Susisuchus anatoceps is a neosuchian crocodylomorph lying outside the clade Eusuchia, and associated with the transition between basal and advanced neosuchians and the rise of early eusuchians. The specimen MPSC R1136 comprises a partially articulated postcranial skeleton and is only the third fossil assigned to this relevant taxon. Thin sections of a right rib and right ulna of this specimen have been cut for histological studies and provide the first paleohistological information of an advanced non-eusuchian neosuchian from South America. The cross-section of the ulna shows a thick cortex with 17 lines of arrested growth (LAGs), a few scattered vascular canals, and primary and secondary osteons. This bone has a free medullary cavity and a spongiosa is completely absent. Thin sections of the rib show that remodeling process was active when the animal died, with a thin cortex and a well-developed spongiosa. In the latter, few secondary osteons and 4 LAGs were identified. According to the observed data, Susisuchus anatoceps had a slow-growing histological microstructure pattern, which is common in crocodylomorphs. The high number of ulnar LAGs and the active remodeling process are indicative that this animal was at least a late subadult, at or past the age of sexual maturity. This contradicts previous studies that interpreted this and other Susisuchus anatoceps specimens as juveniles, and suggests that full-grown adults of this species were relatively small-bodied, comparable in size to modern dwarf crocodiles.
Citation: Sayão JM, Bantim RAM, Andrade RCLP, Lima FJ, Saraiva AAF, Figueiredo RG, et al. (2016) Paleohistology of Susisuchus anatoceps (Crocodylomorpha, Neosuchia): Comments on Growth Strategies and Lifestyle. PLoS ONE 11(5): e0155297. https://doi.org/10.1371/journal.pone.0155297
Editor: Christof Markus Aegerter, University of Zurich, SWITZERLAND
Received: March 9, 2015; Accepted: April 27, 2016; Published: May 5, 2016
Copyright: © 2016 Sayão 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.
Data Availability: The data are available on figshare at http://dx.doi.org/10.6084/m9.figshare.1507480.
Funding: Funding was provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPqto JMS (CNPQ Proc.n°458164/2014-3), RAMB, RCLPA, FJL and RGF. Additional funding was provided by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES and the Fundação de Amparo à Pesquisa do Estado do Espírito Santo—FAPES. AAFS acknowledge the Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico—FUNCAP. 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.
Crocodylomorpha is the most common group of Mesozoic tetrapods in Brazil [1, 2]. Fossils described in the past years indicate that the maximum diversity of this group took place during the Cretaceous Period (145–66 Ma), when the Notosuchia dominated the ancient Brazilian terrestrial landscapes (e.g., [3–9]), along with other less specialized crocodylomorphs (e.g. ). On the other hand, the fossil record of Neosuchia is relatively poor for Cretaceous deposits of this country. This latter clade is often represented by dubious and poorly-preserved material such as those of “Goniopholis paulistanus” and “Hyposaurus derbianus" [11–13]. The gigantic Sarcosuchus hartii is an important neosuchian from Brazil; however, a revision of the material and taxonomic status of this taxon is badly needed [14, 15].
Among the few taxa that are represented by better preserved material is Susisuchus anatoceps, a small “advanced neosuchian” (Fig 1) known by well-preserved specimens from the Crato Formation (Aptian-Albian) Konservat-Lagerstätte of the Araripe Basin, Brazil [16–19]. A second, but less complete species of this genus, Susisuchus jaguaribensis, was later described from the Early Cretaceous (Berriasian-Barremian) Lima Campos Basin .
(A) Hypothesis of Fortier & Schultz (2009) shows susisuchids as an "advanced neosuchian" and the sister-taxon of Eusuchia. (B) Hypothesis of Turner & Pritchard (2015) displays a more basal positioning for S. anatoceps along with the Australian species I. duncani.
S. anatoceps shows a combination of plesiomorphic and derived features that makes it a key taxon for the understanding of the early evolution of the Eusuchia [16, 19, 21–23]. Some morphological innovations of the Eusuchia are already present in S. antoceps, such as the incipient procoelous cervical vertebrae . Yet, other features suggest a more basal position for this taxon, excluding it from the Eusuchia. This is the case of the anterior projection of the frontal bone, which separates the nasals, the absence of both the antorbital and mandibular fenestrae, and the leveling between the quadrate condyles and the occipital condyle [16, 22]. The exquisite three-dimensional preservation of the specimens is typical from the lacustrine paleoenvironment of the Crato Formation, and favors the preservation of much of the internal bone microstructures .
