Paleogene equatorial penguins challenge: biogeography, diversity, and cenozoic climate change (página 2)

Fig. 1. Holotype of P. devriesi (MUSM 889). (a) Skull in dorsal view. (b) Skull in left lateral view. (c) Left dentary in medial view. (d) Skull in ventral view. (e) Right humerus in anterior and posterior views. ( f) Right humerus in proximal view. (g) Right humerus in distal view. (h) Left carpometacarpus in distal view. (i) Left carpometacarpus in ventral and dorsal views. (j) Synsacrum, ilia, and left femur in dorsal view. (k) Right femur in posterior and anterior views. (l) Left tarsometatarsus in plantar and dorsal views. All to same scale except j (scale for j is at lower left); f, g, and h are enlarged in the line drawings to show detail. Anatomical abbreviations: ac, acetabulum; art, articular surface for antitrochanter; at, antitrochanter; cb, coracobrachialis insertion; d, depression on lingual surface of dentary; f, femur; fmn, articular facet for first digit of metacarpal III; fmj, articular facet for first digit of metacarpal II; ld, scar for latissimus dorsi; mtr, middle trochlear ridge; nc, nuchal crest; po, postorbital process; pp, pisiform process; ps, expansion of parasphenoid; ptr, posterior trochlear ridge; sc, scar for supracoracoideus; sf, fossa for salt gland; sp, supracondylar tubercle.

Diagnosis. P. devriesi is diagnosed by the following four autapo- morphies relative to all other Sphenisciformes: (i) postorbital process directed anteroventrally; (ii) marked anterior expansion of the parasphenoid rostrum; (iii) posterior ridge forming the humeral scapulotriceps groove projects distal to the middle ridge and is conformed as a large, broadly curved surface; and (iv) femur with a convex articular surface for the antitrochanter. Additional differential diagnosis is given in supporting informa- tion (SI) Appendix.

Description. Deep temporal fossae excavate the skull roof, and the frontal is beveled for a supraorbital salt gland. A pronounced sagittal crest meets the nuchal crest at a 90° angle. The postor- bital process is directed anteroventrally, unlike the ventral orientation in other penguins. The nasal–premaxilla suture is obliterated. The left mandible indicates a straight, elongate beak with a short symphysis. A lingually directed flange from the dorsal margin expands to create a flat surface, a feature previ- ously unknown in penguins. An elongate depression on the lingual surface of the dentary is similar to a feature present in Gaviidae (loons) and to an Eocene sphenisciform mandible (18); extant penguins lack this morphology.

The humerus is flattened and pachyostotic. The dorsal tuber- cle is nearly level with the apex of the head. The shaft is narrow and lacks notable distal expansion. The coracobrachialis impres- sion is developed as a shallow oblong fossa, and the tricipital fossa is deep, undivided, and apneumatic. The supracoracoideus insertion scar is elongate and well separated from the small, circular latissimus dorsi insertion scar. A compact dorsal supra- condylar tubercle is present, a feature absent in all other penguins except the basal taxon Waimanu (10). The ulnar condyle is damaged, but a wide shelf-like surface is preserved adjacent to the posterior margin of the condyle. Metacarpal II is anteriorly bowed, with a broad anterior margin unlike the compressed sharp edge present in extant penguins. The pisiform process is a low but distinct ridge. The articular facet for the phalanx of metacarpal III is plesiomorphically subtriangular. The sutures between the ilia and the sacrum are open. The femur has a very weak trochanteric crest and is nearly straight for its preserved length. A proximally placed fibular crest and narrow supratendinal bridge are discernable on the tibiotarsus. The tarsometatarsus is short, with a straight metatarsal IV and a shallow dorsal sulcus between metatarsals II and III. The medial proximal vascular foramen and the distal vascular foramen are absent.

Icadyptes salasi. New genus, new species.

Holotype specimen. MUSM 897, comprising a skull, axis, and eight additional cervical vertebrae, partial right and left coracoids, cranial end of the left scapula, left humerus, radius, ulna, proximal carpals, carpometacarpus, and phalanges (Fig. 2).

Etymology. ""Ica"" refers to the Department of Ica, ""dyptes"" is Greek for diver. ""Salasi,"" honors Rodolfo Salas for his important contributions to vertebrate paleontology in Peru.

