Plateau and the strengthening of the monsoon, the climate environment of the Linxia Basin gradually became wet and cold, resulting in the extinction of the Linxia sand cockroach in this area.

The most recent intercontinental spread of the family was an intraspecific spread of the spotted sandhopper. It is the most widely known distribution of the living sand worm, spanning three continents: Europe, Asia and Africa.

As for the red sandworm, it did not spread from West Asia to northwest China until the late Early Pleistocene 1.4 million years ago, much later than when the Linxia sandhopper came to Asia from Africa.

This study points out that global or regional environmental aridification and the expansion of deserts and grasslands may be the main external factors contributing to the spread of the sand grasshopper family. (Source: Hu Minqi, China Science News)

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