The study of bone microstructure is a powerful tool that complements the traditional morphological descriptions and allows the inference of important information about the biology of extinct animals, such as growth rates, lifestyle adaptations, and ontogenetic stages [24–29]. The paleohistology of neosuchian taxa, however, remains still largely unexplored. Up to date, the only published thin sections regarding these animals are those of the dyrosaurids Dyrosaurus phosphaticus and Guarinisuchus munizi [28, 29]. Here we provide an histological study of the ulna and one rib of a referred specimen (MPSC R1136) of Susisuchus anatoceps. This is the first histological study of an “advanced neosuchian”.
The Araripe Basin is located in northeastern Brazil, in the central part of the Borborema Province . It is an intracratonic basin and the most extensive of the interior basins in northeastern Brazil . The stratigraphy of the Araripe Basin is very complex and remains controversial (e.g., see [30–42]). In this paper, we follow the terminology proposed by Neumann and Cabrera (1999) . These authors carried out a detailed stratigraphic review of the Araripe Basin, elevating the former Santana Formation to the status of Group, and the Crato, Ipubi and Romualdo members to the status of formations (see  for more details). The Crato Formation is the lower most stratigraphic unit in the Santana Group . It consists mainly of micritic laminated gray and cream limestones with halite pseudomorphs . The Crato Formation (lacustrine-carbonatic) together with the upper part of the underlying Barbalha Formation (deltaic) constitute the lacustrine Aptian_Albian sequence of the post-rift phase of the Araripe Basin [39, 44]. The fossiliferous record of this formation is abundant and diverse . The fossils are found in laminated limestones of lacustrine environments that developed under tropical, arid and semi-arid climatic conditions, with long intervals of dry weather and periodic precipitation . The Crato Formation has produced an immense variety of fossils of both, fauna and flora, including plants [43, 45–48], insects , ostracods , conchostracans  fishes (e.g. [52, 53]), amphibians (e.g. [54, 55]), pterosaurs [56–62], crocodylomorphs [17, 19] and feathers (e.g. ). The preservation of is the material is often exceptional, conferring to the Crato Formation the status of Konservat Lagerstätte [57, 64–68].
Materials and Methods
No permits were required for the present study, which complied with all regulations. The specimen MPSC R1136 is housed in the paleontological collection of the Museu de Paleontologia da Universidade Regional do Cariri (Santana do Cariri, Ceará State, Brazil). The material was previously described and assigned to the species Susisuchus anatoceps by Figueiredo et al. 2011 . MPSC R1136 is recognized as the third specimen of S. anatoceps on the basis of at least three diagnostic features shared with the holotype, SMNK PAL 3804 (Staaliches Museum für Naturkunde Kalsruhe, Germany) . Furthermore, the referred material comes from the same stratigraphic unit as the holotype, which is an important aspect for systematic purposes regarding fossils (e.g. [69, 70]). Despite the preservation of an almost complete articulated skeleton, only the middle shaft of the right ulna and one right thoracic rib were used in this study.
We followed the osteohistological terminology of Francillon-Vieillot et al. (1990)  and used the phylogenetic relationships of Turner & Pritchard (2015) . General features of the cross-section are described, then microstructures are discussed in detail, from the endosteal margin to the periosteal surface.
For this analysis, the mid- diaphysis of the right ulna and one thoracic rib were sectioned (Fig 2). A 0.5 cm sample was obtained from each specimen in order to prepare the histological slides.Prior to sampling, all bones were mechanically prepared with the use of airscribes and manual tools. Molds in silicon rubber (RTV CAL/N—ULTRALUB QUÍMICA LTDA, São Paulo, Brazil) and resin casts (RESAPOL T-208 catalyzed with BUTANOX M50—IBEX QUÍMICOS E COMPOSITOS, Recife, Brazil) were produced to preserve the external morphological information of the specimens. The bones were subsequently measured and photographed according to the protocol proposed by Lamm 2013 .
(A) General view of the specimen. Red (marked B) and green (marked C) arrows (corresponding to rib and ulna respectively) indicate where the cut were made for the sample collection. (B) View of the cross section of the ulna. (C) View of the cross section of the rib. Scale bar 50 mm in A; 5 mm in B; C.
