A NEW SPECIMEN OF SAURIPTERUS TAYLORI ...educ.jmu.edu/~davis4mc/download/davis_etal2004a.pdfA NEW...

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Journal of Vertebrate Paleontology 24(1):26–40, March 2004q 2004 by the Society of Vertebrate Paleontology

A NEW SPECIMEN OF SAURIPTERUS TAYLORI (SARCOPTERYGII, OSTEICHTHYES) FROMTHE FAMENNIAN CATSKILL FORMATION OF NORTH AMERICA

MARCUS C. DAVIS1, NEIL SHUBIN1, and EDWARD B. DAESCHLER2

1Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637, [email protected];2Department of Vertebrate Biology, Academy of Natural Sciences of Philadelphia, Philadelphia, Pennsylvania 19103

ABSTRACT—A new specimen of the rhizodontid sarcopterygian, Sauripterus taylori (Sarcopterygii, Osteichthyes),from the Late Devonian (Famennian) Catskill Formation of Pennsylvania consists of a well-preserved right pectoralfin, girdle, and associated scales. Rhizodontid affinity is supported by the unique pattern of overlap between the clavicleand cleithrum, the robustness of the pectoral girdle, the presence of elongate and unjointed lepidotrichia in the pectoralfin, and the pattern of connectivity between the endochondral bones of the pectoral fin. The new specimen of Sauripterusprovides a three-dimensional and articulated view of basal rhizodontid pectoral girdle and fin anatomy and preservesthe first known scapulocoracoid for a Devonian rhizodontid. The glenoid fossa faces posteroventrally, in contrast tothe more posterolaterally facing glenoids of other tetrapodomorph fish. The humerus is short proximodistally andpossesses an expansive ventral flange for the attachment of the adductor, flexor, and rotatory musculature of the fin.Together, these data suggest that the pectoral fins of Sauripterus were optimized for adduction and flexion against thewater column or a solid substrate. To maximize the propulsive force of these motions the distal fin was stiffened bythe presence of eight radials, similar in position to the autopodials of tetrapods. However, unlike the digits of tetrapods,the unit that interacts with the environment in rhizodontids is a combined structure consisting of both dermal andendochondral fin skeletons. As recent phylogenetic hypotheses do not support a sister-group relationship betweenrhizodontids and tetrapods, the similarities between the endoskeletal radials of Sauripterus and the autopodium oftetrapods are independently derived.

INTRODUCTION

Rhizodontids were medium-sized (1–3 m) to enormous (7 m)predatory fish that inhabited freshwater ecosystems from thelate Middle Devonian until the Early Carboniferous. They forma monophyletic group of sarcopterygians and have often beenat the center of discussions of tetrapod origins. Characters sup-porting rhizodontid monophyly include: lepidotrichia with elon-gate, unjointed proximal segments; a depressed posterior flangeon the cleithrum; and the pattern of overlap between the clav-icle and cleithrum. Devonian taxa include Sauripterus from theLate Famennian Catskill Formation of North America and theEast Gondwanan taxa Gooloogongia (from New South Wales,Australia; late Frasnian) and Aztecia (from Victoria Land, Ant-arctica; Givetian). Carboniferous forms include Barameda fromthe Mansfield Formation of Australia and the taxa Rhizodus,Strepsodus, and Screbinodus from the UK. Fragmentary mate-rial of Carboniferous rhizodontids has also been discovered inAustralia, Colombia, Russia, Turkey, and the United States.

Hall described the holotype specimen of Sauripterus in 1843,suggesting that the proximal elements of the fin endoskeletonwere homologous with the humerus, radius, and ulna of tetra-pod forelimbs. Sauripterus became the fossil intermediate parexcellence between fins and limbs until more derived tetrapo-domorph fish (e.g., Eusthenopteron) were adequately describedin the early 20th century (see Patten, 1912, and Watson, 1913).The discovery of more complete material of rhizodontids (An-drews and Westoll, 1970b; Andrews, 1985; Long, 1989; Johan-son and Ahlberg, 1998) and other tetrapodomorph fish (e.g.,Panderichthys; Vorobyeva and Schultze, 1991) have led to newphylogenetic hypotheses that exclude rhizodontids from a sistergroup relationship with tetrapods (Ahlberg & Johanson, 1998;Johanson & Ahlberg, 2001). As such, rhizodontids have effec-tively disappeared from discussions on the origin of the tetrapodlimb. Here we describe a new, articulated specimen of Saurip-terus taylori that offers a detailed look at the anatomy of rhi-

zodontid pectoral fins and provides insights into the evolutionof the pectoral appendage in sarcopterygians and tetrapods.

Abbreviations

Institutional AMNH, American Museum of Natural His-tory, New York, New York, U.S.A.; ANSP, Academy of Nat-ural Sciences of Philadelphia, Philadelphia, Pennsylvania,U.S.A.; MHNM, Musee d’Histoire Naturelle de Miguasha,Nouvelle, Quebec, Canada.

Anatomical apr sco, anterior process of the scapulocora-coid; ar R, articulation for the radius; ar sup, articulation forsuppinator muscle; ar U, articulation for the ulna; ca H, caputhumeri; clm, cleithrum; clm dl, dorsal lamina of cleithrum; clmvl, ventral lamina of cleithrum; clv dl, dorsal lamina of clavicle;clv sp, clavicular spine; clv vl, ventral lamina of clavicle; d ri,dorsal ridge of humerus; dpr sco, dorsal process of the sca-pulocoracoid; dr1-dr8, distal radials 1–8; ect, ectepicondyle;ent, entepicondyle; f ent, foramen of the entepicondyle; f vfl,foramina of the ventral flange; gl, glenoid; H, humerus; i, in-termedium; kl r, anterior keel on the radius; lep, lepidotrichia;m ri, ridge on the posteromedial margin of the cleithrum; mprsco, medial process of the scapulocoracoid; pr pct, pectoralprocess; R, radius; sc, scale; sco, scapulocoracoid; sym clm,symphysis between left and right cleithra; sym clv, symphysisbetween left and right clavicles; U, ulna; ul, ulnare; v ri, ventralridge.

GEOLOGY

ANSP 20581 was collected in 1995 at Powys Curve alongPennsylvania Route 15, 10 miles north of the city of Williams-port, Pennsylvania (N 418 20.8409, W 778 05.6149) (Fig. 1). Thespecimen was found in a block of rock on a talus slope thatwas created when a large road cut was made and the excavatedrock material spread out for the new highway roadbed in 1992–1993. The lithology of the block from which the fossil was

27DAVIS ET AL.—NEW SPECIMEN OF SAURIPTERUS TAYLORI

FIGURE 1. Map of Sauripterus localities in north-central Pennsyl-vania. Stars indicate fossil localities for the holotype specimen, AMNH3341 (1840 site), and the new specimen, ANSP 20581 (1995 site). Scalebar equals 1 km.

extracted is identical to in-situ material from the road cut lessthan 100 meters away. Although more precise stratigraphicplacement is not possible, the entire exposure of the nearby roadcut is mapped as the Sherman Creek Member of the CatskillFormation (Upper Devonian; Famennian) (Faill et al., 1977).

The Sherman Creek Member is characterized by laterallypersistent, several-meter-thick, fining-upward cycles of silt-stones and fine to medium sandstones. These lithologies indi-cate that the Sherman Creek Member formed in a delta plainsetting characterized by low gradient, high sinuosity, shallowchannels near the coastline (Sevon, 1985). Root traces are abun-dant and suggest stable, vegetated interfluves (Harvey, 1998).Other vertebrate fossils found at the Powys Curve locality in-clude the antiarch placoderm Bothriolepis, the porolepiformsarcopterygian Holoptychius, and an unidentified acanthodian.Numerous large scales were also found closely associated withthe rhizodontid pectoral fin (Fig. 2).

