Chinese scholars and overseas collaborators proposed a new mechanism for the origin of vertebrate paired appendages

Fig.
1 Photograph of the orthomorphic specimen of the Lingdong Tujia fish (taken by Peng Lijian) and explanatory sketch (illustrated by Shi Aijuan)

Fig.
2 Computational fluid dynamics simulation results of paired fin folds of flexible tujia fish (courtesy of Humberto G.
Ferrón)

With the support of the National Natural Science Foundation of China (grant numbers: 42130209, 41972006, 42072026), the research team of researcher Gai Zhikun and academician Zhu Min of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, cooperated with British scholars to make new progress
in the field of the origin of vertebrate paired appendages 。 The findings, titled "Galeaspid anatomy and the origin of vertebrate paired appendages," were published
as a cover article in the journal Nature on September 28, 2022.
Article link: https://doi.
org/10.
1038/s41586-022-04897-6
.

Vertebrate paired appendages mainly include the even-fins (pectoral and ventral fins) and quadrupeds (forelimbs and hind limbs)
of aquatic fish.
From the successful landing of meat-finned fish on land 400 million years ago and evolved into quadrupeds, to our humans freeing our hands and becoming the masters of the earth, it is inseparable from the evolution of vertebrate pairs of appendages
.
Therefore, the emergence of paired appendages is of great significance, a hot issue in evolutionary-developmental biology, and a century-old problem
that has plagued the academic community.

In the past 20 years, the research team has traveled through more than 200 field sections of almost all the Silurian Devonian in China that may contain armor fish fossils, and finally found the world's first armor fish fossil with complete body preservation in Xiushan Tujia and Miao Autonomous County in Chongqing, and gave it a very vivid name - Lingdong Tujia fish! The discovery of the Spirit Tujia fish reveals for the first time the full picture
of the body of the armored fish.
It is covered with diamond-shaped oblique rows of scales, has a flat abdomen adapted to benthic life, and has an anus in the center of the abdomen, but no fins; The dorsal body has three dorsal fins, the first two of which are close to each other and have the same forked lower crooked tail
as the allosaurus and the scaled fish.
Particularly surprising is the presence of a pair of ventral fin folds that run through the entire body from the back of the head to the tip of the tail (Figure 1).

This discovery not only provides the most critical fossil evidence for the "fin fold theory" more than 100 years ago, but also is highly consistent
with the data of modern molecular developmental biology.
The cross-confirmation of paleontological fossil evidence and developmental biology makes the study of the origin of paired appendages of vertebrates as the study of jaw origin a classic case
of evolutionary developmental biology.

Conventional theories suggest that paired vertebrate appendages may have originated to improve motor capacity
.
However, the research team found that the ventral fin fold of the dragon fish that runs through the whole body is formed by a series of V-shaped exoskeleton bone units superimposed and combined, without muscle and endoskeleton support, and can not move freely and control direction
like real even-fins and limbs.
So why do vertebrate paired appendages appear in such seemingly non-functional ways? To this end, the research team used computational fluid dynamics simulation experiments to simulate the three-dimensional virtual model of the flexible tujia fish with and without paired fin folds, and analyzed the functional performance
of the ventral fin folds at different water flow speeds and different angles of attack.

The results show that in addition to the 0° angle of attack, the flexible Tujia model with paired fin folds can generate considerable upward force when the water flows through, and can counteract its own gravity without any input of muscle force, thus achieving more efficient swimming (Figure 2).

This principle is very similar
to the hydrofoil or "hydrofoil" of a fighter jet in modern speedboats.
In layman's terms, this pair of ventral fin folds is equivalent to inserting "wings" on the flat belly of the fish, and after using water to flow through the "hydrofoil", the lower surface pressure is greater than the upper surface pressure, thus generating lift, thus achieving efficient seabed cruising
.
This hidden function of the ventral fin fold was very important for these early "armored" cumbersome benthic fish, because without the help of movable even fins, it would be very difficult for them to get out of the water by the propulsion of the tail fin alone
.

The research team proposed a new hypothesis about the origin of vertebrate paired appendages from the functional mechanism, that is, the formation of adult appendages initially only used the water flow to generate lift to counteract its own gravity, rather than enhancing the driving force and control direction, which provided a new perspective
for vertebrate evolution research.