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“Most primitive” known four-legged animal described

June 26, 2008
World Science staff

A new re­port analyzes fos­sils of what sci­en­tists de­scribe as the most prim­i­tive four-leg­ged an­i­mal known. The find­ings fur­ther fill in the al­ready shrink­ing gaps in in the fos­sil rec­ord of an ep­och­al tran­si­tion in an­i­mal evo­lu­tion—the shift from sea to land, re­search­ers said.

Artist's concept of the head of Ven­ta­stega. (Im­age by Phi­lip Renne)
The ma­jor hole in that rec­ord seems to have been plugged in 2006, with the fos­sil dis­cov­ery of a crea­ture al­most ex­actly mid­way be­tween land an­i­mal and fish. 

Even so, bi­ol­o­gists said a gap re­mained be­tween that crea­ture—known as Tik­taa­lik—and the ear­li­est true tet­ra­pods, or an­i­mals with limbs in place of fins.

Sci­en­tists have now pre­sented a de­scrip­tion of an an­i­mal that would seem to stand mid­way be­tween Tik­taa­lik and the most prim­i­tive tet­ra­pods pre­vi­ously rec­og­nized, in par­tic­u­lar crea­tures by the names Acan­thos­tega and Ich­thy­os­tega.

 The newly stud­ied an­i­mal is called Ven­tas­tega. It was “first de­scribed from frag­men­tary ma­te­ri­al in 1994,” said Per Ahlberg of Upp­sa­la Uni­ver­s­ity, Swe­den, an au­thor of the re­port. “S­ince then, ex­cava­t­ions have pro­duced lots of new su­perbly pre­served fos­sils, al­low­ing us to re­con­struct the whole head, shoul­der gir­dle and part of the pel­vis.”

Ven­tastega was more fish-like than its con­tem­po­rar­ies, such as Acan­thos­tega, ac­cord­ing to Ahlberg’s group: Ven­tas­tega shows a tet­ra­pod-like low­er jaw, but more fish-like fangs. The find­ings al­so point to skull changes dur­ing the tran­si­tion, they said: the eyes and snout be­came larg­er, but the skull over­all be­gan to shrink.
Reconstruction of Ventastega (Courtesy Per Ahlberg)


The study re­veals tet­ra­pod lin­eages di­ver­si­fied much ear­li­er than pre­vi­ously thought, since oth­er con­tem­po­rary fos­sils show quite dif­fer­ent fea­tures, the re­search­ers ar­gued. “The trans­forma­t­ion from paired fins to limbs had al­ready oc­curred” with Ven­tastega, said Ahl­berg, but it seems “dif­fer­ent parts of the body evolved at dif­fer­ent speeds dur­ing the tran­si­tion,” which oc­curred dur­ing the Late De­vo­ni­an per­i­od, about 360 to 380 mil­lion years ago.

Ven­tastega was repre­sented in the new study by re­mains of a skull, brain­case, shoul­der gir­dle and par­tial pel­vis found in Lat­via. The find­ings ap­pear in the June 26 is­sue of the re­search jour­nal Na­ture.

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A new report describes fossils of what researchers call the most primitive four-legged animal known. The findings further fill in the already shrinking gaps in in the fossil record of an epochal transition in animal evolution—the shift from land to sea, scientists said. The major hole in that record seems to have been plugged in 2006, with the fossil discovery of a creature almost exactly midway between land animal and fish. Even so, biologists said a gap remained between that creature—known as Tiktaalik—and the earliest true tetrapods, or animals with limbs in place of fins. Scientists have now presented a description of an animal that would seem to stand midway between Tiktaalik and the most primitive tetrapods previously recognized, in particular creatures by the names Acanthostega and Ichthyostega. The newly studied animal is called Ventastega. It was “first described from fragmentary material in 1994,” said Per Ahlberg of Uppsala University, Sweden, an author of the report. “Since then, excavations have produced lots of new superbly preserved fossils, allowing us to reconstruct the whole head, shoulder girdle and part of the pelvis.” Ventastega was more fish-like than its contemporaries, such as Acanthostega, according to Ahlberg’s group: Ventastega shows a tetrapod-like lower jaw, but more fish-like fangs. The findings also point to skull changes during the transition, they said: the eyes and snout became larger, but the skull overall began to shrink. The study reveals tetrapod lineages diversified much earlier than previously thought, since other contemporary fossils show quite different features, the researchers argued. “The transformation from paired fins to limbs had already occurred” with Ventastega, said Ahlberg, but it seems “different parts of the body evolved at different speeds during the transition,” which occurred during the Late Devonian period, about 360 to 380 million years ago. Ventastega was represented in the new study by remains of a skull, braincase, shoulder girdle and partial pelvis found in Latvia. The findings appear in the June 26 issue of the research journal Nature.