Dispatch R513

Dispatches

Palaeontology: Turtles in Transition

One of the major remaining gaps in the vertebrate fossil record concerns the origin of turtles. The enigmatic little reptile Eunotosaurus could represent an turtles [13,14], while analyses important transitional form, as it has a rudimentary shell that resembles the focused on identifying turtle turtle carapace. relatives within diapsids excluded poorly known anapsids (such as Michael S.Y. Lee of the amniote radiation. One anapsid Eunotosaurus) [11,12]. lineage, the ‘parareptiles’, includes This Gordian knot was recently cut Turtles (tortoises, terrapins and sea three historical contenders for turtle when the striking similarities between turtles) have a very bizarre and highly relatives: the procolophonids — Eunotosaurus and turtles were modified anatomy that has long lizard-shaped reptiles with often reiterated [15,16], and the two taxa hindered attempts to decipher their spinose skulls [5]; the pareiasaurs — were finally simultaneously included in evolutionary origins and relationships. large, stout animals varyingly covered rigorous phylogenetic analyses [15]. The most notable feature of their highly with armour plates [6]; and, The results were intriguing (Figure 1A). aberrant body plan is the external shell, Eunotosaurus — an odd little creature When turtles were added to analyses which incorporates vertebrae, ribs, with a short rigid body encased in wide of anapsids, they fell next to shoulder and sometimes the pelvis. leaf-shaped ribs [7]. However, all Eunotosaurus (and thus within This highly derived anatomy means putative anapsid relatives fell from parareptiles in general). When that morphological traits are often not favour when genomic data robustly Eunotosaurus was added to analyses readily comparable between turtles placed turtles within diapsid reptiles, focusing on diapsids, it again fell with and their putative relatives, leading to usually as sister-group to archosaurs turtles, this pairing again nesting within numerous disputed homologies. (birds and crocodiles) [8–10]. This parareptiles. There was a consistent Therefore, turtles have been arrangement implied that turtles could morphological signal uniting turtles particularly difficult to place within the not be related to any primitively anapsid with Eunotosaurus in particular, and reptile evolutionary tree [1,2]. Now, reptiles: rather, their anapsid-like skulls with parareptiles generally. writing in this issue of Current Biology, must be secondary (atavistic) rather Lyson and colleagues [3] undertake a than representative of the primitive Eunotosaurus: No Longer a re-evaluation of the neglected Permian reptilian condition, and their nearest Pariah-Saur (w260 million year old) fossil relatives should be sought amongst The potential importance of Eunotosaurus. Their analysis reveals diapsid reptiles, notably Eunotosaurus as a transitional taxon that this small, stiff-bodied terrestrial sauropterygians, an extinct clade has spurred a detailed reassessment reptile possessed an expanded ribcage that includes marine reptiles such of the carapace-like structure of this that shares many detailed similarities as plesiosaurs, placodonts and neglected reptile, known from a with the turtle carapace [3]. The overall ichthyosaurs [11]. The sauropterygian handful of good specimens from South morphology of Eunotosaurus is also hypothesis raised the possibility that Africa. In this issue of Current Biology, consistent with that of a turtle ancestor turtles evolved in the ocean, boosted by Lyson and colleagues [3] now predicted by recent ontogenetic the recent discovery of the most document additional turtle-like features studies. These discoveries should shed primitive known turtle, the small aquatic in Eunotosaurus, which encompass light on the broader phylogenetic Odontochelys [12]. However, this gross anatomy as well as fine structural relationships of turtles, and the hypothesis has some inconsistencies: detail. As in turtles, the trunk region evolutionary origins of their highly for instance, while genomic data place of Eunotosaurus is wide and stiff, distinctive body plan. turtles with archosaurs [8–10], consisting of only 9 elongate vertebrae sauropterygians are generally each with a pair of broadened Evolutionary Relationships Turning considered related