Locality and age. MUSM 897 is from a tuffaceous, diatomaceous, fine-grained sandstone that is part of the basal transgressive

Fig. 2. Holotype of I. salasi (MUSM 897). (a) Skull in lateral view. (b) Mandible in dorsal view. (c) Left quadrate in lateral view. (d) Left humerus in posterior and anterior views. (e) Left ulna in ventral view. ( f) Left radius in ventral view. (g) Left carpometacarpus and phalanges in ventral view. (h) Left coracoid in ventral view. All are to the same scale except c, which is enlarged to show detail. Anatomical abbreviations: ac, acrocoracoid process; af, anteorbital fenestra; am, tubercle for m. adductor mandibulae externus; atr, anterior trochlear ridge; cb, coracobrachialis insertion; cf, ovoid coracoid fossa; fa, distal facet of first metacarpal; j, jugal; lc, lateral cotyle of mandible; mc, medial cotyle of mandible; mtr, middle trochlear ridge; n, nares; ol, olecranon; ptr, posterior trochlear ridge; qc, quadratojugal cotyle; qt, tubercle on optic process; sc, scar for supracoracoideus; sf, fossa for salt gland; ss, sesamoid of m. scapulotriceps tendon; tf, temporal fossa; tr, tricipital fossa; ts, transverse sulcus.

sequence of the Otuma Formation (16)l exposed in the lower Ullujaya Valley of the Rio Ica, Department of Ica (14°37'S, 75°37'W). The fossil-bearing unit lies 70 m above an angular unconformity marking the base of the depositional sequence. Specimens of the gastropods Peruchilus culberti and Xenophora carditigera establish a late-middle to late Eocene age for this sequence, whereas microfossils from a contiguous and continu- ous section indicate a more exact age of =36 Ma for the strata containing MUSM 897.l Additionally, ash beds collected near the base of the same depositional sequence 5 km southwest across the R´io Ica yielded 40Ar/39Ar dates of 37.2 and 36.5 Ma, whereas an ash bed higher in the section, correlated with beds above the horizon bearing MUSM 897, yielded an age of 35.7 Ma.k

Diagnosis. I. salasi possesses four proposed autapomorphies rel- ative to all other penguins: (i) a beak forming more than two-thirds of the skull length, (ii) fusion of the premaxillae and palatines, (iii) axis with an elongate hypophysis terminating in a greatly mediolaterally expanded disk-like plate, and (iv) a deep ovoid fossa on the lateral surface of the acrocoracoid process. I. salasi is also differentiated from all other penguins by the following combination of postcranial characters: humerus with a straight, broad shaft (midshaft width/length = 0.22); metacar- pal I with a flat carpal trochlea and distinct distal terminus; and

lDeVries, T. J., XII Congreso Peruano de Geolog´ia, Oct. 26 –29, 2004, Lima, Peru´ (ext. abstr.).

metacarpals II and III subequal in distal extent. Additional differential diagnosis is given in SI Appendix.

Description. The skull of I. salasi is the first complete specimen reported for a basal penguin. Striking is the hyperelongate, spear-like beak, unlike that of any previously known extinct or extant penguins. Fusion of the palatal elements and premaxillae creates a powerfully constructed upper jaw with a flat ventral surface bounded by lateral ridges and inscribed with reticulate vascular sucli. Similar vascular texturing is seen in Sulidae (boobies and gannets) but is not present in other penguins, suggesting a distinct rhamphotheca in I. salasi. The mandible has an extensive symphysis with a corresponding flat dorsal surface. The cranium is extremely narrow, with deep temporal fossae meeting at a midline sagittal crest. A supraorbital shelf for the salt gland is present. The external nares extend posterior to the anterior margin of the antorbital fenestra. The jugal bar is straight. The pterygoid is rod-like, lacking the fan-like anterior expansion present in Spheniscidae. The otic process of the quadrate is shorter than the optic process. The quadrate shaft bears a tubercle for the adductor mandibulae externus attach- ment that abuts the squamosal capitulum. A retroarticular process is present.

