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The Tetrapod Caerorhachis Bairdi Holmes and Carroll from the Lower

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The Tetrapod Caerorhachis Bairdi Holmes and Carroll from the Lower Transactionsof the Royal Society of Edinburgh: Earth Sciences, 92, 229±261, 2002 (for 2001) Thetetrapod Caerorhachis bairdi Holmes and Carroll from the Lower Carboniferous of Scotland MarcelloRuta, Andrew R.Milnerand MichaelI. Coates ABSTRACT:The tetrapod Caerorhachisbairdi ,probablyfrom the Pendleian Limestone Coal Groupin the Scottish Midland Valley, is rediagnosed and redescribed, and its a nities are discussed. Caerorachis wasoriginally interpreted as a temnospondylamphibian, based on charactersthat are now regarded as primitive for tetrapods, or of uncertain polarity. Several features of Caerorhachis (e.g.gastrocentrous vertebrae, curved trunk ribs, reduced dorsal iliac blade,L-shaped tarsal intermedium) are observed in certain primitive amniotes. In particular, Caerorhachis resembles ‘anthracosaurs’,generally considered to be among the most primitive of stem-groupamniotes. Thephylogenetic position of Caerorhachis isconsidered in the light of recentlypublished cladistic analysesof Palaeozoictetrapods. Most analyses place Caerorhachis atthe base of, or within, ‘anthra- cosaurs’.When multiple, equally parsimonious solutions are found, its ‘anthracosaur’a nities are shownin at least some trees, and are supported by several informative and, generally, highly consistentcharacters. Alternative phylogenetic placements (e.g. sister taxon to temnospondyls) are usuallyless well corroborated. Ifthefundamental evolutionary split of mostearly tetrapods into stem-group lissamphibians (e.g. temnospondyls)and stem-group amniotes (e.g. ‘anthracosaurs’)is accepted, then the revised interpretationof Caerprhachis shedslight on near-ancestralconditions for Amniota. KEYWORDS: amniotes, ‘anthracosaurs’,characters, lower jaw, palate, relationships, skull. Inthe present work, we reconstructand redescribe Caerorha- nostrilsare similar to those of baphetids(=loxommatids), an chis bairdi,oneof theproblematic fossil tetrapods ‘...fall[ing] enigmaticclade of large Carboniferous tetrapods with key- intothe nebulous area which surrounds the crown group node’ hole-shapedorbits (Beaumont 1977; Beaumont & Smithson (Ahlberg1998, p. 104). Originally described by Holmes & 1998;Milner & Lindsay1998), variously regarded as stem- Carroll(1977), Caerorhachis wasplaced in the temnospondyls, grouptetrapods (e.g. Milner et al.1986;Ahlberg & Milner adiversegroup of LowerCarboniferous to LowerCretaceous 1994)or stem-group amniotes (e.g. Panchen & Smithson tetrapodsoften considered to be ancestralto someor alllissam- 1988;Clack 1998a,b, in press). More recently, Coates (1996) phibians(e.g. Bolt 1979, 1991; Milner 1988, 1990; Milner & hypothesisedthat Caerorhachis mayrepresent a basalplesion Sequeira1994). Several classi® cations of earlytetrapods place inthe amniote stem-group, as indicated by its gastrocentrous Caerorhachis eitherin a basalposition within temnospondyls, vertebrae,transverse pelvic ridge, and L-shaped tarsal interme- oras their plesiomorphic sister-taxon, sometimes in a mono- dium.Indeed, various cranial and postcranial characters of typicfamily Caerorhachidae (e.g. Godfrey et al.1987;Carroll Caerorhachis resemblemorphological conditions found in 1998;Milner 1988, 1990, 1993a). This interpretation has been severalmembers of the ‘anthracosaurs’(e.g. Holmes 1984, acceptedby most workers until recently (e.g. Holmes 2000). 1989;Panchen 1985; Smithson 1986, 1994; Clack 1987a,b, However,some of the characters employed by Holmes & 1994,1998a,b, 2001; Panchen & Smithson1987, 1988; Smith- Carroll(1977) to assign Caerorhachis totemnospondyls are son et al.1994;Lebedev & Coates1995; Coates 1996; Paton broadlydistributed among Palaeozoic tetrapods (e.g. radiating et al. 1999). ‘Anthracosaurs’,used throughout sensu Panchen dermalsculpture), whereas others are of dubious polarity (e.g. &Smithson(1988), include Permo-Carboniferous tetrapods rigidcontact between skull roof and cheek) (Milner & Sequiera placedby various authors among the most basal of stem- 1994;Coates 1996; sections 5.1 and 5.2). A. R.Milner(1980) groupamniotes (e.g. embolomeres and gephyrostegids; see ®rstsuggested that hardly any anatomical feature of Caerorha- alsoSa Ève-SoÈderbergh1934; Carroll 1970; Heaton 1980; Smith- chis canbe regarded as asynapomorphyof this tetrapod with son1986, 1994; Gauthier et al.1988;Clack 1994, 1988a,b; temnospondyls,apart from its rather small and narrow inter- Smithson et al.1994;Coates 1996; Paton et al.1999).In con- pterygoidvacuities. In particular, the presence of large and trast,Laurin’s (1998a± c), Laurin & Reisz’s(1997, 1999), and horseshoe-shapedpleurocentra distinguishes Caerorhachis Laurin et al.’s(2000a,b) novel taxonomic use restricts the fromeven