Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx Gregory M. Erickson1,6*, Oliver W. M. Rauhut2, Zhonghe Zhou3, Alan H. Turner4,6, Brian D. Inouye1, Dongyu Hu5, Mark A. Norell6 1 Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America, 2 Bayerische Staatssammlung fu¨r Pala¨ontologie und Geologie and Department of Earth and Environmental Sciences, LMU Munich, Mu¨nchen, Germany, 3 Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology & Paleoanthropology, Chinese Academy of Science, Beijing, China, 4 Department of Anatomical Sciences, Stony Brook University, Stony Brook, New York, United States of America, 5 Paleontological Institute, Shenyang Normal University, Shenyang, China, 6 Division of Paleontology, American Museum of Natural History, New York, New York, United States of America Abstract Background: Archaeopteryx is the oldest and most primitive known bird (Avialae). It is believed that the growth and energetic physiology of basalmost birds such as Archaeopteryx were inherited in their entirety from non-avialan dinosaurs. This hypothesis predicts that the long bones in these birds formed using rapidly growing, well-vascularized woven tissue typical of non-avialan dinosaurs. Methodology/Principal Findings: We report that Archaeopteryx long bones are composed of nearly avascular parallel- fibered bone. This is among the slowest growing osseous tissues and is common in ectothermic reptiles. These findings dispute the hypothesis that non-avialan dinosaur growth and physiology were inherited in totality by the first birds. Examining these findings in a phylogenetic context required intensive sampling of outgroup dinosaurs and basalmost birds. Our results demonstrate the presence of a scale-dependent maniraptoran histological continuum that Archaeopteryx and other basalmost birds follow. Growth analysis for Archaeopteryx suggests that these animals showed exponential growth rates like non-avialan dinosaurs, three times slower than living precocial birds, but still within the lowermost range for all endothermic vertebrates. Conclusions/Significance: The unexpected histology of Archaeopteryx and other basalmost birds is actually consistent with retention of the phylogenetically earlier paravian dinosaur condition when size is considered. The first birds were simply feathered dinosaurs with respect to growth and energetic physiology. The evolution of the novel pattern in modern forms occurred later in the group’s history. Citation: Erickson GM, Rauhut OWM, Zhou Z, Turner AH, Inouye BD, et al. (2009) Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx. PLoS ONE 4(10): e7390. doi:10.1371/journal.pone.0007390 Editor: Robert DeSalle, American Museum of Natural History, United States of America Received June 6, 2009; Accepted September 16, 2009; Published October 9, 2009 Copyright: ß 2009 Erickson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research was generously funded by an DFG grant RA 1012/4 to OWMR, NSF DBI 0446224 and EAR 04418649 grants to GME, an NSF ATOL 0228693 grant to MAN, and the Major Basic Research Projects [2006CB806400] of MST of China to ZZ. The funders had no role in study design, data collection, and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction Avialae as a whole. (Although, whether that condition was comparable to living birds is a subject of debate [6,12–15].) The genesis of birds was a key event in vertebrate history. The We used dissecting microscopy to directly examine the long descendants of these animals have dominated aerial niches from bone periosteal and fracture surfaces spanning the entire size the Late Mesozoic until today, where they are the most speciose range of Archaeopteryx (Eichsta¨tt, Munich, Ottmann & Steil, amniote clade (,10,000 species) [1,2]. Skeletal, feather, and London, and Solnhofen specimens; Table 1), the oldest known behavioral evidence conclusively place birds within the evolution- bird [16]. (Note: we follow Chiappe’s (2) interpretation that all ary radiation of theropod dinosaurs, rightly making birds the only specimens are referable to a single species, Archaeopteryx lithographica; living dinosaurs [2–4]. Growth patterns, revealed through long see Materials and Methods.) In the two largest individuals bone osteohistology can be traced phylogenetically through this delamination of cortical bone consistent with growth line interfaces transition and used as a proxy for inferring physiological changes. were found, as anticipated (Figure 2). However, all specimens Non-avialan dinosaur bones show a