Is Palaeospondylus gunni a Fossil Larval Lungfish? Insights From Neoceratodus forsteri Development JEAN JOSS 1Ã AND ZERINA JOHANSON 1,2 1 Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia 2 MUCEP, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia ABSTRACT The enigmatic Devonian fossil Palaeospondylus gunni was identified as a larval form, metamorphosing into the lungfish Dipterus valenciennesi. Morphological features used to identify P. gunni as a larval lungfish include enlarged cranial ribs, rudimentary limb girdles, and absence of teeth. However, this combination of features does not characterize the extant lungfish Neoceratodus forsteri, even at very young stages, nor early stages of Devonian and younger fossil lungfish. Absence of teeth is problematic because early ontogenetic stages of fossil and living lungfish possess full dentitions including marginal teeth. Also problematic are cranial ribs as a defining character of lungfish, as these also occur in certain actinopterygians. It is argued that Neoceratodus is an obligate neotene (reproductively mature larva), with the implication that metamorphosis was a feature of the ontogeny of early lungfish. Pedomorphic characters have been recognized in Neoceratodus and other post-Devonian lungfish, including large cells and correspondingly large genome size; these latter characters correlate with neoteny in salamanders. Small cells preserved in fossil bone suggest that Devonian lungfish had a smaller genome than post-Devonian lungfish, implying that they were not neotenic. As fossil lungfish cell sizes (and genomes) increased in the late Paleozoic, the diversity of lungfish morphologies decreased, so that taxa like Sagenodus and Conchopoma show morphological similarity to Neoceratodus, marking a point in phylogeny at which metamorphosis was potentially lost. Since ancestral larval characters are retained in neotenic adults, we predict that Devonian larvae should resemble these post-Devonian taxa, a prediction which Palaeospondylus does not fulfill. J. Exp. Zool. (Mol. Dev. Evol.) 308B:163–171, 2007. r 2006 Wiley-Liss, Inc. How to cite this article: Joss J, Johanson Z. 2007. Is Palaeospondylus gunni a fossil larval lungfish? Insights from Neoceratodus forsteri Development. J. Exp. Zool. (Mol. Dev. Evol.) 308B:163–171. The enigmatic fossil vertebrate Palaeospondylus gunni Traquair, 1890 is only known from a single locality: the Middle Devonian Old Red Sandstone, Achanarras Quarry, Caithness, Scotland (Fig. 1). It is found in association with a variety of jawed vertebrates (placoderms, acanthodians, actinop- terygians (ray-finned fish), and sarcopterygians (lobe-finned fish)). This final group includes the lungfish Dipterus valenciennesi, which is among the most abundant vertebrates at the Achannaras Quarry (Forey and Gardiner, ’81; Sarcopterygii; Trewin ’86). P. gunni is known from hundreds of specimens, ranging in size from 5 to 60 mm, and showing no morphological differences from the smallest to the largest size (Sollas and Sollas, ’03; Thomson et al., 2003). Its unusual morphology has led to a wide variety of identifications (summar- ized by Forey and Gardiner, ’81), including a larva of D. valenciennesi (Kerr, ’00; Miller, ’30; Forey and Gardiner, ’81; Thomson et al., 2003; Thomson, 2004). Lungfish characters of P. gunni include large cranial ribs articulating to the independent occipital region of the skull and the pectoral girdle, ring-shaped vertebral centra and a median vomer. These characters, and the absence of overlapping sizes between Palaeospondylus and Published online 26 October 2006 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jez.b.21125. Received 20 February 2006; Accepted 6 June 2006 Ã Correspondence to: J. Joss, Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia. E-mail: jjoss@rna.mq.edu.au r 2006 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 308B:163–171 (2007)