The pectoral fin from Powys Curve shares several uniquefeatures with the holotype of Sauripterus taylori Hall, 1843(AMNH 3341). Remarkably, both specimens preserve a pec-toral girdle and the right pectoral fin exposed in ventral view,as well as associated scales. The new specimen, however, offersa wealth of additional data because of its exceptional preser-vation and completeness.

AMNH 3341 was collected in 1840 near Blossburg, TiogaCounty, Pennsylvania (about 40 km north of the recent discov-ery). The exact site and stratigraphic position of the 1840 dis-covery has been unclear for many years. The labels that accom-pany AMNH 3341 and associated blocks read only ‘‘nearBlossburg, PA.’’ Leviton and Aldrich (1992) recount the historyof the discovery and early publications on Sauripterus taylori.They were under the impression that the strata near Blossburgwere restricted to the Huntley Mountain Formation and theoverlying succession of Mississippian and Pennsylvanian strata.The Huntley Mountain Formation spans the Devonian-Missis-sippian boundary, so they concluded that Sauripterus tayloriwas very latest Devonian, or most likely Mississippian (LowerCarboniferous) in age. However, AMNH 3341 is in a red-brown, medium sandstone, which is not typical of the HuntleyMountain Formation or overlying formations.

Examination of the geological quadrangle map for the regionaround Blossburg (Berg and Dodge, 1981; Blossburg Quadran-gle, 7.5 minute series) shows that an abandoned railroad bedheading north from Blossburg along the Tioga River intersectsthe Catskill Formation only a few miles north of Blossburg.This was the Blossburg-Corning Railroad and was traveled byJames Hall in July of 1839 as he sought to clarify some NewYork geological boundary issues in Pennsylvania (Leviton andAldrich, 1992). Our field reconnaissance located a substantialmanmade exposure of Catskill Formation along this railroadbed between Blossburg and Covington (N 418 42.9899, W 77804.8389) (Fig. 1). The lithology of this railroad cut is very sim-ilar to the matrix of the original Sauripterus taylori specimen.It seems very likely that this is the site, or very near the site,where the original specimen was found. This locality suggeststhat the original material of Sauripterus taylori is from the Cat-skill Formation (which is undifferentiated by member desig-nations in this region) and thus Upper Devonian in age. Thefossiliferous nature of the rocks along the railroad bed betweenBlossburg and Covington is confirmed by specimens of cf.Eusthenodon at the American Museum of Natural History withlabels indicating that they were collected ‘‘at the railroad cutbetween Blossburg and Covington’’ (Thomson, 1976). TheAMNH collection also houses numerous Bothriolepis and Hol-optychius specimens in red sandstone from a locality designatedas ‘‘near Blossburg, PA.’’ These specimens probably also camefrom outcrops of the Catskill Formation along the railroad bednorth of Blossburg.

SYSTEMATIC PALEONTOLOGY

Class OSTEICHTHYESSubclass SARCOPTERYGII Romer, 1955

Order RHIZODONTIDA Andrews and Westoll, 1970bFamily SAURIPTERIDAE fam. nov.

Included Genera Sauripterus (Hall, 1843) and Aztecia (Jo-hanson and Ahlberg, 2001).

Diagnosis A clade of rhizodontids exhibiting the followingsynapomorphy: anteriorly expanded pectoral process on the hu-merus; absence of a distinct ‘neck’ separating the caput humerifrom the humeral shaft.

Genus SAURIPTERUS Hall, 1843SAURIPTERUS TAYLORI Hall, 1843

Diagnosis Rhizodontid affinities supported by the follow-ing features: cleithrum overlapping ventral lamina of clavicle(in non-rhizodontid tetrapodomorphs, ventral lamina of clavicleoverlaps cleithrum); laterally expanded ventral lamina of clav-icle and cleithrum; rostrally expanded ventral lamina of cleith-rum (relative to condition seen in other tetrapodomorphs); sca-pulocoracoid sitting dorsal to junction of dorsal and ventral

28 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 24, NO. 1, 2004

FIGURE 2. Sauripterus taylori, ANSP 20581, showing right pectoral fin and girdle in ventral view. Scale bar equals 1 cm.

laminae of cleithrum; lepidotrichia of pectoral fins unjointed formost of their length; branched radials in pectoral fin; no post-axial process on ulnare. Diagnostic characters of Sauripterusare: posteriorly extended cleithral symphysis; no posteriorflange on dorsal lamina of cleithrum (also in Gooloogongia);dorsally expanded process of scapulocoracoid; glenoid fossafacing posteroventrally; expanded ventral ridge on humerus; ra-dius anteroposteriorly broader than ulna (compared to antero-posteriorly narrow radius of other rhizodontids); radius articu-lating distally with more than one radial; ulnare articulatingwith more than three distal radials.

Referred Material Specimens referred to Sauripterus tay-lori: ANSP 20581, well preserved articulated right pectoral finand girdle with associated material (Figs. 2, 3); AMNH 3341,type block of Sauripterus taylori (Hall, 1843) consisting of ar-ticulated right pectoral fin and girdle; AMNH 3340, scales as-sociated with type block. Specimens referred to Sauripterus sp.:ANSP 20980, immature specimen lacking most cranial ele-ments and median fins, preserved as part and counterpart;ANSP 20981, smaller individual consisting of left dentary,paired gulars, pectoral girdle and fins, preserved on reverse sideof ANSP 20980 counterpart block.

DESCRIPTION

Unless otherwise noted, anatomical descriptions refer toANSP 20581.

Pectoral Girdle

A relatively complete right cleithrum and clavicle and frag-ments of the left ventral cleithrum and clavicle are preserved(Fig. 4). Ornamentation on the ventral surfaces consists ofclosely packed sinuous ridges that extend mediolaterally. As inother rhizodontids, the ventral laminae of the cleithra are an-teroposteriorly broader than the dorsal laminae. The right ven-tral lamina is also expanded medially. The overall proportionsof the ventral laminae indicate that the cleithra met along all ormost of their medial margins. Further support for this interpre-tation comes from the articulated shoulder girdle of an imma-ture specimen of Sauripterus sp. (Davis et al., 2001:fig. 4). Aposteriorly extended cleithral symphysis is unique for rhizo-dontids (compare with Rhizodus and a specimen comparable toStrepsodus; Andrews, 1985:figs. 13c, f) and sarcopterygians ingeneral (e.g., Eusthenopteron; Andrews and Westoll, 1970a:fig.2e).


FIGURE 3. Illustration of Sauripterus taylori, ANSP 20581, showing right pectoral fin and right clavicle and cleithrum in ventral view. A bodyscale overlying the ulna-intermedium articulation has been removed for clarity. Abbreviations listed in text. Scale bar equals 1 cm.

The lower half of the dorsal lamina of the cleithrum is or-namented with a continuation of the same ridges present on theventral surfaces. This ornamentation terminates at the level ofthe scapulocoracoid. Above this point, the surface of the cleith-rum is smooth and covered with round sensory pores (Fig. 4A).These pores are of uniform size, being roughly 0.5 mm in di-ameter. Similar sensory pores have been observed in other rhi-zodontids (Jeffery, 2001; Johanson and Ahlberg, 2001) and arelikely a synapomorphy of the Rhizodontida.