The axis is mediolaterally compressed and elongate compared with extant penguins, and the remaining cervical vertebrae are robust, particularly compared with the narrow skull. The cora- coids preserve deep scapular cotylae, and the scapula shares the expanded proximal end present in extant penguins. The humeral supracoracoideus scar parallels the long axis of its shaft, and the

Fig. 3. Recovered penguin phylogenetic relationships, including placement of new species P. devriesi and I. salasi (in red) and showing stratigraphic and latitudinal distribution of species against 018O values as a proxy for changes in global temperature over the last 65 Ma (from ref. 12). The strict consensus cladogram of the four most parsimonious trees (4,356 steps) is shown. Black bars indicate stratigraphic range. No neospecies has a fossil record extending beyond

latissimus dorsi insertion is displaced distally to near midshaft. The tricipital fossa is undivided. Two distal trochlea are present for the humerotriceps and scapulotriceps sesamoids. The ulnar olecranon process is tab-like and proximally located, as in Waimanu (10) and Palaeeudyptes (19). The radius bears a roughened anteroproximal brachialis insertion surface but lacks a proximally directed process on the anterior border. Metacar- pals II and III are equal in distal extent as in some other Paleogene species, whereas in extant and most extinct species metacarpal III extends farther. A free alular digit is absent. Phalanx II-1 is longer than phalanx III-1, unlike in extant penguins in which these two digits are subequal in length.

Penguin Evolution and Cenozoic Climate Change

Phylogenetic results from analysis of the largest data set yet compiled for penguins, following the combined analysis methods of ref. 20, identify P. devriesi as a deep divergence within penguins and I. salasi as part of a paraphyletic assemblage of giant Eocene–Oligocene taxa closer to Spheniscidae, the pen- guin crown clade (Fig. 3). Relationships among extant genera are those recovered from less inclusive molecular data sets in maximum-likelihood and Bayesian analyses (4).

The Peruvian species inform estimates of Pacific biotic con- nections during greenhouse-to-icehouse earth transition. Two equatorial ingressions by Paleogene penguins are supported (Fig. 3). On the basis of ancestral area reconstructions (see SI Appendix), one dispersal from Antarctic regions is inferred by the middle Eocene and a second from New Zealand by the late Eocene. Whether this biogeographical pattern reflects pro- posed, but controversial, late Eocene (=41 Ma; ref. 21) begin- nings for reorganization of southern ocean circulation patterns and isolation of Antarctica (21) deserves further investigation. We find no fossil evidence for the extant penguin radiation, Spheniscidae, earlier than a species of Spheniscus =8 million years in age (22); all pre-late Miocene taxa are placed outside this radiation (Fig. 3). By contrast, molecular sequence diver- gence estimates identify the origin of this radiation at 40 Ma (4). Although there is an extensive sphenisciform fossil record from multiple continents between 40 and 8 Ma, no fossil from this interval has been placed as part of Spheniscidae (refs. 6 and 23 and Fig. 3).

Calculations from modified scripts for MSM*range (ref. 24 and see Materials and Methods) indicate that accommo- dation of the molecular divergence estimates would require an additional 164.1–334.2 million years of missing fossil record (a 172–205% increase) compared with the inferred missing record (""ghost lineages"") given only cladogram topology and fossil ages. The fossil record of penguins is approximately three times as incomplete when divergence dates are required to be true and would be strikingly biased toward recovery of only stem taxa. Extant penguin diversification may be related to later Neogene global cooling phases (4, 12), but there is no fossil evidence to support a crown radiation in the Paleogene concomitant with the initiation of the circum-Antarctic current, initial onset of Cenozoic global cooling, or at the proposed extinction of giant penguins (in contrast to ref. 4). An Oligocene fossil cited as representing the extant genus Eudyptula in support of a Paleo- gene crown radiation (4) is not assignable to that genus (6). Although stratigraphic data can only falsify a divergence date when a fossil discovery is older than estimated, the large amount of implied missing fossil record suggests that proposed crownpenguin divergence times may be too old. Future analyses using calibration points within the crown clade (e.g., refs. 22 and 25) could improve the fit of these estimates.

Based on inferred patterns linking latitudinal expansion, increases in extant species diversity, and phases of Cenozoic global cooling, it was predicted (4) that global warming could trigger an opposite pattern in the penguin lineage, with equa- torial species retreating to higher latitudes and high-latitude species facing extinction. This prediction is supported by data on some extant species (1, 3). The new Peruvian species indicate that early in penguin evolution there was a more complex relationship between global temperature and diversity than previously recognized. Additional factors such as upwelling (26) and ocean circulation patterns (6, 20, 23) were also critical in the evolution of the penguin lineage. Because the Peruvian species are members of the stem sphenisciform lineage and current global warming would be on a distinct, significantly shorter time scale, the data from these basal forms should not be used to refute predictions of changes in extant penguin distribution with continued warming. Given the different interactions between climate and distribution seen in basal penguins and in extant species, the physiological and ecological factors that could drive the response of extant penguins to climate change (1, 3, 13) may well be restricted to the crown clade.

