Gogonasus, the Tetrapodomorpha and the Paratetrapoda

Holland and Long 2009
found “an unexpected affinity” between the lobefins, Gogonasus Elpistostege and Tiktaalik (Figs. 1–3).

Figure 1. Gogonasus to scale with Tiktaalik and Elpistostege.
Figure 1. Gogonasus to scale with Tiktaalik and Elpistostege.

The authors wrote:
“Within the Tetrapodomorpha, the Late Devonian Gogonasus andrewsae of the Gogo Formation, Gogo, Western Australia, has occupied an uncertain phylogenetic position. Following the description of several well-preserved three-dimensional skulls and pectoral girdles, the discovery of the first complete specimen (NMV P221807) made Gogonasus one of the best-known tetrapodomorph fish. Recent studies of pectoral fin structure and the spiracular opening of Gogonasus have suggested an unexpected affinity with ‘elpistostegalid’ fish such as Tiktaalik. New phylogenetic analyses of the Tetrapodomorpha show a revised phylogenetic
position of Gogonasus as being deeply nested within the Tetrapodomorpha, crownward of
Osteolepis and Megalichthys , but basal to Eusthenopteron + ‘elpistostegalids’.”

Unfortunately,
taxon exclusion mars the Holland and Long study. Several basal tetrapods with tiny legs tested in the large reptile tree (LRT, 1870+ taxa; Figs. 2, 3) were not included in the Holland and Long analysis. Paratetrapods evolved limbs and digits from lobefin ancestors like Gogonasus independently from the clade Tetrapoda which arose from flatter, more derived lobefins, like Pandericthys. Clack 2009 likewise omitted many pertinent taxa as it inaccurately nested Acanthostega and Ichthyostega as basal tetrapods.

Figure 2. Subset of the LRT focusing on basal Tetrapods and their ancestors.
Figure 2. Subset of the LRT focusing on basal Tetrapods and their ancestors.

Sharp-eyed readers will also notice
the flat-headed tetrapod with lateral ribs and tiny digits, Greererpeton, now moves basal to Trypanognathus as the most basal tetrapod.

Figure 3. Members of the Paratetrapoda to scale include Gogonasus, Colosteus, Deltaherpeton and Pholidogaster. These taxa developed legs but did not lead to tetrapods. These taxa are moderately flat-headed. Tetrapoda basal members were much more flat-headed.

Holland and Long 2009 placed great importance
on the spiracular notch and chamber in Gogonasus. The authors report, “It is the radical restructuring of a set of anatomical features that enable a new functional adaptation to the animal, here assumed to be enhanced breathing ability via the enlarged spiracle and modified hyomandibular.”

The LRT pays little attention to the spiracular notch. Rather it scores for 235 other multi-state characters from snout to tail tip. Gogonasus pectoral fins have no derived traits that indicate they are precursors to the four-fingered limbs found in descendant Colosteus and Pholidogaster. Most of the traits that nest paratetrapods with Gogonasus are in the skull. The expansion of the posterior maxilla in Gogonasus is an apomorphy of this taxon not seen in related taxa.

Late Devonian Gogonasus is a late-survivor
of earlier radiations, based on the Early Devonian appearance of the eplistostegid, Brindabellaspis stensioi.

References
Clack JA 2009. The fish-tetrapod transition: New fossils and interpretations. Evo Edu Outreach 2:213–223.
Holland T and Long JA 2009. On the phylogenetic position of Gogonasus andrewsae Long 1985, within the Tetrapodomorpha. Acta Zoologica (Stockholm) 90 (Suppl. 1): 285–296.
Long JA, Barwick RE and Campbell KSW 1997. Osteology and functional morphology of the osteolepiform fish Gogonasus andrewsae Long, 1985, from the Upper Devonian Gogo Formation, Western Australia. Records of the Western Australian Museum Supp. 53:1–89.
Long JA 1985. A new osteolepidid fish from the Upper Devonian Gogo Formation of Western Australia, Recs. Western Australia Mueum 12: 361–377.
Long JA et al. 1997. Osteology and functional morphology of the osteolepiform fish Gogonasus Long, 1985, from the Upper Devonian Gogo Formation, Western Australia. Recs. W. A. Mus. Suppl. 57, 1–89.
Long JA, Young GC, Holland T, Senden TJ and Fitzgerald EMG 2006. An exceptional Devonian fish from Australia sheds light on tetrapod origins. Nature 444(9):199–202.

wiki/Gogonasus

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