The cleithrum widens from a minimum of 36 mm at the levelof the glenoid fossa to a maximum of 45 mm at the dorsalmargin. This ‘waisting’ of the cleithrum at the level of the sca-pulocoracoid is not as pronounced as that of Aztecia (Johansonand Ahlberg, 2001), Barameda (Long, 1989), and other Car-boniferous forms (Andrews and Westoll, 1970b). In contrast,the Late Devonian Gooloogongia lacks this narrowing altogeth-er (Johanson and Ahlberg, 2001), as do non-rhizodontid tetra-podomorphs (e.g., Eusthenopteron; Andrews and Westoll,1970a) and more basal sarcopterygians (e.g., Holoptychius; Jar-vik, 1972).

In Sauripterus, the posterior margin of the cleithrum lacks amedially offset flange, a condition shared with Gooloogongiaand non-rhizodontid sarcopterygians (Johanson and Ahlberg,1998). In contrast, Aztecia possesses a shallow flange along theposterior margin (Johanson and Ahlberg, 2001) and all Carbon-iferous rhizodontids possess a distinct and medially offsetflange (see Johanson et al., 2000, for a recent review of Car-

boniferous taxa). The presence of a medially offset flange onthe posterior margin of the dorsal cleithrum has often been citedas a rhizodontid synapomorphy (Cloutier and Ahlberg, 1996;Jeffery, 2001), but is clearly absent in some Devonian rhizo-dontids.

The branchial margin of the dorsal cleithrum is ornamentedwith a pattern of minute grooves. A similar surface is describedfor the branchial margin of the dorsal cleithra in Eusthenopter-on (Andrews and Westoll, 1970a:fig. 1, ar. mem) and in anundescribed Devonian osteolepidid, ANSP 21159, from theCatskill Formation. This area has been interpreted as the at-tachment point for the soft tissues of the posterior gill chamber,providing a flap to seal the posterior margin of the operculumagainst the body (Jarvik, 1944). This type of opercular flap isseen in the extant coelacanth Latimeria (Millot, 1954:fig. 30).The dorsal margin of the cleithrum is fractured and incomplete.Skeletal elements of the supercleithral series are not preserved,nor are they present in the holotype. However, an articulatedsupercleithral series is present in an immature specimen of Sau-ripterus sp., ANSP 20980 (Davis et al., 2001). In this specimen,a subdermal anocleithrum is present, medially overlapping thedorsal cleithrum and the supracleithrum. This condition isshared with Gooloogongia (Johanson and Ahlberg, 2001) andmore basal sarcopterygians such as porolepiforms (Ahlberg,1989). In more derived tetrapodomorphs, such as tristichopter-ids (Ahlberg and Johanson, 1997), the anocleithrum articulateswith the cleithrum posteriorly and the supracleithrum anteriorly,


FIGURE 4. Shoulder girdle of Sauripterus taylori, ANSP 20581. A, lateral view; B, dorsal view; C, medial view; D, posterior view; E, ventralview. Abbreviations listed in text. Scale bar equals 1 cm.

preventing contact between the cleithrum and supracleithrum.This condition has been cited as a synapomorphy of Osteole-piformes and higher level taxa by Long (1985). A ridge on themedial side of the posterior margin of the cleithrum resemblessimilar ridges described for rhizodontids (e.g., Rhizodus, An-drews and Westoll, 1970b) and tristichopterids (Andrews andWestoll, 1970a), and may be a site of origin for pectoralis mus-culature. This ridge is most prominent in the curvature formedwhere the dorsal and ventral laminae of the cleithrum meet.

The pattern of overlap between clavicle and cleithrum in Sau-ripterus is shared with other rhizodontids and is unique amonggnathostomes. In rhizodontids, the dorsal lamina of the cleith-rum is overlapped laterally by the clavicle and the ventral lam-ina of the clavicle is overlapped laterally by the cleithrum. Incontrast, the clavicles of porolepiforms (e.g., Holoptychius; Jar-vik, 1972) and more derived tetrapodomorphs (e.g., Eusthen-opteron; Andrews and Westoll, 1970a:fig. 1) laterally overlap

the cleithra both dorsally and ventrally and there is no demar-cation between dorsal and ventral halves of the contact. Thecomplex double articulation between the clavicle and cleithrumin rhizodontids is further strengthened by a modified clavicularspine, which is rotated such that the ‘‘medial’’ surface faceslaterally and contacts the anteromedial surface of the dorsalcleithrum. An elongate interclavicle is preserved in associationwith the clavicles of two immature Sauripterus sp. specimens(Davis et al., 2001), although an interclavicle is not associatedwith the clavicles of ANSP 20581 or the holotype specimen.

The right scapulocoracoid is uncrushed and lies in articula-tion with the medial surface of the dorsal cleithrum. Charac-teristic of rhizodontids, the scapulocoracoid sits dorsally on thecleithrum, in contrast to the more ventral position of the sca-pulocoracoid in tristichopterids (e.g., Eusthenopteron; Andrewsand Westoll, 1970a:fig. 2) and porolepiforms (e.g., Glyptolepis;Ahlberg, 1989). The scapulocoracoid exhibits the triradiate at-


tachment to the cleithrum similar to that of other sarcoptery-gians. However, the three buttresses are greatly expanded an-teriorly and dorsally, almost mirroring the shape of the dorsalpart of the cleithrum (Fig. 4C, D). In contrast, the scapulocor-acoid of Glyptolepis is a dorsoventrally constricted, almostwing-like structure projecting from the medial wall of thecleithrum. Although small anterior and dorsal buttresses in-crease the area of contact with the cleithrum, the glenoid fossais left unsupported laterally, as the posterior part of the glenoidbuttress does not contact the cleithrum (Ahlberg, 1989:fig. 3).In Eusthenopteron, the glenoid does not contact the cleithrumdirectly but attaches by three slender buttresses (see Andrewsand Westoll, 1970a:figs. 3, 4). The scapulocoracoids of speci-mens of Eusthenopteron are rarely found uncrushed, leadingAndrews and Westoll (1970a) to speculate that the core of thebone was unossified or, at least, weakly ossified. The glenoidfossa of Sauripterus is oval in shape, in contrast to the roundfossae of Rhizodus and Strepsodus (Andrews, 1985) and the‘pear-shaped’ glenoid fossa of Eusthenopteron (Andrews andWestoll, 1970a:fig. 4). The orientation of the glenoid fossa rel-ative to the dermal pectoral girdle is also unique. In general,the glenoid fossa of tetrapodomorphs (rhizodontids included)faces posterolaterally. However, in Sauripterus the glenoid fac-es posteroventrally (Fig. 4C, D). Lack of preservation in Azteciaand Gooloogongia prevents the comparative assessment of thisfeature in Devonian rhizodontids.

Endochondral Fin Skeleton

The fin endochondral skeleton possesses a very tetrapod-likepattern of articulation between individual elements. A singlefirst mesomere, the humerus, articulates with the shoulder girdleas in all other sarcopterygians (Figs. 5, 6). Distally the humerusarticulates with two elements, the radius and ulna. The ulnaarticulates distally with the ulnare and intermedium. There areeight distal radials present, two articulating with the radius, twowith the intermedium, and four with the ulnare.