Body Size Evolution

The phylogenetic results are consistent with a single origin of extremely large size in the penguin lineage (in contrast to ref. 27) by the early Eocene and retained in Icadyptes. Giant size persisted into the Oligocene (6, 19) across the Eocene– Oligocene climate transition (Fig. 3). Intriguingly, over late Tertiary global cooling, the average size of penguins has become smaller (Fig. 3).

Icadyptes was far larger than any living penguin, whereas Perudyptes was approximately the size of the extant king penguin. Compared with the three largest previously described extinct taxa, the I. salasi holotype humerus is =5 mm shorter than the largest exemplars of Pachydyptes ponderosus (19) and Anthro- pornis nordenskjoeldi (8, 28) and exceeds the largest of Palae- eudyptes klekowskii (28) by =9 mm. Regressions based on hindlimb measurements estimated the standing height and mass of A. nordenskjoeldi (1.66 –1.99 m and 81.7–97.8 kg, respectively) and of P. klekowskii (1.47–1.75 m and 56.0 – 65.7 kg, respectively) (29). Humeral dimensions indicate that the holotype individual of I. salasi would be intermediate in size between these two taxa, yielding a conservative minimum standing height of 1.5 m.

When the body size of low-latitude I. salasi is compared with all higher latitude taxa with overlapping observed and inferred stratigraphic ranges (Fig. 3), I. salasi is larger or approximately the same size. Although body size distribution in birds has been suggested to be largely consistent with the proposed biological law ""Bergmann"s rule,"" relating cooler temperatures and higher latitudes with large body size (e.g., refs. 30 and 31; although, see ref. 32), the warm water foraminiferan Asterigerina has been recovered in matrix associated with giant-form P. ponderosus, a form approximately the same size as Icadyptes (19). Giant size in the penguin lineage does not appear to be correlated with either cooler temperatures or higher latitudes. A period of increased late Eocene upwelling (11) and ocean productivity that would have impacted the Peruvian coast (26) should be investigated as the Pleistocene (6). Extinct taxa are indicated with a †. Branch color indicates the latitude of extant taxon breeding territories and the paleolatitude of fossil taxon localities, with ancestral latitude ranges reconstructed along internal branches from downpass optimization: 0 –30°S latitude (yellow), 30 – 60° (green), and 60 –90° (blue). Silhouettes reflect body size; small silhouettes indicate taxa smaller than extant Aptenodytes patagonicus (king penguin), medium silhouettes indicate taxa intermediate between A. patagonicus and Aptenodytes forsteri (emperor penguin), and large silhouettes indicate taxa larger than A. forsteri. The plot of mean 018O values and estimated mean ocean water temperature scale (only valid for an ice-free ocean, preceding major Antarctic glaciation at =35 Ma) are from ref. 12 and give an indication of changing conditions across the Cenozoic.

a potentially important factor driving low-latitude penguin spe- cies diversity and body size.

Implications for the Evolution of Penguin Morphology

The Peruvian fossils provide insight into early penguin cranial anatomy and the evolution of the penguin flipper apparatus. An elongate, powerfully constructed beak unknown in extant penguins is present in both Perudyptes and Waimanu (10) and, in an extreme form, in Icadyptes. This morphology can now optimized as ancestral for penguins. Undescribed long-billed Oligocene fossils have also been reported (6, 10), revealing that this morphology was retained for a significant portion of early penguin evolution and through the greenhouse-to-icehouse transition (12).

Perudyptes displays a unique combination of traits that fills an important gap between the wing morphology of the basal-most penguin, Waimanu, and living penguins. The extant penguin wing is highly modified into a stiffened, paddle-like structure equipped with a greatly reduced set of intrinsic wing muscles (33) and exhibiting the lowest degree of intrinsic joint mobility of any extant avian group (34). The presence of a conspicuous dorsal supracondylar tubercle and distinct pisiform process in Per- udyptes are consistent with retention of functioning major in- trinsic wing muscles whose attachment surface (extensor carpi radialis) and pulley-like guide (flexor digitorum profundus) these processes represent. In penguins closer to and including the crown clade, the dorsal supracondylar tubercle is absent, with indication of muscle origin reduced to a weak, flat scar. The pisiform process is reduced to a barely perceptible ridge. Like other basal penguins, Perudyptes retains a prominent ulnar condyle that potentially permitted greater flexion at the humer- us/ulna joint, although the large shelf adjacent to the condyle would restrict flexion during the flight downstroke (23). New data from the Peruvian fossils invite further biomechanical research to discern implications for locomotor differences and feeding ecology in basal penguins.

