Painted reconstruction by Maggie Newman, of the tetrapods Umzantsia (left) and Tutusius (right) from Waterloo Farm. Tutusius is investigating Godwanascorpio on a log. Serenichthys juveniles are seen in the foreground, amongst the charophyte meadows. Three Groenlandaspis placoderms are cruizing along in the background, and to bottom left is a Bothriolepis placoderm.
In the Mid to Late Devonian, the first tetrapods (four-legged animals) appeared though they are thought to have been primarily aquatic (1). Two new species described from Waterloo Farm, represent the only high palaeolatitude Devonian tetrapods known. This occurrence shows that the presumed tropical origin of tetrapods was based on sampling bias.
A short summary of the significance of the findings: "When we think of Devonian tetrapods, the ancestors of all modern vertebrates, we tend to picture amphibian-like creatures emerging from the water into a wet tropical forest or swamp. Indeed, all previously described specimens of this group have been recovered from the tropics. Gess and Ahlberg now describe two fossil tetrapods from Devonian Antarctica. Thus, the distribution of tetrapods may have been global, which encourages us to rethink the environments in which this important group was shaped." (Gess and Ahlberg, 2018, Science) The remains included a shoulder bone (cleithrum) from a more than one metre long individual, which belonged to a new genus, Tutusius (named after Archbishop Desmond Tutu). A second type of cleithrum was associated with several skull and lower jaw bones, and it was named Umzantsia (meaning 'of the south' in isiXhosa). The shapes and surface patterns on the bones suggest that Tutusius may be likened to fairly advanced Devonian tetrapods like Acanthostega and Ventastega, whilst Umzantsia probably emerged from the base of the digitate tetrapod tree(see infographic below). This is implied by extensive dermal ornament on the cleithrum of Umzantsia. This ornamentation occurs on fish bones which contact the skin, including cleithra, but is lost in the cleithra of almost all tetrapods. The only other exception is the far smaller ornamented area on Parmastega.
The cliethra distinctively bear a posteriorly oriented buttress for the attachment of the enlarged scapulocoracoid, which supported the bulked-up shoulders powering the forelimbs of these early four-legged animals. According to Gess and Ahlberg (2018), both of these tetrapod species would have had four limbs with digits, but likely retained their fish-like tails and predominantly aquatic habits. Although popular conception holds that tetrapod limbs and air-breathing capacities may have enabled access to invertebrate prey on the banks of waterbodies, an alternative hypothesis advanced by Dr. Rob Gess is that the limbs and air-breathing of tetrapods were an adaptation to navigating the shallows of estuaries (and other suitable waterbodies), to prey on small fish (3). References:
1) Clack J. A., (2012). Gaining ground: the origin and evolution of tetrapods. Indiana University Press. 2) Gess, R., & Ahlberg, P. E. (2018). A tetrapod fauna from within the Devonian Antarctic Circle. Science, 360(6393), 1120-1124. 3) Gess, R. W., & Whitfield, A. K. (2020). Estuarine fish and tetrapod evolution: insights from a Late Devonian (Famennian) Gondwanan estuarine lake and a southern African Holocene equivalent. Biological Reviews, 95(4), 865-888. |