Humerus

In overall proportions and shape, the humerus of Sauripteruscompares favorably with that of Aztecia (Johanson and Ahlberg,2001). Both taxa possess the hemispherical caput humeri typicalof rhizodontids, but there is no distinct ‘neck’ separating thecaput humeri from the humeral shaft as in Barameda (Long,1989), Strepsodus (Johanson and Ahlberg, 2001), and Rhizodus(Jeffery, 2001). When viewed proximally, the caput humeri ofSauripterus is oval in shape, being wider anteroposteriorly thandorsoventrally, mirroring the corresponding proportions of theglenoid fossa (Fig. 6D). The body of the humerus is broadanteroposteriorly (Fig. 6A). The proximodistal axis of the boneis compressed when compared to that of other rhizodontids(e.g., Strepsodus; Vorobyeva, 2000) and tristichopterids.

The facets for the radius and ulna are strongly convex andconfluent with each other. There is a small triangle of periostealbone along the ventral margin of the facets that partially dividesthe radial and ulnar sections (Fig. 6B). Confluent radial andulnar facets are also present in Aztecia (Johanson and Ahlberg,2001). The distal facets of other rhizodontids and Eusthenop-teron are separated by a narrow band of unfinished bone or bya distinct change in curvature. Anteriorly, the margin of theradial facet is separated from the pectoral process by a deepnotch. In the articulated fin, this notch provides clearance forthe expanded leading edge of the radius. A small groove sep-arates the postaxial margin of the ulnar condyle from the prox-imal entepicondyle.

One of the unique features of the humerus of Sauripterus isthe disproportionately large ventral flange (Fig. 6). The ventral

flange is composed of three continuous structures: ventrally ex-panded entepicondyle, ventral ridge, and pectoral process.These structures can be homologized with corresponding struc-tures in Rhizodus (Jeffery, 2001), Eusthenopteron (Andrewsand Westoll, 1970a) and basal tetrapods (e.g., Acanthostega;Coates, 1996).

The humerus of Sauripterus possesses an elongate entepi-condyle similar in proportions to that of Aztecia (Johanson andAhlberg, 2001:figs. 15a, 16a). The entepicondyle extends 25mm posteriorly from the ulnar facet. However, the distal tip ofthe entepicondyle was lost during preparation. The actual lengthof the entepicondyle is estimated to be roughly 29 mm (seeDaeschler and Shubin, 1998:figs. 1a, b). The ventral margin ofthe entepicondyle possesses a beveled edge, 5 mm in width,which extends from the ventral ridge to the preserved end ofthe entepicondyle (Fig. 6B, C). The plane of the entepicondyleis approximately 30 degrees offset from the planes of the prox-imal and distal articulations of the humerus. As such, the pre-served tip of the entepicondyle sits several millimeters distal tothe radial/ulnar condyles. An entepicondylar foramen piercesthe base of the entepicondyle ventral to the ulnar condyle. Thedorsal edge of the entepicondyle meets the posterior margin ofthe ulnar condyle in a small raised lip.

The ventral ridge (humeral ridge of Andrews and Westoll,1970a, b) is a thin plate expanded ventrally. The posterior mar-gin of the ridge is continuous with the entepicondyle. A smallarea of fractured bone, situated where the ventral ridge andentepicondyle meet, exaggerates the contour of this area. Theventral ridge is curved proximodistally such that, at its apex,the ridge is at the same proximal level as the radial and ulnarcondyles (Fig. 6C, E). A number of small foramina pierce thebase of the ventral ridge (Fig. 6B, f vfl). A similar series offoramina was described for a specimen of Rhizodus (Andrewsand Westoll, 1970a:pl. 9e) but have not been identified for otherrhizodontid taxa. A nearly identical series of perforations isseen on the humerus of Eusthenopteron (Andrews and Westoll,1970b) and on an undescribed osteolepiform humerus from theCatskill Formation (ANSP 21176).

In Sauripterus, the pectoral process (preaxial flange of Jo-hanson and Ahlberg, 2001) is anteriorly expanded and confluentwith the ventral ridge. The pectoral process of Aztecia is sim-ilarly expanded anteriorly (Johanson and Ahlberg, 2001:fig.16a, pl. fl). In contrast, the pectoral process of Strepsodus (An-drews and Westoll, 1970a:pls. 12N, O) and Rhizodus (Jeffery,2001) is a robust, square-shaped process capped with unfinishedbone.

Dorsal to the pectoral process is a well-formed ridge of bonethat runs orthogonal to the ‘‘shaft’’ and parallel to the radial/ulnar condyles. Previous authors have attempted to homologizethis ridge in rhizodontids with the distinct ectepicondylar, su-pinator, and deltoid processes seen in Eusthenopteron (Andrewsand Westoll, 1970a). In Sauripterus, however, there is no di-vision of the dorsal ridge into distinct processes and the ecte-picondyle is a separate structure. Jeffery (2001) reached a sim-ilar interpretation for Rhizodus. Vorobyeva (2000) denotes aseparate deltoid and supinator process but these appear to besimply two ends of a continuous ridge. In Sauripterus, a deepgroove runs parallel to, and between, the dorsal ridge and theradial/ulnar facet. This structure has been interpreted in Strep-sodus as being a channel housing the supinator muscle (Voro-byeva, 2000:fig. 2, ar. m. sup.), although Jeffery (2001) inter-prets this groove as an artifact of preservational damage. InSauripterus this groove is prominent and clearly not the resultof damage. The ectepicondylar region is a flattened patch ofrugose bone proximodorsal to the ulnar condyle (Fig. 6A) andappears similar to the ectepicondyle described in Aztecia (Jo-hanson and Ahlberg, 2001:fig. 16a). There is no foramen of the


FIGURE 5. Humerus of Sauripterus taylori, ANSP 20581. A, dorsal view; B, distal view; C, ventral view; D, proximal view; E, anterior view;F, posterior view. Scale bar equals 1 cm.


FIGURE 6. Line drawings of the humerus of Sauripterus taylori, ANSP 20581. A, dorsal view; B, distal view; C, ventral view; D, proximalview; E, anterior view; F, posterior view. Abbreviations listed in text. Scale bar equals 1 cm.

ectepicondyle, similar to the condition in Rhizodus (Jeffery,2001). In contrast, an ectepicondylar foramen is present inEusthenopteron and divides the ectepicondyle from the supi-nator process (Andrews and Westoll, 1970a:fig. 10).

Radius

The radius of Sauripterus is unique in both relative size andin its articulation with the humerus and distal radials. The radius


FIGURE 7. Distal fin of Sauripterus taylori, ANSP 20581. A, ventral view (boxed areas refer to location of Figs. 7C and 7D); B, proximalview of radius and ulna (matrix removed for clarity); C, close-up of lepidotrichia and dr1 in distal view; D, close-up of dorsal side. Abbreviationslisted in text. Scale bars equal 1 cm.

forms a broad plate that is both longer and wider than the ulna.When viewed proximally and anteriorly, the radius is seen toconsist of a thickened shaft and a thin, anteriorly expanded keel(Fig. 7B). This contrasts with the rod-shaped radii of Barameda(Long, 1989) and Strepsodus (Andrews and Westoll, 1970b). InSauripterus, the proximal articular surface of the radius is trap-ezoidal (Fig. 7B). The proximal articulation is deeply concaveand fits tightly with the corresponding distal articulation of thehumerus. The outer margin of this articulation is also concavewhen viewed anteriorly and ventrally (Fig. 7A). As a result, thehumero-radial articulation is essentially a ball-and-socket joint.Humero-radial articulations in other tetrapodomorph fish aremildly concavo-convex (i.e., Eusthenopteron; Andrews & Wes-toll, 1970a:fig. 13; and Panderichthys; Vorobyeva, 2000:fig. 3).A series of anterodistally oriented ridges ornaments the ventralsurface of the radius. These processes are, presumably, attach-ment sites for the ventral flexor and rotator musculature. Theanterior margin of the radius is nearly twice as long as theposterior margin due to the step-like facets for the two distalradials. The anterior-most of these radials (dr1) is wider distallythan proximally (Fig. 7A). This radial appears to articulate withthe radius both proximally and obliquely. Though the distalmargin is incomplete, it is unlikely that other elements articu-lated with dr1 because the bone in this area is extremely thin