Thin sections were produced using standard fossil histology techniques [72, 73]. The samples were embedded in epoxy clear resin RESAPOL T-208, catalyzed with BUTANOX M50, and cut with a diamond-tipped blade on a saw (multiple brands). The mounting-side of the sections were wet-ground using a metallographic polishing machine (AROPOL-E, Arotec LTDA) with Arotec abrasive papers of increasing grit size (60/P60, 120/P120, 320/P400, 1200/P2500) until a final thickness of 30–60 microns was reached.
Imaging and Image Analysis
Histological structures were observed with an optical microscope in transmitted light mode. Parallel/crossed nicols and fluorescence filters were used to enhance birefringence. Histological images were taken using an AxioCam digital sight camera (Zeiss Inc., Barcelona, Spain) mounted to an Axio Imager.M2 transmitted light microscope (Zeiss Inc. Barcelona, Spain). Images were taken at 56 and 106 total magnification.
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The endosteal margin in the ulna is surrounded by the endosteal lamellae. The most striking feature is the complete absence of spongy tissue, giving rise to a free medullary cavity which is 730 μm in diameter (Fig 3). The marrow cavity extends to the first quadrant (superolateral portion) of the cross-section. The compact cortex is composed of primary parallel-fibred bone tissue that is 1.110 μm in diameter.The vascular network is present only in the inner and mid-portion, whereas the outer portion is free of vascularization. The vascular canals show a random distribution along the cortex, and some of these canals anastomose and run obliquely (Fig 3C). This tissue is characterized by growth cycles (zones-annuli-LAGs). A single primary osteon appears in the deep cortex between the fourth and fifth LAG. There is no evidence of secondary osteons or spongy bone. There are some small erosion rooms (Fig 3B) near the medullary cavity, which represent signal of remodeling process. These structures are more numerous in the outer cortex than in the deep cortex. The shape of the osteocytes varies along the tissue, being more flatten periostealy in comparison to the more rounded ones endostealy. Their orientation follows the same pattern of the fibrillar and lamellar organization.
(A) View of the cross section. Black boxes indicate where the related images were taken. (B) View of the cortex exhibiting vascular canals parallel-fibered bone embedded with osteocyte lacunae. Seventeen simple lines of arrested growth—LAGs (partially covered by lines) can be observed from the inner cortex (endosteal region) to the outer surface (periosteal region) and seven annulus (marked by A), the white arrows indicate erosion rooms. (C) Detail of the primary bone tissue, showing the few scattered simple and anastomosed vascular canals composesing the vascular network.
The growth marks are widespread in the primary cortex. There are 5 complete growth cycles in the inner cortex (zone-annuli and LAG), followed by a thicker zonecontaining two closely annuli but no LAGs. The next growth marks is a row of three closely-spaced LAGs with zones between them. After these triple LAGs, an annulus follows it representing a low bone deposition with another growth cessation marked by a LAG. The next growth cycle starts with an annulus and is followed, again, by three close-spaced LAGs. The six last cycles are represented by zone and LAGs. There was no external fundamental system (EFS) preserved as already observed in other basal Neosuchia (see  for a review).
The rib exhibits a parallel-fibered histological pattern that is similar to that of the ulna, yet it shows some important differences. In general, the cortex in the rib isthinner than in the ulna, which is 310 μm in diameter. The most notable difference is the deposition of a dense spongy tissue, which is absent in the ulna. The rib also exhibits a different pattern of cortical LAGs compared to the ulna; only five of them can be observed in the rib cortex. LAGs can be observed spreading all over the cortex.
Few secondary osteons are present. They are located in the inner and outer cortex, indicating that the process of bone remodeling was active in this individual. Therefore, the possibility of loss of other LAGs by bone resorption cannot be ignored. There was no deposition of the avascular bone lamellae called external fundamental system (Fig 4).
(A) View of the cortex exhibiting parallel-fibred bone embedded with osteocyte lacunae and a deposition of spongy tissue highlighted by erosion rooms (ER). Five simple lines of arrested growth—LAGs (marked by lines) can be observed in the cortex. (B) View of the complete transect. Black boxes indicates respectively, were the related images were taken. (C) Detail of the outer cortex showing the intense remodeling process and the presence of few secondary osteons (black arrows) and an isolated simple vascular canal (red arrow).
