The First Tetrapod Finds from the Devonian (Upper Famennian) of Latvia

Abstract

Ventastega curonica, from the Upper Famennian Ketleri Formation, is the first tetrapod find from the Upper Devonian of Latvia, and only the fourth adequately represented Devonian tetrapod genus to be described. The taxon is represented by disarticulated cranial and postcranial elements from two localities, Ketleri on the Venta River and Pavari on the Ciecere River. A second tetrapod, represented by a single mandibular fragment, appears to be present at Ketleri. The lower jaw of Ventastega is strikingly primitive in retaining fangs on the coronoid series, but shares many characters with those of other known Devonian tetrapods. Some of these features are interpreted as basal tetrapod synapomorphies; they provide a new data set for the identification of isolated tetrapod jaw fragments, and confirm the (previously disputed) tetrapod status of Metaxygnathus. The upper jaw bones of Ventastega are broadly similar to those of Acanthostega, Ichthyostega and Tulerpeton, as is the narial region. The lateral rostral bone is either very small or absent. A preopercular bone is present in the cheek, and the lacrimal is excluded from the orbit. The palate is closed. Palatine and vomer bear fangs which are set in the marginal tooth row. An isolated iliac blade from Pavari, probably attributable to Ventastega, resembles that of Acanthostega but may not have carried a dorsal process. Two clavicles from Pavari and Ketleri which may also belong to Ventastega are of a typical early tetrapod pattern, similar to Greerpeton but with a broader ventral blade. Non-attributable or doubtfully attributable bones from Ketleri include a probable tetrapod postorbital and a possible limb bone. Ventastega appears to be a tetrapod of the same broad `grade' as Ichthyostega and Acanthostega, but is arguably more primitive than either.

Ventastega curonica and the origin of tetrapod morphology

“…A lamina extending down from the dorsal margin of the squamosal forms part of the lateral wall of this notch. The posterior ramus of the pterygoid is narrow as in Acanthostega, indicating the same type of spiracular architecture 21,27 . The increase in size of the spiracular opening across the transition has been interpreted to indicate increased reliance on air-breathing among the tetrapod stem members [27][28][29] .…”

“…The preserved pectoral girdle of Ventastega comprises interclavicle, clavicle, cleithrum, scapulocoracoid and anocleithrum (Figs 2 and 3d, e); the cleithrum was previously misidentified as an ilium 21 , but a real ilium has since been collected and the discovery of a cleithrum with attached scapulocoracoid confirms its identity. Of these elements the interclavicle, clavicle and anocleithrum resemble those of Acanthostega 6 rather than Ichthyostega.…”

“…Although the braincase of Tiktaalik has not yet been described in detail, the published figures show a basicranial fenestra and a posteriorly Ventastega has pterygoid-pterygoid contact and a tetrapod lower jaw albeit with coronoid fangs 19,21,22 .…”

The origin and early radiation of terrestrial vertebrates

“…Upper Devonian amphibians were diverse, both structurally and in terms of geographical distribution, being known from Scotland (Ahlberg, 1998), central Russia (Lebedev and Coates, 1995), Latvia (Ahlberg et al, 1994) eastern United States (Daeschler et al, 1994) and Australia (Campbell and Bell, 1977). The best known are Ichthyostega and Acanthostega from East Greenland (Clack, 2000).…”

Dynamical, biological and anthropic consequences of equal lunar and solar angular radii

“…By contrast, Ichthyostega [18] and Acanthostega [19], the earliest known tetrapods with fairly complete body fossils (later than Elginopteron, for which only fragmentary remains are known) are associated with a non-marine inland basin (Greenland), a more ambiguous tidal zone. Remains of the approximately contemporaneous tetrapod Ventastega [20] were found in river bed tidal deposits (Latvia). Note that Ventastega, Panderichthys and Livoniana all left remains in the massive Baltic Devonian Delta system described above.…”