(,1 mm; see Fig. 7C). The more posterior radial (dr2) articu-lates with the radius more proximally than does dr1. This radialalso contacts the radius anteriorly along roughly a third of itslength. Unlike the simple and flat joint between dr1 and theradius, the articulation between dr2 and the radius appears morerobust. The posterodistal portion of the radius is swollen ven-trally to meet a similarly expanded proximal dr2. The articu-lation also appears to be mildly concavo-convex. At least oneelement lies distal to dr2. This element is only one half thewidth of dr2, suggesting that dr2 articulated with more than oneelement distally. Other rhizodontids appear to possess elementsdistal to the radius as well. An articular facet on the distal radiusis described for Rhizodus and Strepsodus (Jeffery, 2001) andfor Barameda (Long, 1989). However, incomplete preservationof the fins in these taxa makes it difficult to determine howmany radials were present. More derived tetrapodomorph fish,such as Eusthenopteron (Andrews and Westoll, 1970a) andPanderichthys (Vorobyeva, 2000), possess terminal radii. Thus,among tetrapodomorphs, only rhizodontids and tetrapods pos-sess a radius with distal articulations.

Ulna

The ulna is a relatively square bone, being slightly wideranteroposteriorly than proximodistally. The proximal articular


facet is deeply concave (Fig. 7A, B). The anterior margin ofthe facet is unpreserved and the posterior margin tapers to apoint. Distally the ulna possesses facets for the intermediumand ulnare. The facet for the intermedium appears to be convex,though bone in this area is poorly preserved (Fig. 7D). Thefacet for the ulnare is wider than the facet for intermedium. Theproximal ulnare possesses a deeply concave facet, suggestingthat the distal ulna is convex at this articulation.

Ulnare and Intermedium

The ulnare is anteroposteriorly wider distally than proximal-ly, and is of roughly the same surface area as the ulna. Distallythe ulnare articulates with four radials. In the holotype speci-men of Sauripterus taylori (AMNH 3341, see rediagnosis be-low), Gooloogongia (Johanson and Ahlberg, 2001) and possiblyStrepsodus (Jeffery, 2001) the ulnare articulates with three ra-dials distally. The ulnare lacks a posterior flange, or ‘pisiforme’process; this absence is shared with other rhizodontids forwhich an ulnare is known (Andrews and Westoll, 1970b; Long,1989; Johanson and Ahlberg, 1998). A posterior flange is pre-sent in tristichopterids but is lacking in Panderichthys and basaltetrapods. The lack of a posterior flange on the ulnare in rhi-zodontids, Panderichthys, and basal tetrapods may be correlatedwith an increase in the number of radials or the elaboration ofthe autopodial region. In Sauripterus, the intermedium is hour-glass shaped, similar in shape to that of Rhizodus (Jeffery,2001). In contrast, the intermedium of Barameda (Long, 1989:fig. 11, Ra2) is less elongate and ‘unwaisted.’ Distally the in-termedium possesses facets for articulation with two radials, acondition shared with Rhizodus and Barameda.

Distal Radials

There are eight separate radials articulating with the moreproximal elements described above. Two radials articulate withthe radius, two articulate with the intermedium, and the re-maining four articulate with the ulnare. Several of these radialsarticulate distally into a further pair of elements. At least oneset of these secondary segments articulates with a more distalpair of elements (Fig. 7A, distal to dr5). Radial thickness variesfrom roughly 2 mm for more proximal elements to less than0.5 mm for distal elements. Due to lack of preservation distally,it is difficult to determine the total number of elements articu-lating with the eight radials. Proximal and distal articular sur-faces are not concavo-convex, but flat.

Dermal Fin Skeleton

The lepidotrichia are round in cross section and unjointed forall of their preserved length. The fin rays are incompletely pre-served distally, and would have likely extended well beyondthe preserved ‘lobe’ of the fin. When viewed in cross sectionmost of the lepidotrichia appear to be hollow, although theirwalls are thick (Fig. 7C). There is no evidence of lepidotrichiabecoming flattened or ‘‘gutter-shaped’’ distally as has been at-tributed to basal sarcopterygians by Goodrich (1904). Compar-ison with the pectoral fins of immature specimens of Saurip-terus suggest that the lepidotrichia remained unjointed for mostof their total span, with only a distal fringe of jointed ray seg-ments and/or actinotrichia (Davis et al., 2001). It cannot beruled out, however, that lepidotrichia may be remodeled duringontogeny, as specimens of Eusthenopteron appear to remodelthe unjointed lepidotrichia of immature specimens into seg-mented rays (MCD, pers. obs.). Lepidotrichia on the anteriorside of the fin, particularly those overlapping the radius, appearto be more robust than lepidotrichia closer to the posterior mar-gin. Excavation of the dorsal side of the new specimen revealsthat lepidotrichia extend proximally to the articulation between

the ulna and intermedium (Fig. 7D). More anterior lepidotrichiaextend even farther proximally onto the radius. As a result, allendochondral elements, except for the humerus and ulna, liewithin a sandwich of lepidotrichia.

Scales

A number of large scales is associated with ANSP 20581.The scales range from roughly 44 mm to 52 mm in diameterand are cycloid to oblong in shape. A scale that appears to bepart of the squamation of the fin lobe overlaps the ulna-inter-medium articulation ventrally (Fig. 7A, sc). Part of a thickenedfulcral scale sits anterior to the radius. In general morphology,all scales compare favorably to the body scales described forStrepsodus and Rhizodus (Andrews, 1985) and immature spec-imens of Sauripterus (Davis et al., 2001). Scales show a numberof concentric growth rings surrounding a central scale primor-dium (base). As in other rhizodontid taxa, and tristichopterids,a fusiform-shaped boss is present on the internal side of thescales. Some of the scales associated with the fin lobe are mostlikely displaced body scales. This interpretation is supported bythe presence of secondary tubules of the lateral line system onsome of the scales.

Reassessment of the Holotype

The holotype specimen of Sauripterus taylori consists of anisolated right pectoral fin and cleithrum (Fig. 8). The cleithrumis preserved in medial view and appears complete dorsally. Theventral lamina of the cleithrum is incomplete. The scapulocor-acoid is absent, although a strip of bone along the posteriormargin of the cleithrum, next to the proximal humerus, may bea crushed portion of the glenoid (Fig. 8). The position of thescapulocoracoid can be inferred from the rugosities that spanthe anteroposterior width of the cleithrum. These rugosities ex-tend anteriorly and dorsally, comparing favorably with the an-teriorly and dorsally expanded buttresses of the scapulocoracoidin ANSP 20581.