to the other major leaf-shaped ribs (Figure 1a). Other Turtle living branch of diapsids, the reptiles typically have over twenty short Despite their uniquely specialised lepidosaurs (lizards, snakes and vertebrae, each with narrow cylindrical bodies, turtles have rather primitive tuataras) [11,12]: if both relationships ribs. The similarities also extend to the ‘anapsid’ skulls, characterised by a are true, then turtles and underside. Most reptiles have multiple solid cheek region, an arrangement sauropterygians cannot be close kin. longitudinal rows of rod-like bones resembling that of early reptiles. In The emerging consensus that turtles along their belly (gastralia), whereas contrast, all other living reptiles have were aberrant diapsid reptiles stymied Eunotosaurus has only two rows, more advanced ‘diapsid’ skulls with further consideration of anapsid-grade perhaps a precursor to the turtle two large openings (fenestrae) in the relatives, including all parareptiles. plastron which similarly consists of two cheek region [4] (Figure 1). Based on Eunotosaurus was thus overlooked in rows of fused bony plates. skull morphology, the search for turtle recent debates on turtle origins. Eunotosaurus also appears to have ancestors historically focused on Analyses focused on relationships lost intercostal muscles (which extinct anapsid-grade reptiles, the among Eunotosaurus and other normally extend between the ribs earliest and most primitive members anapsids explicitly excluded and are involved in breathing and Current Biology Vol 23 No 12 R514

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Anapsid skull a striking similarities between Parareptiles Eunotosaurus and turtles are currently arboniferou 359 Carboniferous C all associated with a single adaptive Current Biology complex (the shell), raising the possibility of convergent evolution. Figure 1. Phylogenetic relationships and scenarios for turtle evolution. Further study of this reptile is required, Affinities of Eunotosaurus and turtles, suggested by morphology alone (left) and morphology in to identify turtle-like features the context of genomic evidence (right). Anapsid-skulled taxa in blue, diapsid-skulled taxa in (synapomorphies) in other anatomical red; extinct taxa in light shading, living taxa in darker shading. (A) Morphology alone suggests areas, especially in the poorly-known that Eunotosaurus is a stem-turtle, and places both taxa within anapsid-skulled reptiles called skull region [16]. Such traits would parareptiles, outside the clade of all diapsid reptiles (archosaurs, lepidosaurs and sauroptery- more robustly corroborate the link gians) [3,15]. However, this position conflicts with robust genomic evidence placing turtles between Eunotosaurus and turtles. within diapsids, next to archosaurs [8–10]. If one accepts the latter evidence, two possible res- olutions to this dilemma are: (B) Eunotosaurus is a stem-turtle with a primitive carapace, Second, the position of the and thus falls within the turtle-archosaur clade, perhaps along with sauropterygians; Eunotosaurus turtle clade within (C) alternatively, Eunotosaurus is an anapsid-grade parareptile, distant from the turtle-archo- parareptiles is acknowledged to saur clade, and has convergently evolved numerous carapace-like traits. be unstable, alternating between near the lizard-shaped millerettids [15] locomotion), again a novel condition ancestral turtle lineage just before and the large stout pareiasaurs [3]. characteristic of turtles. The histology Odontochelys (most primitive ‘true’ These positions have implications of the ribs of Eunotosaurus reveals turtle) elucidates the order in which for wider homologies of some major Sharpey’s fibres (indicating muscle turtles evolved their novel adaptations turtle novelties, e.g. some attachments) are only present on [3]. Notably, the short trunk region, millerettids have broadened ribs and the ventral (i.e. internal) surface, expanded ribs (i.e. costal bones in the possible precursors of the turtle strongly suggesting that there were turtle carapace), and reorganisation of plastron [3,14,15], while pareiasaurs no muscles extending between intercostal musculature evolved very have reduced vertebral numbers and adjacent ribs. This inference is early, in the common ancestor of possess an ‘acromion process’, consistent