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The new amniote is a marine reptile from Hubei Province in China (amniotes include reptiles, birds and mammals). Its most remarkable feature is the number of its digits: the forelimbs bear seven and the hindlimbs have six (Fig. 1). The extra digits on both fore- and hindlimbs are well developed and the bones are arranged normally as distal carpal/tarsal (distal carpals 3 and 4 are fused as a single large carpal in the forelimb of the new amniote), metacarpal/metatarsal and phalanges (see supplementary information).

Figure 1: The Triassic polydactylous amniote, housed at the Shanghai Science and Technology Museum (specimen SSTM 5025).
figure 1

It is represented by the part and counterpart of an almost complete skeleton from which the anterior end of the snout and the tail tip are missing. The specimen was collected from the marine Jialingjiang Formation (late Early Triassic11) near Xunjiansi, Nanzhang County, Hubei Province, China. Taxonomically, SSTM 5025 is referred to Nanchangosauridae, Wang, 1959 of Hupehsuchia Young & Dong, 1972 in Diapsida Gauthier et al., 1988 of Reptilia Gauthier et al., 1988. Further details are available from X.-C. W. Scale bar, 10 cm.

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Ichthyosaurs of the Mesozoic era (250–65 million years ago) had porpoise-like bodies, with dorsal and tail fins and often polydactylous limbs. This polydactyly, however, was quite different from that of the Devonian tetrapods. Modern pandas and moles2, humans2,3 and cats4 occasionally have extra preaxial digits, but these are rarely morphologically or structurally comparable with a normal digit2,3,5.

Almost all polydactyly in tetrapods can be referred to one of three types. In postaxial polydactyly, the extra digits are posterior to digit V, as seen in the Late Devonian Tulerpeton6, some frog individuals and even humans2,3. In preaxial polydactyly, the extra digits are anterior to digit I, as seen in the fore- and hindlimbs of the new amniote, and in the hindlimbs of the Late Devonian Ichthyostega and forelimbs of the Late Devonian Acanthostega1. In bilateral polydactyly, the extra digits are anterior to digit I and posterior to digit V, as seen in the forelimbs of the ophthalmosaurian ichthyosaurs7 and today in some polydactylous Indian families3; the extra digits on the hindlimbs of the Devonian Acanthostega8 also appear to be of this type.

Other types of polydactyly can occur, for example in the forelimbs of many non-ophthalmosaurian ichthyosaurs7. Most occur by interdigital or postaxial phalangeal bifurcation7. Of the known polydactylous tetrapods, the new amniote is the only one that has both fore- and hindlimbs that are preaxially polydactylous, matching the current limb-development model9 (see supplementary information).

The new amniote was a secondarily aquatic reptile and its polydactylous limbs are derived from adaptation to its aquatic life. Its manus and pes are short and wide, and generally resemble those of the Late Devonian Ichthyostega and Acanthostega. They are also comparable in shape to the limb-like paired fins of extant frogfishes8,10. The limbs of this amniote may have functioned in a similar way to those of the Devonian tetrapods or to the paired fins of frogfishes when moving across underwater substrates. In its morphology and way of life, the new amniote provides a good example of evolutionary convergence with the earliest tetrapods, as well as an analogy with frogfishes in vertebrate evolution.