The humerus of the holotype has been a source of confusion.As preserved, the humerus appears to possess a large, albeitbroken, anterior process. Such an anterior process on the hu-merus would be unique among sarcopterygians. Furthermore,the humerus appears to lack an entepicondyle, in sharp contrastto the well-developed entepicondyles of other rhizodontids andEusthenopteron (Andrews and Westoll, 1970a:fig. 10). Uponcloser inspection, it is clear that the radius, ulna, and distal finhave been dislocated from the humerus and rotated such thatthe ventral fin sits in the same plane as the medial face of thecleithrum. The humerus appears to have maintained its life po-sition relative to the cleithrum. As a result, much of the hu-merus would have sat orthogonal to the fin, and either did notpreserve or was lost in the counterpart block to AMNH 3341,which was never recovered. What is preserved of the humerusis a ‘section’ through the caput humeri and ulnar facet, and partof the proximal face of the pectoral process and entepicondyle(compare with ANSP 20581, Figs. 5, 6). Thus, AMNH 3341does preserve an entepicondyle and this is not a ‘preaxial’ ex-tension of the humerus as has been proposed by many (An-drews and Westoll, 1970b; Johanson and Ahlberg, 2001).

As in ANSP 20581, the radius is anteriorly expanded into apaddle. Two prominent ridges on the ventral surface of the ra-dius, positioned anterodistally, are identical to those of ANSP20581. There are at least two radials articulating distally withthe radius. Lack of preservation makes it difficult to determinewhether the radius articulated with a third radial anteriorly orwhether the radius possessed an anterodistal extension as inANSP 20581 (Fig. 7A, dr1). The intermedium articulates withtwo radials, and these, in turn, appear to articulate with distalelements. Three radials articulate with the ulnare. However, the


FIGURE 8. The holotype specimen of Sauripterus taylori, AMNH 3341, a right cleithrum and pectoral fin. The cleithrum is preserved in medialview. The humerus is preserved in oblique posterior view with portions of the ventral ridge and entepicondyle crushed into the plane of cleavage.The distal fin is preserved in ventral view. The exact number of distal radials present in the holotype specimen is unclear due to lack of preservationalong the anterior and posterior margins of the fin. Abbreviations listed in text. Scale bar equals 1 cm.

posterior margins of the ulna and ulnare are broken, and it ap-pears that these elements may have continued posteriorly intothe missing counterpart block. Likewise, lepidotrichia along thetrailing edge of the fin may have expanded farther posteriorlyand a fourth radial may have articulated with the ulnare as inANSP 20581. The distal ends of the lepidotrichia preservednear the ulnare are expanded, suggesting that the lepidotrichiawere segmented distally. Thickened and curved lepidotrichia sitat the anterior margin of the radius along with what is either abroken portion of the distal entepicondyle or a fulcral scale.This reanalysis of the holotype specimen suggests that there areno distinctive characters of the new specimen, ANSP 20581, tojustify erection of a new species designation. The primary dif-ferences between ANSP 20581 and the holotype specimen areinterpreted as relating to differences in preservation.

FUNCTION

Morphometric analyses of extinct and extant sarcopterygianssuggest that fin to body length ratios remain the same through-out ontogeny. For example, pectoral and pelvic fin length in-creases isometrically with body length in embryos and adultsof the coelacanth Latimeria (McAllister and Smith, 1978). Like-wise, Schultze (1984) found that fin position along the bodyremained invariant with increasing body length in an ontoge-

netic series of Eusthenopteron foordi. Individuals assumed tobe immature (small size, lesser degree of ossification) possessedthe same relative fin positions on the body as adults (larger,more completely ossified specimens). Based on these observa-tions, and the lack of any discernable larval characteristics invery small specimens (,3 cm in length), Cote et al. (2002)consider Eusthenopteron to be a direct developer. A series ofEusthenopteron from the MHNM and AMNH collections (sizerange 29 cm to 300 cm) reveals that pectoral fin length alsoincreases isometrically with total body length (MCD, unpubl.data). Furthermore, the ratio of fin endoskeleton to total finlength remained invariant across all sizes. When placed in aphylogenetic context, these data suggest that fin to body lengthisometry is the primitive condition for tetrapodomorphs. To thisend, total body length for the adult specimens of Sauripterustaylori can be estimated by ‘‘scaling up’’ immature specimensof Sauripterus described in Davis et al. (2001). Total bodylength for the immature specimen ANSP 20981 is 254 mmwhile total pectoral fin length is 62 mm. These values result ina total fin length of (260–270 mm) for ANSP 20581. The re-sulting total estimated adult body length for Sauripterus tayloriis between 1.0 and 1.3 meters.

At an estimated 25% of total body length, the pectoral finsof Sauripterus are disproportionately large for a sarcopterygian.


Most tetrapodomorph fishes possess pectoral fins that areroughly 15% of the total body length. More basal sarcoptery-gians, such as coelacanths and dipnoans, may possess propor-tionately larger fins (McAllister and Smith, 1978; MCD, pers.obs.). The large pectoral fins of Sauripterus are correlated withother rhizodontid specializations of the pectoral appendage aswell. The robust shoulder girdle, specialized fin endoskeleton,and elongate and unjointed lepidotrichia form an integrated lo-comotor system optimized for flexion and extension of largefins against a solid substrate or water column.

Girdle

The expansive contact between the scapulocoracoid and thecleithrum seems ideally suited for resisting dorsally and ante-riorly directed forces from the pectoral fins. Likewise, the broadareas of contact between the dermal bones of the shoulder gir-dle provided a rigid ‘‘hoop’’ through which motions of the pec-toral fins could be transmitted to the body. The clavicles areanteriorly expanded, providing a large area of contact along theventral midline. In contrast to other rhizodontids and tetrapo-domorphs, the ventral laminae of the cleithra are expanded me-dially, such that left and right cleithra contact along all or mostof their medial margins. Sauripterus possesses the ‘‘rhizodon-tiform’’ complex double articulation between the clavicle andcleithrum that is further strengthened by contact between theclavicular spine and the anteromedial surface of the cleithrum.Rhizodontid scapulocoracoids are typically robust elements,contacting the medial wall of the cleithrum along a broad, ex-pansive surface (e.g., Strepsodus; Jeffery, 2001). This conditiondiffers from the comparatively gracile scapulocoracoids of po-rolepiforms (i.e., Glyptolepis; Ahlberg, 1989) or tristichopterids(i.e., Eusthenopteron; Andrews and Westoll, 1970b). Whereasthe scapulocoracoid of Sauripterus still exhibits the triradiateattachment seen in these taxa, the three buttresses are greatlyexpanded anteriorly and dorsally, almost mirroring the shape ofthe dorsal part of the cleithrum (Fig. 4C, D). This large area ofcontact between the scapulocoracoid and the medial wall of thecleithrum would have been necessary to transfer forces result-ing from movements of the large pectoral fins to the head andrest of the body. In particular, the anterior and dorsal processesof the scapulocoracoid and the posteroventral orientation of theglenoid fossa seem designed to oppose ventral and posteriormotions of the fins.

Humerus

The short, stout humerus, with expanded sites of muscle at-tachment ventrally, and the unique orientation of the shoulder/humerus articulation suggest that the pectoral fins of Sauripte-rus were optimized for flexion and pronation. In sharp contrastto the posterolaterally facing glenoid of other rhizodontids (Jef-fery, 2001), and Eusthenopteron (Andrews and Westoll, 1970a),the glenoid fossa faces posteroventrally in Sauripterus. Artic-ulation of the humerus with the glenoid reveals that the proxi-modistal axis of the humerus runs ventrolaterally. As a result,the articular facets for the radius and ulna also face ventrolat-erally. In Eusthenopteron, the proximodistal axis of the humer-us and, thus, the distal facets would face posterolaterally in life.In Sauripterus, the ventral flange on the humerus provided sur-face area for the attachment of musculature that presumablywould have played a major role in flexing and pronating thefin. Muscles originating from the ventral portion of the scapu-locoracoid and the posterior margin of the cleithrum wouldhave inserted onto the proximal face of this ventral flange.