with the short, relatively rigid Eunotosaurus and all turtles. The dorsal which in turtles connects the shoulder ribcage of Eunotosaurus. Turtles, of plates over the vertebrae (i.e. neural girdle with the shell [6]. course, have immobile ribs integrated bones along the carapace midline) However, any proposed position into the carapace, and have evolved later, in the common ancestor for Eunotosaurus and turtles accordingly lost intercostal of all turtles. This was followed by within anapsid-grade parareptiles musculature. Finally, microanatomy appearance of dermal bones along the (Figure 1A) conflicts with the robust of cross-sections further suggests edges of the carapace (peripheral genomic evidence that turtles are that the ribs of Eunotosaurus bones), and envelopment of the shell related to archosaurs and thus nested passed through ontogenetic stages around the shoulder girdle, which within diapsid reptiles [8–10]. If one similar to those in embryonic turtles: the appeared in turtles later than accepts that the molecular evidence is ribs begin life as rod-like elements, and Odontochelys. correct, and turtles are modified only later develop the leaf-shaped This proposed evolutionary diapsids, there are two likely external expansions. The turtle-like sequence closely matches the order possibilities: Eunotosaurus could features thus encompass both in which these traits appear during indeed represent the beginnings of the ontogeny and adult morphology. the embryology of living turtles. The turtle carapace: if so, it too must fall lateral elements (costals) of the within diapsids, and any anapsid-like Ontogeny Reflects Phylogeny carapace develop first, followed by features (notably in the skull) would be Based on these new analyses, the the midline elements (neurals), and evolutionary reversals (Figure 1B); position of Eunotosaurus along the finally the entire developing carapace alternatively, Eunotosaurus might be a Dispatch R515 genuinely anapsid-grade reptile which 2. Lee, M.S.Y. (2001). Reptile relationships turn 13. Tsuji, L.A., Mu¨ller, J., and Reisz, R.R. (2012). turtle. Nature 389, 245. Anatomy of Emeroleter levis and the phylogeny has convergently evolved several 3. Lyson, T.R., Bever, G.S., Scheyer, T.M., of the nycteroleter parareptiles. J. Vert. Paleont. carapace-like traits (Figure 1C). These Hsiang, A.Y., and Gauthier, J.A. (2013). 32, 45–67. scenarios could be investigated by Evolutionary origin of the turtle shell. Curr. Biol. 14. Cisneros, J.C., Rubidge, B.S., Mason, R., and 23, 1113–1119. Dube, C. (2009). Analysis of millerettid applying a genomic scaffold to the 4. Gauthier, J.A., Kluge, A.G., and Rowe, T. (1988). parareptile relationships in the light of new phylogenetic analyses: enforcing Amniote phylogeny and the importance of material of Broomia perplexa Watson, 1914, fossils. Cladistics 4, 105–209. from the Permian of South Africa. J. Syst. relationships among living taxa to 5. Laurin, M., and Reisz, R.R. (1995). A Palaeontol. 6, 453–462. conform to the molecular evidence (e.g. reevaluation of early amniote phylogeny. Zool. 15. Lyson, T.R., Bever, G.S., Bhullar, B.A.S., turtles as sister-group to archosaurs), J. Linn. Soc. 113, 165–223. Joyce, W.G., and Gauthier, J.A. (2010). 6. Lee, M.S.Y. (2001). Molecules, morphology and Transitional fossils and the origin of turtles. and then using morphological data to the monophyly of diapsid reptiles. Contrib. Biol. Lett. 6, 830–833. best place all fossil taxa within this Zool. 70, 1–22. 16. Carroll, R.L. (2013). Problems of the ancestry 7. Watson, D.M.S. (1914). Eunotosaurus of turtles. In Morphology and Evolution of framework. africanus (Seeley) and the ancestors of the Turtles, D.B. Brinkman, P.A. Holroyd, and Whether or not the affinities of Chelonia. Proc. Zool. Soc. London 11, J.D. Gardner, eds. (Dordrecht: Springer), Eunotosaurus with turtles are 1011–1020. pp. 19–36. 