characteristic well-vascular- unexpectedly showed exceptionally sparse longitudinal vasculari- ized, rapidly formed woven-fibered matrix that is often interrupted zation visible on the periostal surfaces and within the semi- by growth lines [5–7] (Figure 1). Similar broad-scale histological transparent bones (Figure 3). Furthermore, transversely running attributes have been reported in Mesozoic bird lineages close to fracture faces showed a circumferential fabric characteristic of the the base of the avialan tree [8–11] (Figure 1). This has contributed dense lamellar or parallel-fibered bone-types of living non- to the paradigm that dinosaurian physiology is ancestral for dinosaurian reptiles [17]. PLoS ONE | www.plosone.org 1 October 2009 | Volume 4 | Issue 10 | e7390 Archaeopteryx Growth These observations conflict with the hypothesis that dinosauri- an-type growth physiology is primitive for the first birds. Alternative explanations are: 1) Archaeopteryx grew aberrantly, 2) basalmost avialan growth was substantially decelerated, perhaps in response to the reallocation of resources from growth to powered flight, or 3) that current phylogenetic resolution of histological character states are insufficient to correctly infer the basalmost avialan condition. To test these hypotheses we used a comparative phylogenetic approach. We were given the opportunity to study the long bone histology of a juvenile specimen of Archaeopteryx (The Munich Specimen, BSPG 1999 I 50; Figure 4) from which we describe the taxon’s peculiar osseous microstructure. We also sampled the successive crownward lineages, Jeholornis (another very primitive long-tailed avialan; [18]), and Sapeornis (the primitive short-tailed avialan that retains three fingers and teeth; [19]), to provide broader representation of avialan histology in truly basal lineages (Table 1). (Initial studies utilized the histology of Rahonavis, that was Figure 1. Long bone histology of a non-avialan dinosaur and a Mesozoic bird viewed with polarized microscopy. (A) The once thought to occupy a similar position [5,20] but is now femoral microstructure of the small alvarezsaurrid, Shuvuuia deserti considered a non-avialan dinosaur [21,22]; and the birds (IGM 100–99) is compared to the tibial histology (B) of the Early Confuciusornis, Enantiornithes, and Patagopteryx [5,9,20,23] that are Cretaceous avialan, Confuciusornis sanctus (IVPP V11521). Both show now placed well crownward from the basal avialan node [24].) well-vascularized bone owing to the presence of numerous primary Interpreting our basal avialan findings in a phylogenetic context vascular canals (large black structures), and woven fibered matrix spanning the avialan transition also necessitated broader histolog- characterized by oblong, randomly oriented osteocyte lacunae (numerous small black structures). Arrows point to growth lines in ical sampling of the proximate outgroup maniraptoran clades, the form of a line of arrested growth (left) and an annulus (right). Scale Oviraptorosauria and Deinonychosauria ([22,24]; Figure S1). bars = 0.5 mm (A) and 0.1 mm (B). Initial examinations across the avialan transition had either no doi:10.1371/journal.pone.0007390.g001 samples ascribed to these clades, or just one representative – Table 1. Specimen identifications and sizes. Taxon Specimen Number(s) Max. Femoral Length (mm) Citipati osmolskae IGM 100–1004* 405 Conchoraptor gracilis IGM 97–212* 250 Caudipteryx zoui IVPP 11819* 149 Troodon formosus MOR 748* 483 Byronosaurus jaffei IGM 100–984* 150 Troodontidae (undescribed) IGM 100/1129* 84 Troodontidae (undescribed) IGM 100–1323* 80 Utahraptor ostrommaysi BYUVP 15465* 600 Velociraptor mongoliensis IGM 100–982* 184 Mahakala omnogova IGM 100–1033* 76.2 Sapeornis chaoyangensis LPM B00166* 80 Jeholornis prima IVPP 13274*, IVPP 13353* 75 Archaeopteryx lithographica JM 2257, BSPG 1999 I 50* 65.7 BMMS 500, BMNH 37001 Ottmann & Steil—uncataloged TSMHN 6928/6929** WDC-SG-100** HMN 1880/1881** Maxberg—uncataloged** 8th Exemplar—uncataloged** *Histologically sampled. **Examined using research photographs from colleagues and/or through high-resolution research casts. Institution designations: BMMS = Bu¨rgermeister-Mu¨ller Museum, Solnhofen; BMNH = Museum of Natural History, London; BSPG = Bayerische Staatssammlung fu¨r Pala¨ontogie und Geologie, Munich; BYUVP = Brigham Young University Earth Science Museum, Provo; HMN = Museum fu¨r Naturkunde, Humboldt-Universita¨t, Berlin; IGM = Institute of Geology, Mongolian Academy of Sciences, Ulaanbaatar;