New information on the enigmatic tetrapodomorph fishMarsdenichthys longioccipitus(Long, 1985)

Abstract

ABSTRACT-The tetrapodomorph fish Marsdenichthys longioccipitus from the Givetian-Frasnian of Mt. Howitt, Victoria, Australia, occupies an uncertain phylogenetic position. Marsdenichthys has been linked to the base of the Tristichopteridae, as well as to the poorly known Northern Hemisphere form Rhiz odopsis, based on the presence of rounded scales bearing a median boss on the internal surface. However, its affinities to these taxa are ambiguous, due to the incomplete preservation of the two Marsdenichthys specimens initially described. For the first time, we present significant information regarding the cheek and palate of Marsdenichthys, based on the descriptions of two new specimens from Mt. Howitt. New autapomorphies for Marsdenichthys are proposed, including the presence of a bar-like maxilla being approximately equal in depth to the dentary, and a rectangular anterior termination of the lacrimal. Several plesiomorphic features are described from the palate, including rounded vomer morphology lacking a posterior process, a relatively short, broad parasphenoid, and a dermopalatine approximately equal in length to the ectopterygoid. No synapomorphies are shared between Marsdenichthys and tristichopterids. Scale morphology is redescribed, and shown to exhibit concentric rings on the external surface, as in Rhiz odopsis. However, differences in skull morphology, such as the lack of an external opening for the pineal foramen in Rhiz odopsis, suggest that this scale morphology may have evolved independently.

A Gigantic Sarcopterygian (Tetrapodomorph Lobe-Finned Fish) from the Upper Devonian of Gondwana (Eden, New South Wales, Australia)

“…26A), perhaps approaching the condition in Mandageria where the median extrascapular narrows almost to a point [16]. A specimen of Marsdenichthys recently described (NMV 179619) seems not accurately restored ([7]: fig. 3A), as the latex cast shows a central part of the posterior skull margin which evidently overlapped the median extrascapular, with a distinct process and lateral notch where the posterior pitline, which passed across onto the lateral extrascapular, the latter bone evidently overlapping the skull.…”

“…Marsdenichthys is another Australian taxon originally assessed as a primitive sister taxon to tristichopterids [6], and recent revision [7] retains the idea that it may lie outside the group, whereas Snitting [28] placed the Greenland taxon Spodichthys (which shares with Marsdenichthys a lateral extratemporal bone) as the sister group to tristichopterids.…”

“…The most typical and best studied tristichopterid is Eusthenopteron foordi from the Late Devonian (Frasnian) of Miguasha, Canada [2]–[5]. Marsdenichthys Long, 1985 [6] from rocks of similar age in Victoria, Australia, was described as a possible very primitive tristichopterid from the Southern Hemisphere (recently redescribed [7]), and Notorhizodon Young et al, 1992 [8] is a very large sarcopterygian from the Middle Devonian (Givetian [9]) Aztec Siltstone of southern Victoria Land, Antarctica (initially assigned to the family Rhizodontidae; later re-interpreted as a tristichopterid [10]). …”

Upper Devonian osteichthyan remains from the Genoa River, Victoria, Australia

“…This taxon has also been associated with the Rhizodopsidae (Long, 1999), although new material described by Holland et al (2010) shows no affinities to this clade or the Tristichopteridae. In addition, the lower jaw of Marsdenichthys (Holland et al, 2010: fig. 2) lacks the posteroventral embayment of NMV P198470 and some tristichopterid taxa.…”

“…Mt. Howitt, Holland et al, 2010). Thus, isolated elements such as these must be interpreted with caution, because they may belong to different taxa.…”