the most primitive temnospondyls (Milner & term ‘anthracosaurs’to a cladeincluding Solenodonsaurus , Sequeira1994, 1998). Milner & Sequeira(1994) discussed diadectomorphsand crown-group amniotes (but see comments Caerorhachis inthe context of ananalysisof characterdistribu- in Coates et al. 2000). tionin basal temnospondyls, and concluded that it should be Givensuch contrasting views on the a nities of Caerorha- removedfrom this group. According to Milner & Sequeira chis,aredescriptionis necessary in order to provide a more (1994), Caerorhachis isbest regarded as atetrapodof uncertain sharplyde® ned picture of this taxon. Re-examination of anities. However, they noted that its palate and internal Caerorhachis bringsinto focus some additional characters 230 MARCELLORUTA ET AL. thatcomplement its original description (Holmes & Carroll toa paraphyletic ‘anthracosaurs’.This placement may re¯ ect 1977),and prompts a revisedinterpretation of several other eitherpaucity of key features linking Caerorhachis unambigu- characters.Recent advances in our understanding of early ouslyto other taxa, or the primitive nature of its lower jaws. tetrapodcomparative anatomy reveal the importance of this Similarresults are obtained by analysing Laurin’s (1998a) fossil:as in the case of the recently described Eucritta (Clack matrix.However, manipulations of Ahlberg & Clack’s(1998) 1998b,2001), Caerorhachis showsan interesting combination andLaurin’s (1998a) data sets, based on taxon and/ orcharacter ofplesiomorphiesand apomorphies that may explain the con- inclusion,place Caerorhachis within ‘anthracosaurs’(see ¯ictsurrounding its interpretation. Therefore, a reappraisalof ‘Anities’ below). thisanimal may cast new light on morphological conditions TheAppendix includes the coding for Caerorhachis relative occurringat the evolutionary split between di Verentlineages toseven recent analyses (Carroll 1995; Coates 1996; Ahlberg ofearly tetrapods. This study examines only some aspects of &Clack1998; Clack 1998a,b; Laurin 1998a; Paton et al. theanatomy of Caerorhachis .Comparisonswith other early 1999;see Appendix). However, only four of theseare discussed, tetrapodspermit a revisedinterpretation of suture patterns namelyCoates (1996), Ahlberg & Clack(1998), Laurin (1998a), andbone proportions on the skull roof and palate. A thorough and Paton et al.(1999).These were chosen according to two redescriptionof the lower jaw is provided in the light of criteria.First, Laurin’s (1998a) and Paton et al.’s(1999) data Ahlberg& Clack’s(1998) recent work on mandibular charac- setssupersede Carroll’s (1995) and Clack’s (1998a,b), respec- tersin early tetrapods. Several characters of the palate, man- tively,in terms of character inclusion and taxon sampling. dibleand postcranial skeleton that di Verfrom those of Therefore,they are likely to provide a morestringent test of temnospondylsare discussed in detail (e.g. spindle-shaped thea nities of Caerorhachis .Second,Ahlberg & Clack’s palatalvacuities, morphology of choanae, overall con® guration (1998)and Laurin’s (1998a) works o Vera novelperspective ofmandibular ramus, shape of basal plate of parasphenoid, onPalaeozoictetrapod phylogeny, by placingmost groups on gastrocentrousvertebrae, transverse pelvic ridge, etc.). New thetetrapod stem. This perspective contrasts with the hypo- featuresof the pelvic girdle and hind-limb are highlighted thesiseddeep evolutionary split of most Palaeozoic tetrapods (e.g.short dorsal iliac blade, pelvic sutures, acetabular shape, intostem-group lissamphibians (e.g. temnospondyls) and ischiotrochanterictuberosity on femur, details of ¯exorsurface stem-groupamniotes (e.g. ‘anthracosaurs’),as advocated by oftibia,details of tarsalelements, etc.). Some characters of the Coates(1996) and Paton et al.(1999).Investigation of con¯ict- axialskeleton and body armour not discussed by Holmes & ingsolutions produced by di Verentmatrices falls outside the Carroll(1977) are mentioned brie¯ y (e.g.haemal spine mor- aimsof this paper, and will bediscussed elsewhere in conjunc- phology,shape of gastralia, etc.). The atlas, axis, fore-limb tionwith a newlarge-scale matrix for early tetrapods. andpectoral girdle do not require a redescription.The new reconstructionof Caerorhachis (Fig. 1A) diVers in some respectsfrom Holmes & Carroll’s(1977) (Fig. 1B). In particu- lar,conservative estimates of lower jaw and presacral column 1.Materialsand methods lengthsindicate that the head is more massive than previously assumed.Stout shape and relatively small size of the femur Caerorhachis ispreserved as acid-etchedmoulds of cranialand (comparedwith mandible length), and the very short tibia postcranialmaterial, probably belonging to asingleindividual and® bula,contribute to the diminutive aspect of the hind- (Holmes& Carroll1977). Silastomer casts of di Verentparts of limbs.Incomplete remains
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