The use of paleohistology allows the identification and characterization of four general signs of biological properties in microscopic structures; i.e. ontogeny, phylogeny, biomechanics and habitat (e.g. ). These are influenced by age, rate of growth, physiology and other factors at different moments of the life of a given taxon, and inferences about them can be made if they are linked to comparative information . Despite the diversity of crocodylomorph taxa, which show a great variety of habitats and life styles, there is a lack of knowledge about their histological patterns throughout the fossil record. Until now, the paleohistological studies of Crocodylomorpha and close-related groups range from the basal Pseudosuchia and Phytosauria  through Notosuchia (Simosuchus clarki, . The Thalattosuchia  and Dyrosauridae [28, 29] represent the only two groups of basal Neosuchia with well-known paleohistological data. Bones of extant crocodylians have also been sectioned, including those of Crocodylus niloticus [78, 79], Crocodylus johnstoni , Alligator mississipiensis [81, 82] and Gavialis gangeticus .
Bone Tissues and Growth Strategies
The histological pattern observed in Susisuchus anatoceps is parallel-fibered (PFB). This tissue is often poorly vascularized or avascular, with simple canals or primary osteons distributed randomly when present. [73,84]. In S. anatoceps the PFP cortex is avascular in the periosteal region, and the vascular canals increase endostealy towards the inner cortex with some anastomoses, as observed in both the ulna and rib. The tissue organization and their fibers can reveal rates of growth and bone depositionin organisms (e.g [85, 86]). The growth rate increases with the amount of vascularization and decreases with the degree of collagenous fibers organization, starting from fibrous to lamellar . This type of tissue is deposited slower than the fibro-lamellar [85, 86]. The latter is often found in mammals, birds, synapsids, dinosaurs and pterosaurs [24, 27, 84, 88, 89].
The presence of parallel- fibered bone tissue indicates slow bone deposition. However, several moments of pause in growth are still present and evidenced by the lines of arrested growth (LAGs). LAGs are regularly formed throughout the animal’s life . The annual cyclicity of LAGs have been proposed before for captive crocodylians exposed to constant temperature, diet, and photoperiod, yet they still exhibit the periodic and cyclical skeletal growth banding of their wild counterparts . As far as we know from extant crocodylians, the LAGs appear to be formed in annual periodicity. In a captive-bred four-years-old Crocodylus siamensis three LAGs were found and one was in process of forming before its death . Crocodylus niloticus have been shown to have LAGs formed cyclically, after the use of fluorescent markers in dermal scutes , as well as in a population of Crocodylus johstoni . An exception is observed in Alligator mississipiensis, in which a less distinctive extra growth mark can be formed under artificially induced periods of cold or heat stress .
Despite the huge difference in the growth history patterns between the ulna and the rib of MPSC R1136, the latter reveals advanced remodeling process due to an enlarged medullary cavity, large erosion rooms and a thin cortex. Therefore, it could not be considered for skeletochronology. A large number of LAGs (17) and annuli (7) were found in the ulna, representing the ciclicity of the growth. The first five cycles (zone-annuli and LAG) are followed by a thicker zone with two closely annuli and no LAG in it, representing the deacrease of bone deposition. The next GM is a row of three close-spaced LAGs with zones between them. After these triple LAGs, there is an annulus, representinga period of low bone deposition rate, that ultimately terminated in a LAG. The next growth cycle starts with slow bone deposition (annulus) and is followed again by three close-spaced LAGs. The last six cycles are represented by zone and LAGs. Considering that such retention in growth occurs annually, than it is possible to infer 17 years to this individual at the moment of death. In this bone the remodeling process was just beginning with three LAGs recovered from the resorption area for the maintenance of the medullary cavity. This is consistent with the current knowledge for this group, in which LAGs formation occurs annually [78, 81, 82].
Despite the small size of the individuals assigned to Susisuchus anatoceps, the parallel-fibered bone with the high number of growth cycles, and the degree of remodeling of the rib, are indicative that MPSC R1136 was an animal of advanced ontogenetic stage (Fig 5). The living genera Paleosuchus and Osteolaemus are considered dwarf crocodylians, with adult average sizes ranging between 1.0 and 1.5 meters and, therefore, similar to Susisuchus . Paleosuchus males reach sexual maturity when they have grown to at least 1.4 meters and females about 1.3 meters; this size category likely corresponds to 10–20 years of age [94–96].The holotype of S. anatoceps is about 60 cm in length, and was supposed to represent a young animal on the basis of some morphological features . However, S. anatoceps does not reach the length observed in the extant species cited above. Based on the number of LAGs, the estimated age for MPSC R1136 is 17 years, within the age range of sexual maturity of dwarf crocodylians. Analysis of growth rates in various groups of living vertebrates suggests that, in general, small species grow more slowly than large species . However, smaller species may reach their mature size earlier than larger ones .