Radius and Ulna

The well-developed humeral ball-and-socket joint revealsthat the radius was capable of both dorsoventral and rotatory

motions. The combined extremes of flexion at the humeral andradial articulations may have allowed the leading edge of thepectoral fin to obtain an almost parasagittal orientation. Thebroad, deeply concave joint between the humerus and ulna sug-gests that the posterior portion of the fin was also capable ofconsiderable flexion and adduction. The anteroposterior breadthof the humeral/ulnar joint may have limited the degree to whichthe ulna could pronate and supinate. The articulations betweenthe ulna and the ulnare and intermedium are simple hinge jointsthat are limited to movements in the dorsoventral plane.

Distal Fin

Articulations between the radials and the proximal elements(two each articulate with the radius and intermedium and fourarticulate with the ulnare) are simple and flat. These joints areof a ginglymoid type, and appear to allow only a few degreesof flexion and/or extension. In contrast to the rotatory abilitiesof the radius, and to a lesser degree the ulna, the endochondralelements distal to the ulna consist of a series of simple hinges.The degree of flexion/pronation possible appears to decreasewith each successively more distal joint, with the distal radialsperhaps capable of only a few degrees of movement. In con-trast, the ulna and ulnare of Eusthenopteron appear capable ofsome degree of rotation as well as flexion. This rotation mayhave been facilitated by muscular attachment to the posteriorprocess on the ulnare. Like tetrapods, rhizodontids lack a pos-terior flange on the ulnare. In rhizodontids, this condition islikely correlated with the simple hinge joint of the ulna andulnare. Likewise, the unjointed lepidotrichia overlapping the ul-nare and intermedium would have greatly limited or preventedrotation at these joints.

Lepidotrichia

In rhizodontids, the lepidotrichia of the pectoral fin are elon-gate and unjointed for most of the length of the fin. In Saurip-terus, these lepidotrichia extend proximally to the joint betweenthe ulna and ulnare/intermedium. More anteriorly, lepidotrichiaextend almost to the base of the radius. As a result, all endo-chondral elements except for the humerus and ulna lie withina ‘‘sandwich’’ of lepidotrichia. This condition differs markedlyfrom that of other sarcopterygians, which possess jointed lepi-dotrichia and exhibit far less overlap between the dermal andendochondral skeletons. In most actinopterygians the lepidotri-chia are terminal and articulate with the distal radials. However,some basal actinopterygians, such as the chondrosteans Acipen-ser and Polyodon, appear to have convergently evolved lepi-dotrichia that overlap the endoskeleton (MCD, unpubl. data).

The elaboration and overlap of the endochondral and dermalfin skeletons, with an intervening layer of muscle, resulted infins that were stiff paddles. In this regard, the pectoral fins ofSauripterus may have functioned more like the flippers of ple-siosaurs and whales, or the stiffened fins of chondrichthyans.The shoulder and ‘‘elbow’’ joints of Sauripterus remained freeof overlapping lepidotrichia. However, it is unlikely that jointsdistal to the ‘‘elbow’’ would have been capable of the samedegree of bending seen in the distal fins of Neoceratodus orLatimeria, or proposed for Eusthenopteron (Andrews and Wes-toll, 1970a). These latter taxa possess fins with lesser degreesof skeletal overlap and with jointed lepidotrichia. Thus, the bio-mechanical function of the digit-like elements in Sauripterus islinked with the dermal skeleton, as changes in the shape of thefin, as well as distal fin movements, would rely on the functionof both skeletal components.

PHYLOGENY AND BIOGEOGRAPHY

Rhizodontids have long been considered a monophyleticgroup based on a suite of synapomorphies, particularly of the


shoulder girdle and paired fins (Andrews and Westoll, 1970b;Andrews, 1985). Subsequent discoveries and phylogenetic anal-yses have continued to support this hypothesis and consistentlyplace Rhizodontida as basal members of the tetrapodomorpha(Cloutier and Ahlberg, 1996; Ahlberg and Johanson, 1998).However, missing data have plagued attempts to elucidate thehigher-level relationships of rhizodontids. Rhizodontids havebeen known since the mid 1800s (e.g., Rhizodus; Owen, 1840;and Sauripterus; Hall, 1843), but this material has generallyconsisted of isolated appendages, scales, and teeth. The numer-ous and weakly articulating skull bones of basal sarcopterygianfishes make the discovery of articulated material a rare occur-rence. Furthermore, the large size of rhizodontids has likelycontributed to this spotty preservation. Large body size maydecrease the likelihood that an animal is buried in situ beforedecay, scavenging, and transport lead to disarticulation of theanimal (Martill, 1991).

Despite the fragmentary nature of early finds, rhizodontidshave figured prominently in discussions on tetrapod origins. In1843, Hall suggested that the three most proximal endoskeletalelements in Sauripterus were homologous with the humerus,radius, and ulna of tetrapods. Gregory (1911) expanded onHall’s hypothesis, attempting to homologize the fin lobe of Sau-ripterus with the forelimb of the Permian temnospondyl Eryops.Later discussions used Sauripterus as the ancestor of tetrapods(see Broom, 1913) or comparatively with Eusthenopteron toillustrate tetrapod limb evolution (Gregory, 1935).

Traquair (1881) erected the family Rhizodontidae for Strep-sodus and Rhizodus, taxa known, at that point, only from scalesand teeth. The complicated history of classification, misidenti-fication, and numerous junior synonyms for early rhizodontiddiscoveries has been discussed in detail by Andrews (1985).Andrews and Westoll (1970b) were the first to propose a group-ing similar to modern phylogenetic hypotheses, excluding taxanow assigned to the Tristichopteridae (i.e., EusthenopteronWhiteaves, 1881). They suggested that Rhizodus, Strepsodus,and Sauripterus should be ‘‘classified together’’ and noted thatthe earliest member of the group, Sauripterus, appeared to bethe most derived. On this basis, the authors erected a new order,the Rhizodontida, to include the newly restricted definition ofFamily Rhizodontidae. Andrews (1985) described the first rel-atively complete and articulated rhizodontid, a specimen attri-buted to Strepsodus (identified as ?Strepsodus anculonamensisin the text). Subsequent description of the Gondwanan Bara-meda provided a more complete picture of rhizodontid cranialand pectoral fin anatomy (Long, 1989). Recently, articulatedmaterial of two Devonian taxa has been recovered: immaturespecimens of the North American Sauripterus (Davis et al.,2001) and relatively complete specimens of the GondwananGooloogongia (Johanson and Ahlberg, 2001). Johanson andAhlberg (2001) have incorporated the descriptions of Gooloo-gongia and Aztecia into an analysis of rhizodontid internal re-lationships. Their analysis results in Gooloogongia being con-sidered the sister group to all other rhizodontids. Sauripterusand Aztecia are united by a character of the humerus, a rela-tively large pectoral process (preaxial flange of Johanson andAhlberg, 2001). The Carboniferous forms Barameda, Rhizodus,Screbinodus, and Strepsodus form a polytomy in the authors’consensus tree. The authors also present one of four most-par-simonious trees, distinguishing Barameda and Strepsodus assuccessive outgroups to a Rhizodus/Screbinodus clade. Perhapsthe most interesting result of this analysis is that the Carbon-iferous taxa form a distinct clade, with Devonian taxa forminga paraphyletic series basal to them. Whereas there is little res-olution supporting sister-group relationships within the Rhizo-dontida, the Carboniferous taxa consistently form a monophy-letic group separate from the Devonian taxa.

