Records of the Western AlIstralzan MlIselllll Supplement No. 57: 169-173 (1999).

New observations on the snouts of rhizodont fishes(Palaeozoic Sarcopterygii)

John A. Long! and Per E. Ahlberg2

1 Western Australian Museum, Francis Street, Perth, WA 6000; email: long@museum.wa.gov.au2 Department of Palaeontology, The Natural History Museum, Cromwell Rd, Kensington, London SW7 5BD, UK

Abstract - Reinvestigation of the snout of the Lower Carboniferous rhizodontfish Barameda decipiens, together with observations on an isolated premaxillaof Strepsodlls sllllroides from Scotland and a new Devonian rhizodont fromCanowindra, Australia, reveals that the rhizodontid snout had only one pairof nostrils rather than two as previously suggested. There are distinct lateralrostra I and tecta I bones surrounding the external naris. The palate of Baramedaalso shows evidence of a choana being present. The protrusion of the lowerjaws in Barameda, previously interpreted as an artefact of preservation, ismatched in the three-dimensional Canowindra material and appears to be areal morphological feature. These data permit a new reconstruction of thehead of Barameda.

INTRODUCTION

The first rhizodontid sarcopterygian to bedescribed from relatively complete cranial remainswas the Early Carboniferous Barameda decipiens,from Mansfield, Victoria, Australia (Long 1989),although a partial head of a juvenile of ?Strepsodllsanclllonamensis from the Early Carboniferous ofFoulden, Scotland, had been reported by Andrews(1985). Long (1989) concluded that Barameda hadtwo pairs of external nares, rather than the singlepair seen in all osteolepiform, elpistostegidpanderichthyid; Schultze 1996) fishes andtetrapods. Andrews (1985) had tentativelyreconstructed ?Strepsodlls with two pairs of externalnares, so Long concluded that this condition wasgeneral for rhizodonts. As two pairs of externalnares is the primitive condition within theOsteichthyes, Long was prompted to placerhizodonts below osteolepiforms and elpistostegidsin the tetrapod stem group.

New work on British Carboniferous rhizodontids,by J.E. Jeffery at the University of Cambridge Museumof Zoology, together with the recent discovery of aprimitive rhizodontid in the Late DevonianCmowindra site in New South Wales Oohanson 1997;Johanson and Ahlberg 1998), prompted us to restudythe snout of Barameda. The original interpretation byLong was based on examination of several latex castsof the skull of NMV P160880 (in the vertebratepalaeontological collection of the Museum of Victoria,Melbourne), in which the exact pattern of the snoutbones was not clear.

We took the opportunity to examine a well­preserved natural mould of a premaxilla ofStrepsodlls sallroides (Binney, 1841) from Airdrie,

Scotland (BMNH P364(2), in the vertebratepalaeontological collection of the Natural HistoryMuseum, London), which clarified the suturalrelationships of this bone to surrounding elements.The information from this specimen and theCanowindra rhizodontid has allowed us toreinterpret the snout pattern and cranialmorphology of NMV P160880, and produce a newskull reconstruction for Barameda (see Figure 4).

THE SNOUT OF RHIZODONTIDS

In Long's (1989) description of Barameda, theanterior nostril was shown as bounded ventrally bythe premaxilla, mediodorsally by the anteriorsupraorbital (= prefrontal in tetrapod terminology),and dorsolaterally by the supraorbitotectal (SO-Teelement of Long 1989, figure 2). The posteriornostril was placed in the anteroventral corner of theorbit, its position indicated by a notch in the dorsalmargin of the lacrimal bone. It was described asbeing bordered anterodorsally by thesupraorbitotectal bone and posterodorsally by theorbit (Long 1989, figures 2, 5). The position of theposterior nostril thus differed from that inferred byAndrews (1985) for ?Strepsodlls, but resembled thatin primitive actinopterygians (Gardiner 1984).However, the material was in part difficult tointerpret. Long (1989: 4) stated that "the position ofthe external nostrils is not clearly seen on NMVP160880 but can be estimated from the notches inthe premaxilla and lacrimal bones".

The anterior naris identified by Long (1989) isindubitably real (Figure 1). However, it is not aspreviously described located in a notch in the

170 J.A. Long, P.E. Ahlberg

postrostrate

externalnaris

nasal

lateralrostral

postrostral

premaxillaeinfraorbitalsensory-line

canal1 cm

pineal -----f--'-.;+-..-l-

maxilla

Figure 1 The snout of Barameda decipiens, NMV P160880. A, latex cast whitened with ammonium chloride showingactual arrangement of bones. B, new interpretation of snout bones. Note the protruding lower jaws shownin A.

premaxilla, but is bounded ventrally by a lateralrostral bone. The recognition of this fact wasprompted by our examination of the premaxilla ofStrepsodus sauroides (Figure 2), which clearly lacks anarial notch but has a large overlap area thatpresumably (by analogy with osteolepiforms)

received the lateral rostral and part of the tectal. Re­examination of NMV P160880 showed that a lateralrostral was present on both sides of the specimen,but that Long (1989) had interpreted its suture withthe premaxilla as a collapse crack over theinfraorbital sensory-line canal. This sensory line

