Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning Jonathan E. Jefferya,1, Glenn W. Storrsb, Timothy Hollandc, Clifford J. Tabind, and Per E. Ahlberge aSchool of Earth Sciences, University of Bristol, BS8 1TQ Bristol, United Kingdom; bCincinnati Museum Center, Cincinnati, OH 45203; cKronosaurus Korner, Richmond, QLD 4822, Australia; dDepartment of Genetics, Harvard Medical School, Boston, MA 02115; and eSubdepartment of Evolution and Development, Department of Organismal Biology, Uppsala University, 752 36 Uppsala, Sweden Edited by Neil H. Shubin, The University of Chicago, Chicago, IL and approved October 3, 2018 (received for review July 3, 2018) All living tetrapods have a one-to-two branching pattern in the Results embryonic proximal limb skeleton, with a single element at the Pelvic Girdle. The pelvic girdle of MCZ 11916 comprises a single, base of the limb (the humerus or femur) that articulates distally long bone on each side, and would have been ∼120 mm long in with two parallel radials (the ulna and radius or the tibia and life. The shaft of the right pelvis is incomplete, but the distal ends fibula). This pattern is also seen in the fossilized remains of stem- of both left and right bones are well preserved and associated tetrapods, including the fishlike members of the group, in which with their respective fin skeletons. Each pelvis has a robust pubic despite the absence of digits, the proximal parts of the fin skeleton ramus with a posterior-facing acetabulum, flanked by lateral clearly resemble those of later tetrapods. However, little is known (“iliac”; ref. 12) and mesial flanges (Fig. 2B and SI Appendix, Fig. about the developmental mechanisms that establish and canalize S2), similar to those described for other fishlike stem-tetrapods this highly conserved pattern. We describe the well-preserved pelvic (Fig. 3A) (9, 12, 24). The pubic ramus is fairly straight and len- fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian ticular in cross-section for much of its length. Anteriorly it ter- (lobe-finned) fish, and member of the tetrapod stem group. In this minates in jagged, unfinished bone, suggesting that the shaft specimen, three parallel radials, each robust with a distinct morphol- ogy, articulate with the femur. We review this unexpected morphol- continued as cartilaginous tissue. The outer (lateral) surface of ogy in a phylogenetic and developmental context. It implies that the the shaft is smooth except for a process near the base of the EVOLUTION developmental patterning mechanisms seen in living tetrapods, mesial flange (more pronounced on the right pelvis). The inner now highly constrained, evolved from mechanisms flexible enough (mesial) surface bears a shallow longitudinal ridge anteriorly. to accommodate variation in the zeugopod (even between pectoral Posteriorly, the shaft thickens to a mesial buttress, triangular in and pelvic fins), while also allowing each element to have a unique cross-section, encompassing the acetabulum. The acetabulum morphology. itself not well preserved (it was likely cartilaginous and not fin- ished bone), and its shape cannot be determined with any cer- zeugopod | pelvis | limb patterning | tetrapodomorph | rhizodontid tainty. The mesial flange is a robust triangular area of bone. The iliac process is broken off at its base on the left side, but on the he evolution and developmental patterning of the tetrapod right side it is a large, flat flange with a rounded tip. limb has been the subject of intense research in recent de- T Pelvic Fins. Both pelvic fins show a well-preserved femur, ∼45 mm cades (1–9). Limbs arose as a modification of the paired fins of long (Figs. 2 and 4). It is a wide bone with distinct preaxial and sarcopterygian fishes, and the skeletal morphology is well known postaxial edges, similar in proportion to that of Eusthenopteron, in several of the fish-like members of the tetrapod stem group, the only other fishlike stem-tetrapod for which a detailed including such Paleozoic genera as Gogonasus (10, 11), Eusthe- nopteron (12), Panderichthys (3, 5), and Tiktaalik (4, 9). De- Significance velopmental data are available from a living sister taxon of the tetrapods, the Australian lungfish Neoceratodus (13). Rhizodus hibberti Research into the evolution of skeletal patterning of limbs has The fossil fish , a member of the tetrapod stem group, shows a unique skeletal pattern in the pelvic fin. focused principally on the origin of the autopod (the ankle/wrist Rather than the highly conserved one-to-two pattern of a fe- and digits) (1, 3, 5, 14–19). In contrast, the pattern in the prox- mur, tibia, and fibula (seen in all known tetrapods, including imal part of the skeleton has been seen as substantially conserved the extinct, fishlike members of the group), the fin of Rhizodus