The material of Sauripterus taylori, and immature specimens

of Sauripterus (Davis et al., 2001), allow for a reassessment ofrhizodontid interrelationships. Many of the character statesmarked as unknown for Sauripterus can now be identified.These characters have been added to Johanson and Ahlberg’s(2001) data set (see Appendix 1). In addition, one character,17, trifurcations in pectoral skeleton, has been removed fromthe original set. The three elements articulating with the me-someres of Porolepiformes are not considered directly compa-rable to the three or more elements articulating with the ulnareof rhizodontids. The trifurcations in Porolepiformes are onlypresent along the fin axis and always consist of a preaxial el-ement, a postaxial element, and a larger mesomere. There areno post-axial radials articulating with the humerus and ulna inrhizodontids. Furthermore, the trifurcation of radials articulatingwith the ulnare of Sauripterus does not include an identifiablemesomere or axis. In fact, the radials of rhizodontids may benovel structures, convergent with the autopodium of tetrapods.

Our reassessment of Johanson and Ahlberg’s data set did notyield any new interpretations of rhizodontid phylogeny (PAUP4.0b10, exhaustive search). The only clear signal is of a mono-phyletic Carboniferous group within a Devonian paraphyleticassemblage. It should be noted, however, that an Aztecia/Sau-ripterus grouping requires an additional transformation of char-acter 9, a depressed flange on the cleithrum. The presence ofthis flange has long been considered a synapomorphy of theRhizodontida. Both Sauripterus and Gooloogongia lack this de-fining rhizodontid character, although Gooloogongia differs inretaining the ‘unwaisted’ cleithrum seen in outgroup taxa. Fur-thermore, the other Devonian form, Aztecia, possesses a shal-low, less-developed flange, suggesting that the prominent flangeof Carboniferous forms is derived, yet exhibits the ‘‘waisted’’cleithrum also seen in Sauripterus. Sauripterus and Aztecia bothpossess humeri of similar proportions, including a large pectoralprocess. Immature specimens of Sauripterus sp. differ fromGooloogongia in possessing unjointed basal lepidotrichial seg-ments in the pelvic and unpaired fins. Sauripterus sp. possessesa diphycercal caudal fin, as does Strepsodus, in contrast to thehypocercal tail of Gooloogongia. In summary, Sauripterus andGooloogongia share characters of the shoulder girdle and thepectoral fin, while Sauripterus and Aztecia possess similar hu-meral morphologies. Unfortunately, there are few overlappingcharacters between Gooloogongia and Aztecia.

More data are needed before hypotheses on Devonian rhi-zodontid biogeography can be tested thoroughly. Sauripterus isthe only Devonian rhizodontid from Laurasia. The other De-vonian taxa, Gooloogongia and Aztecia, are Gondwanan. Bythe Carboniferous, rhizodontids appear to be widely distributed.For example, material referred to the genus Strepsodus isknown from Australia, the United Kingdom, Turkey, Iran, andNorth America. The phylogenetic hypothesis of Johanson andAhlberg (2001) found Gooloogongia to be the most basal rhi-zodontid. Based on this result, the authors propose that rhizo-dontids likely originated in Gondwana. However, Sauripterusshares a number of primitive character states with Gooloogon-gia, such as the morphology of the shoulder girdle.

DISCUSSION

The interpretation of fin function in Sauripterus supports thenotion that some rhizodontids may have been benthic lie-and-wait predators (Johanson and Ahlberg, 1998; Long and Ahl-berg, 1999). The pectoral fins of Sauripterus consisted of flex-ible joints at the shoulder and elbow that would have beenideally suited for raising the front of the fish by adduction andflexion of the fin against the water column or a solid substratesuch as a river bottom. Additionally, these motions would havebeen aided by the expansive surface area of the disproportion-ately large pectoral fins. The bulk of this surface area consisted


of elongate and unjointed lepidotrichia overlapping the elabo-rate distal endoskeleton. This overlap resulted in a stiffeneddistal fin that could resist bending in response to resultant forc-es, efficiently transmitting muscular action into propulsion. Oth-er evidence supporting a benthic predatory mode of life comesfrom the specialized sensory system of rhizodontids, such asthe presence of sensory pores on the cleithrum and multiplelateral lines on the body (Davis et al., 2001; Johanson and Ahl-berg, 2001) that may have been adaptations for locating preyin murky or vegetation-choked waters. In Gooloogongia, theonly Devonian rhizodontid for which cranial material has beendescribed, the skull is dorsoventrally flattened and the orbitsface dorsally, a morphology ideally suited for a bottom-dwell-ing predator (Johanson and Ahlberg, 1998).

The similarities in the pectoral fin endoskeleton of Saurip-terus and the forelimbs of tetrapods are independently derived.This interpretation is supported by current phylogenetic hy-potheses that do not support a sister-group relationship betweenrhizodontids and tetrapods (Ahlberg and Johanson, 1998). De-spite the similarities, the radials of Sauripterus evolved withina very different functional context than the autopodials of tet-rapods. In rhizodontids, the unit that interacts with the watercolumn or substrate is a combined structure consisting of bothdermal and endochondral fin skeletons. The function of the en-dochondral skeleton in changing the shape of the fin duringlocomotion is mediated by the elongate and unjointed lepido-trichia. In contrast, tetrapods lack this dermal skeleton and, con-sequently, the digits interact directly with the environment. Thisimplies that one of the main shifts in the origin of tetrapodsmay not be the origin of digits, per se, but the loss of the dermalrays (Shubin and Davis, 2004). These discoveries suggest thatsome of the patterning mechanisms involved with the formationof the autopod are not unique to tetrapods and that digit-likeelements evolved at least twice within the Tetrapodomorpha.Parallel evolution of similar structures would be the expectedcondition in taxa that evolved within the same environment andshare common developmental mechanisms.

ACKNOWLEDGMENTS

The late Will Downs first exposed ANSP 20581 throughskilled preparation. We thank C. Frederick Mullison and Wil-liam W. Amaral for their contributions to the preparation ofANSP 20581, John Maisey and Ivy Rutsky of the AmericanMuseum of Natural History, for the loan of the holotype spec-imen, and Scott Rawlins for producing the stippled drawing inFigure 3. We thank Zerina Johanson for generously sharing in-formation and casts of Gooloogongia and Aztecia, John Longfor access to material of Barameda, and Anne Warren for thegift of Strepsodus casts. Bob Carroll generously provided ac-cess to specimens of Eusthenopteron in the collection of theRedpath Museum. We thank the Pennsylvania Department ofTransportation for access to road-cut exposures. This work hasbenefited greatly from discussions with Mike Coates, JonathanJeffery, and Farish A. Jenkins, Jr., and from comments on themanuscript by Per Ahlberg and John Long. This research wassupported by the National Geographic Society (5436-95 toEBD) and the National Science Foundation (EAR 98006800and EAR 0208377 to NHS and EAR 0207721 to EBD).

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Received 19 June 2002; accepted 6 February 2003.

APPENDIX

The following characters have been scored for Sauripterus utilizingthe data set of Johanson and Ahlberg (2001). Characters for Sauripterusare described herein or are from Davis et al. (2001) for Sauripterus sp.

Character numbers from Johanson and Ahlberg (2001).9—Depressed flange on cleithrum: 0, absent.13—Overlap relations of cleithrum and clavicle: 1, rhizodont con-

dition.14—Anocleithrum exposed: 0, no.20—Long basal lepidotrichial segments in other fins: 1, yes.21—Main lateral line canal on body: 1, a few parallel.22—Tail shape: 1, diphycercal.24—Lateral line pores on cleithrum: 1, present.

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