Snouts of rhizodont fishes

Icm

171

Figure 2 Premaxilla of Strepsodlls sallroides, BMNH P364(2) from Airdrie, Scotland, showing contact margins for lateralrostra I and possibly part of the tectal.

nasal

supraorbital

lacrymal

maxilla ---..j.;

lcm coronoid

curved laminafor choana

Figure 3 Sketch based on photoigr,lph of anlamina for the choana.

nn>n"r"r;nn of NMV P160880 showing the vomer having a

canal is in fact still visible as a shallow groovewithin the premaxillae, anterior and ventral to thelateral rostraL The dorsal boundary of the naris isless easy to interpret, as the skull is disrupted inthis region on both sides, but a poorly preservedtectal can be identified dorsal to the naris on the left

side of the skulL The arrangement of circumnarialbones thus seems to be identical to that in

(e.g. ElIstlzenopteron, Glyptopomlls,Jarvik 1966, figure 14). This is also true in the newrhizodont from Canowindra (Johanson and Ahlberg1998).

172

Figure 4 New reconstruction of the head of Barameda decipiens in lateral view,

J.A. Long, P.E. Ahlberg

~ ",:- ,.: ...... : ...'t.:":; '..

.~., .. ': . : ' ..

"

A 'posterior naris' of Barameda was interpreted asbeing present by Long (1989, figure 3), based on asingle notch seen on the right lachrymal. Asmentioned above, its supposed positioncorresponds approximately to that in primitiveactinopterygians (Gardiner 1984). Unlike theanterior naris, which is quite deep and easilyrecognized on both sides of the skull, the 'posteriornaris' notch is shallow and poorly defined. This iseven more true of the supposed 'posterior naris' of?Strepsodus (Andrews 1985),

The new rhizodontid from Canowindra has a wellpreserved narial region, and quite clearly lacks aposterior naris (Johanson and Ahlberg 1998). Giventhe poor quality of evidence for such a nostril inBarameda and ?Strepsodus, we conclude that thesegenera most probably also have a single pair of nares.

Latex casts representing the initial stage ofpreparation on the head of NMV P160880 show thatthe vomers and part of the palate were exposed indorsal view before the skull was fully prepared toreveal the level of the skull roof (by negativepreparation, removing the rock to reveal theimpression of the bones). One of these casts hasbeen illustrated here (Figure 3) to demonstrate thepresence on the vomer of a curved descendinglamina of bone and a gently embayed lateral marginbounded anteriorly by a blunt process. Thisembayed margin is clearly the border of the choana,as seen in the vomers of osteolepiform fishes (e.g.Gogonasus, Long et al., 1997; Eusthenopteron, Jarvik,1980).

In the articulated specimen of Barameda illustratedby Long (1989, figures 1,2) the lower jaws protrudemuch further than the upper jaw margins shown bythe limits of the margins of the premaxillae. Thiswas originally interpreted as a preservationalartefact, but as the same condition can be observedin both flattened and fully three-dimensionalspecimens of the new rhizodontid fromCanowindra (Johanson and Ahlberg 1998) we havereinterpreted it as a real morphological feature. Itmay indicate either a possible close affinity betweenthese two taxa, or be a general feature ofrhizodontids. Functionally, it may be related to alife as a benthic ambush predator; a similarmorphology is seen in the living anglerfish Lophius.Based on the new information on the snout andlower jaws of Barameda, we have attempted a newreconstruction of the head (Figure 4),

DISCUSSION

The Rhizodontida are a monophyletic group,distinguished by a unique pectoral fin morphologywith long unjointed lepidotrichia, and complexoverlaps between cleithrum and clavicle (Andrews1985; Long 1989; Johanson and Ahlberg 1998,Ahlberg and Johanson 1998). In the recent analysisof sarcopterygian interrelationships by Cloutier andAhlberg (1996), based on 140 characters and 158apomorphic character states, the Adams and strictconsensus trees placed the Rhizodontida (based ondata from Barameda and Strepsodus spp,) as the

Snouts of rhizodont fishes

plesiomorphic sister group to the Osteolepidida +Elpistostegalia + Tetrapoda. Within this clade,Cloutier and Ahlberg (1996: 467) noted that "theTetrapodomorpha share a single external nariswhich corresponds to the anterior naris, howeverthe condition is unknown in the Rhizodontida." Thenew infonnation on the snout reinforces this viewthat the Rhizodontida is a group within theTetrapodomorpha. Although they appear moretetrapod-like than hitherto recognized, inpossessing a single pair of nares and a choana, theyare nevertheless probably basal members of theTetrapodomorpha (Johanson and Ahlberg 1998,Ahlberg and Johanson 1998).

ACKNOWLEDGEMENTS

We thank Mr Jonathan Jeffery of the University ofCambridge Museum of Zoology for helpfuldiscussions on the snout and palate of rhizodontidfishes, and Or Zerina Johanson for pennission tocite her unpublished Ph.D. thesis. Mrs OanielleWest is thanked for help labelling the figures.

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Manuscript received 31 March 1998; accepted 19 November1998.