Young’s fish reveals the origin and rapid evolution of crustivity in lung fish

Ecological restoration of pioneer Young’s fish (painted by Brian Choo).

Three-dimensional reconstruction model of pioneer Young’s fish specimens. (Courtesy of the research group)

The oldest lungfish stem member pioneer Yang’s fish is one of the typical representatives of primitive flesh-finned fish in the early Devonian Xitun vertebrate fauna, however, in addition to the jaw arch, other anatomical structures related to food habits, such as jaw arch, tongue arch, gill arch and palate structure, are still unknown, so that the origin time and order of occurrence of lungfish shell-eating related features have not been clarified. Two Young’s fish specimens collected in recent fossil excavations have fortunately preserved the tongue arch, gill arch and palate structure, and the relevant research will bring light to the answer to these questions.

Recently, the team of academician Zhu Min of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences published the latest research results on the feeding habits of Yang’s fish in the journal Nature Communications. This progress in-depth study of two specimens preserving anatomical structures related to feeding habits, clearly revealed the crustivity of Young’s fish, and quantitatively analyzed the differentiation time and characteristic evolution rate of lung fish, which not only provided key fossil evidence for revealing the initial changes in the structure related to lung fish and food habits, but also proposed a new hypothesis for the study of the origin of lung fish.

The hope of the early Devonian (about 410 million years ago) is considered the most primitive lungfish, which has acquired typical lungfish shell-eating characteristics, such as well-developed tooth plates and short, stout jaws. The phylogenetic location of Young’s fish lies between the foraminiferous fish (another important branch of the lungfish) and the chia fish, which is more primitive than the kiwi fish. Therefore, Young’s fish is the key to studying the origin of crustivity in lung fish. The two Young’s fish specimens studied in this work were fortunate to preserve the tongue arch, gill arch and palate structure.

Fossil material shows that the adductor fossa in the posterior region of the palate of the Young’s fish is deep and wide, providing a large space for the adductor muscle, and the posterior edge of the palatal bone is nearly vertical, so the Young’s fish should have a strong bite force; its hyoid jaw is short and thick, similar to that of the lungfish; the morphology and arrangement of the teeth on the inner wing bone have been very similar to that of the strange fish and other primitive lungfish, but the Young’s fish still retains the outer wing bone and membranous palate with large teeth, but the teeth are very thick and blunt. Combining these anatomys, the researchers reconstructed the way youngfish eat: mainly using rough and large teeth to crush the prey, while the smaller teeth on the tooth plate play an auxiliary role in immobilization.

Exotic fish and more advanced lungfish have a stronger shell-eating ability. Their palatal bones fuse with the skull, the outer wing bones and membranous palates are lost, and the inner wing bones thicken to form strong tooth plates that can vigorously grind prey. Spoke-finned fish and foraminiferous scales exhibit very different styles: the adductor fossa in the posterior orbital part of the palate bone is short and shallow, the hyoid jaw is rod-shaped and tilted forward, and the large teeth of the upper and lower jaws, outer wing bones, and palatine bones are very sharp. This structure is more suitable for opening the mouth quickly, thereby generating negative pressure to suck the prey into the mouth and swallow it directly into the abdomen, or to pierce and kill the prey with a large tooth before swallowing it.

Lungfish show a high degree of specificity from the very beginning and are very diverse, and are thought to have undergone rapid evolution at the beginning of the evolutionary stage. In this work, the evolution rate of phylogenetic relationships, differentiation time and characteristics of Paleozoic lung fish was analyzed by Bayesian terminal fixed annual quantum, and the results supported the above hypothesis.

Lungfish originated in a window of about 7 million years between the late Silurian period and the end of the Early Devonian Lokhkaov period and underwent rapid evolution, especially features associated with feeding that had a significantly higher rate of evolution than others.

Lungfish have rapidly radiated since their origins, becoming the most diverse meat-finned fish in the Devonian period. In contrast, another major branch of lungfish, the foraminiferous scale fish, was relatively conservative in morphological characteristics and low in diversity, eventually becoming extinct at the end of the Devonian period. The success of lungfish is likely to have been due to the rapid evolution of feeding habits in the early stages of evolution, which opened up new ecological spaces, and the pioneer Young’s fish just recorded the initial state of this key innovation.

The research has been funded by the National Natural Science Foundation of China, the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences, and the High-precision Discipline Construction Project of Geology and Geophysics of the University of Chinese Academy of Sciences. (Source: China Science Daily Cui Xueqin)

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