8. Chiari, Y., Cahais, V., Galtier, N., and Delsuc, F. 17. Kuratani, S., Kuraku, S., and Nagashima, H. eventually confirmed, the novel (2012). Phylogenomic analyses support the (2011). Evolutionary developmental perspective similarities identified in recent studies position of turtles as the sister group of for the origin of turtles: the folding theory for birds and crocodiles (Archosauria). BMC Biol. the shell based on the developmental nature [3,15,16] will ensure that this enigmatic 10, e65. of the carapacial ridge. Evol. Dev. 13, taxon occupies a pivotal position in 9. Crawford, N.G., Faircloth, B.C., 1–14. future investigations. The resurrection McCormack, J.E., Brumfield, R.T., Winker, K., 18. Hiroshi Nagashima, N., Kuraku, S., Uchida, K., and Glenn, T.C. (2012). More than 1000 Kawashima-Ohya, Y., Narita, Y., and of Eunotosaurus from obscurity ultraconserved elements provide evidence Kuratani, S. (2012). Body plan of turtles: an highlights how preconceived that turtles are the sister group of archosaurs. anatomical, developmental and evolutionary Biol. Lett. 8, 783–786. perspective. Anat. Sci. Int. 87, relationships can hinder phylogenetic 10. Wang, Z., Pascual-Anaya, J., Zadissa, A., Li, W., 1–13. analyses (taxa cannot be inferred to be Nimura, Y., Huang, Z., Li, C., White, S., related if they are never simultaneously Xiong, Z., Fang, D., et al. (2013). The draft genomes of soft-shell turtle and green sea Earth Sciences Section, South Australian considered), and how development, turtle yield insights into the development and genomics and the fossil record are evolution of the turtle-specific body plan. Nat. Museum, North Tce, Adelaide 5000, Australia Genet. 45, 701–706. and School of Earth, Environmental and mutually relevant. 11. Rieppel, O., and Reisz, R. (1999). The origin and Landscape Sciences, University of Adelaide evolution of turtles. Annu. Rev. Ecol. Syst. 30, 5005, Australia. References 1–22. E-mail: [email protected] 1. Harris, S.R., Pisani, D., Gower, D.J., and 12. Li, C., Wu, X.-C., Rieppel, O., Wang, L.-T., and Wilkinson, M. (2007). Investigating stagnation in Zhao, L.-J. (2008). Ancestral turtle from the late morphological phylogenetics using consensus Triassic of southwestern China. Nature 456, data. Syst. Biol. 56, 125–129. 497–501. http://dx.doi.org/10.1016/j.cub.2013.05.011

Scaffolding Proteins: Not Such enzymes, including protein Innocent Bystanders phosphatases and cyclic nucleotide phosphodiesterases [3]. This permits local and reversible control of Sequential transfer of information from one enzyme to the next within the signal-dependent responses. In confines of a protein kinase scaffold enhances signal transduction. Though addition, sophisticated mathematical frequently considered to be inert organizational elements, two recent reports modeling has derived algorithms implicate kinase-scaffolding proteins as active participants in signal relay. to simulate how scaffolding and anchoring proteins shape signaling F. Donelson Smith and John D. Scott example, Ste5 in yeast, and the events [4,5]. A common denominator mammalian proteins KSR (kinase has been the notion that scaffolding Signaling networks are exquisitely suppressor of Ras) and JIP and anchoring proteins are passive organized to respond efficiently to (JNK-interacting protein) organize participants that simply hold their external stimuli. Scaffolding and multi-enzyme MAP kinase assemblies enzyme binding partners in place. anchoring proteins provide a that relay phosphorylation-dependent Two papers recently published in molecular framework for the signals to potentiate activation of the Science challenge this concept by integration, processing and terminal ‘transduction’ enzyme [1,2]. demonstrating that certain dissemination of intracellular signals. A variation on this theme is the family ‘scaffolding’ proteins are actually Not surprisingly, the concept of of A-kinase anchoring proteins active elements in the enzyme enzyme scaffolding has profoundly (AKAPs) that compartmentalize complexes that they organize [6,7]. influenced our thinking about how combinations of signaling enzymes In the first of these papers, Rock et al. particular signaling events occur within that respond to distinct inputs. [6] present exciting work on the yeast precise intracellular environments and AKAPs nucleate multimeric protein mitotic exit network (MEN) scaffold are insulated from promiscuous complexes that cluster signal protein Nud1. This protein is an crosstalk. Early work identified activation components, such as important hub in the signaling network scaffolds that consolidate G-protein-coupled receptors and that controls exit from mitosis during kinase-signaling cascades. For protein kinases, with signal termination the cell cycle. Components of the