Conclusions

With the discovery of the new Peruvian species, penguins show a much more complex relationship with climate factors early in their evolution. Inference of ancestral vs. extant ecologies within a lineage, and attention to when in that lineage derived charac- teristics arose, are key to qualifying interpretations relating past biotic events causally to climate shifts, as well as to informing predictions for future responses to change. Although molecular divergence dating approaches offer great insight into the timing and potential causal factors in lineage diversification, case studies evaluating groups with a comparably rich record offer the only available test to these scenarios. In the case of the penguin lineage, a much more complex paleobiogeographical pattern and grossly different timing of key origin and radiation events are indicated, at odds with current divergence dating estimates.

Materials and Methods

Phylogenetic Analysis and Ancestral Area/Latitude Reconstruction. The data set comprised 194 morphological characters sampled for 43 fossil and living penguin taxa and =6.5 kbp from five genes (RAG-1, 12S, 16S, COI, and cyt-b) for all extant penguins. Direct optimization methods and search strategy follow ref. 20, with 200 tree bisection and reconnection replicates conducted in POY (version 3.0.11; American Museum of Natural History). See SI Data Set and SI Appendix for matrix, GenBank accession num- bers, Bremer support values, area and latitude categories, and references for fossil ages.

Missing Fossil Range Calculation. MSM*range (24), a method used to compare the temporal order of successive branching events with the age of appearance of terminal taxa in the stratigraphic record, was adapted to analyze the effects of requiring the divergence times estimated from calibrated molecular sequence data (4). Ancestral nodes, and associated inferred divergence dates, were incorporated into the analyzed Newick tree. The range of missing record reflects the iterative first appearance datum sampling procedure used by MSM*range and different resolutions of polytomies in our consensus tree.

We thank T. DeVries for stratigraphic data and for many contributions to Peruvian geology and paleontology; R. Salas, N. Valencia, D. Omura,

W. Aguirre, and E. D´iaz, for fieldwork assistance and fossil preparation;

• L. Brand and C. Aguirre for contributions to fieldwork; K. Lamm for illustrations; T. Ando and R.E.F. for information on Waimanu; P. Goloboff for supercluster access; and the editor and two anonymous reviewers for comments that improved our manuscript. We are grateful for support provided by the National Science Foundation Office of International Science and Engineering, National Geographic Society Expeditions Council, North Carolina State University, and North Caro- lina Museum of Natural Sciences (J.A.C.); The Frank M. Chapman Memorial Fund, The Doris O. and Samuel P. Welles Research Fund, and American Museum of Natural History Division of Paleontology (D.T.K.); and the National Science Foundation Assembling the Tree of Life Project: Archosaur Phylogeny.

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Julia A. Clarkea,b,c,d, Daniel T. Ksepkac, Marcelo Stucchie, Mario Urbinaf, Norberto Gianninig,h, Sara Bertellii,g, Yanina Narva´ ezj, and Clint A. Boyda

aDepartment of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box 8208, Raleigh, NC 27695; bDepartment of Paleontology, North Carolina Museum of Natural Sciences, 11 West Jones Street, Raleigh, NC 27601-1029; Divisions of cPaleontology and gVertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024; eAsociacio´ n para la Investigacio´ n y Conservacio´ n de la Biodiversidad, Los Agro´ logos 220, Lima 12, Peru´; fDepartment of Vertebrate Paleontology, Museo de Historia Natural, Universidad Nacional Mayor de San Marcos, Avenida Arenales 1256, Lima 14, Peru´; hProgram de Investigaciones de Biodiversidad Argentina (Consejo Nacional de Investigaciones Cienti´ficas y Te´ cnicas), Facultad de Ciencias Naturales, Instituto Miguel Lillo de la Universidad Nacional de Tucuma´ n, Miguel Lillo 205, CP 4000, Tucuma´ n, Argentina; iThe Dinosaur Institute, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007; and jDepartment of Geology, Centro de Investigacio´ n Cienti´fica y de Educacio´ n Superior de Ensenada, Kilometer 107 Carretera Tijuana–Ensenada, 22860 Ensenada, Baja California, Me´ xico

Edited by R. Ewan Fordyce, University of Otago, Dunedin, New Zealand, and accepted by the Editorial Board May 21, 2007 (received for review December 14, 2006)