across the fish-tetrapod transition (4, 20), comprising a single comprises a femur articulating distally with three bones, each basal element (the humerus or femur) articulating distally with with a distinct morphology. This reveals an early stage in the paired elements (the radius and ulna or tibia and fibula). evolution of limb development, in which the processes pat- Pelvic material of stem-tetrapods is rare (compared with pectoral terning the proximal parts of the embryonic fin/limb (the sty- material), and few examples have been described to date (9, 20). lopod and zeugopod) were not constrained in the way seen in Specimen MCZ 11916 from the Museum of Comparative Zoology, living tetrapods and could produce more varied skeletal pat- Harvard University is a large oil shale nodule from the Asbian terns in the adult. Wardie Shales (Viséan, Early Carboniferous, 339.4–336 Mya) of Wardie Beach near Edinburgh, United Kingdom (21) (Fig. 1) Author contributions: J.E.J. designed research; J.E.J., C.J.T., and P.E.A. performed research; J.E.J., G.W.S., T.H., C.J.T., and P.E.A. analyzed data; and J.E.J., G.W.S., T.H., C.J.T., and containing a near-complete skeleton of the rhizodontid stem- P.E.A. wrote the paper. tetrapod Rhizodus hibberti (22, 23) (SI Appendix,Fig.S1). The authors declare no conflict of interest. Rhizodus is the largest known sarcopterygian fish (24, 25) and This article is a PNAS Direct Submission. MCZ 11916 was a medium-sized individual, ∼3.5 m long. Both This open access article is distributed under Creative Commons Attribution-NonCommercial- pelvic fins are articulated and are preserved in natural association NoDerivatives License 4.0 (CC BY-NC-ND). with the spine, and the dorsal and anal fins (Fig. 2 and SI Ap- 1To whom correspondence should be addressed. Email: [email protected]. pendix,Fig.S1B). This exceptional preservation offers a unique This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. insight into the morphology of the pelvic region at an early stage 1073/pnas.1810845115/-/DCSupplemental. of tetrapod evolution. www.pnas.org/cgi/doi/10.1073/pnas.1810845115 PNAS Latest Articles | 1of6 Downloaded by guest on September 24, 2021 A Tib. Int. Granton Harbour Shale with fish nodules N Shale with no fish nodules Coal Seatclay Sandstone Mes. 100m Mud Sewage Right pelv. pipe Wardie Beach 3 6 7 5 Fem. Fib. 4 3 2 2 Fib. fla. mainly 6 3 2 Fibe. 4 5 shingle sand 1 I. Fig. 1. Map of Wardie Beach, Edinburgh, United Kingdom (55.98N, 3.21W). B Art. Tib. Art. Fib. Art. Tib. C Although the original collection notes are vague (22), all more recent dis- coveries of Rhizodus material at Wardie have been made in the large ex- posure of bed 2 (21). Data from ref. 21. Ridge description is available (Fig. 3B) (12, 26). The ventral face bears two muscle attachment processes and a longitudinal fossa (Fig. 4 Foss. B–D, F, and G). A similar fossa is seen in Eusthenopteron (Fig. 3B) (12, 26), and there is also a longitudinal ridge in the same position as one of the processes seen in Rhizodus (26). The femur Cap. Hum. terminates distally in an expanded region of unfinished bone, Fig. 3. Eusthenopteron foordi, pelvic fin and girdle. (A) Right pelvic fin and divided into three facets, each of which articulates with a robust girdle in dorsal view. Note that the long pubic ramus is not shown. Repro- endoskeletal fin radial. These three radials have distinct indi- duced by permission of The Royal Society of Edinburgh from ref. 12. (B) vidual morphologies, which match perfectly between the left and Reconstruction of the left femur in posteroventral view. (C) Reconstruction right fins. The most external (anatomically anterior) radial tapers of the left femur in dorsolateral view. Reproduced with permission from ref. slightly distally and does not appear to have articulated with more 26. I., illiac flange of pelvis; Mes., mesial flange of pelvis; Art. Fib., articulation for the fibula; Art. Tib., articulation for the tibia; Cap. Hum., caput humeri; Fib. distal radials. The middle radial has a waisted shaft, and the in- fla., postaxial flange on the fibula; Fibe., fibulare; Int., intermedium. ternal (anatomically posterior) radial bears a thick postaxial flange. Both the middle and internal radials have expanded distal ends, and on the right fin they both articulate with a single distal The unfinished bone at the proximal and distal ends of the radial of similar size; on the left fin, the three radials contact the femurandradialsissimilarinitspreservationtothebonesof edge of the nodule, and nothing more distal is preserved. All these apectoralfinofRhizodus from the same locality (NMS G radials are essentially cylindrical “long” bones (sensu refs. 7, 17, 1972.27.434c) (25). The bone degrades and merges with the 18, and 20), with a complete periosteum along their shafts.