Comparative size diagram of Brazilian fossil crocodylomorphs, showing the dwarfism of Susisuchus anatoceps with 70 cm of maximum length. Ilustration by Aline M. Ghilard.
Within Crocodylomorpha, some extant (Paleosuchus and Osteolaemus) and fossil (e.g. Atoposauridae and Susisuchus) taxa are considered dwarves when adult individuals are less than 2 m long and apparently retain a certain number of juvenile characters in adulthood, usually through pedomorphogenic processes [99,100]. This is one possible interpretation for the set of characteristics observed in Susisuchus anatoceps. In the original description of MPSC R1136, Figueiredo et al. (2011)  stated that some appendicular indicate it is a juvenile, whereas other traits observed in the osteoderms and axial skeleton were more suggestive of an adult morphology. Because these conditions are also found in the holotype of Susisuchus, Figueiredo et al. (2011)  concluded that both MPSC R1136 and the holotype were not completely mature individuals [17, 20]. Salisbury et al. (2003)  identify some features in the skull (e.g. large orbits, short posterior process of the quadrates, feeble ornamentation) and postcranium (e.g. poorly-torsioned humerus, indistinct muscle scars on the forelimbs) of the holotype of S. anatoceps that are associated with juveniles in most extant crocodylians, but also with mature individuals of dwarf taxa such as Osteolaemus tetraspis and Paleosuchus spp. The osteohistological features observed here agree with this last interpretation, and MPSC R1136 cannot be regarded as a young individual as previously suggested. The bone microstructure of S. anatoceps presents a pattern consistent with a late subadult animal due to the absence of EFS, and morphologically their bones show patterns of juvenile/adult transition. This suggests that S. anatoceps reaches some degree of cranial skeletal maturity before the growth of the appendicular skeleton was completed.
The absence of deposition of an external fundamental system (EFS) layer in MPSC R1136 could complicate the ontogenetic interpretation. The presence of such layers in the bones of crocodylomorpha is controversial. The EFS layer has been reported in many different taxa, such as in Lepidosauria, non-crocodylomorph pseudosuchians, Pterosauria, and Dinosauria ( TB Kellner 2013). However, the presence of EFS layers in crocodylomorphs is comparatively rare in the evolutionary history of the group. So far, this record of asymptotic growth has only been found in basal Pseudosuchia , the eusuchian Alligator  and in the neosuchian Dyrosauridae . The absence of an EFS indicates that this animal had not reached full size or the end of its active growth phase at the moment of its death. Because the outermost zones are all approximately the same width and do not decrease approaching the periosteum (Fig 3B), it was likely capable of further growth potentially lasting many more years.
S. anatoceps was considered a freshwater semi-aquatic animal based on its external morphology and general bauplan [18, 21]. The histological pattern of a given species also provides information about the body adaptations to different life styles [25, 28, 87]. The thickening of the cortex is often considered an adaptation for buoyancy in aquatic animals [101–105]. Dense bones have been reported in aquatic animals such as the basal diapsid Claudiosaurus, the placodont Placodus, and some derived mosasaurids [87,106]. It is notable that the increase in bone mass and density are common skeletal modifications in terrestrial vertebrates transitioning to a semiaquatic existence . Recently, a semiaquatic habit for the theropod dinosaur Spinosaurus was proposed by Ibrahim et al. (2014)  based on its enlarged midline display structures, the lack of free/open medullary cavities in the long bones, and increased bone density. In the extant crocodylians Alligator mississippiensis and Osteolaemus tetraspis, the heavy limbs are used to stabilize the body in water [109, 110].
The ulna of S. anatoceps has a very thick cortex and a narrow free medullary cavity, which characterizes an osteosclerotic bone pattern. This type of bone comprises an inner compaction of the bone structure [25, 111], resulting in increased skeletal mass. It is considered to play the functional role of ballast for buoyancy control and hydrostatic regulation of body trim [28, 106, 111]. If other limb bones present the same pattern observed in the ulna, the presence of an osteosclerotic limb allied to a lighter axial skeleton (represented/sampled by the rib of MPSC R1136) could be related to buoyancy control and swimming capabilities. Those features are already known for this group and this pattern is present in groups that have to maintain heavy limbs to control the position of the head above the water, which is also observed in living species [78, 81].
Crocodylians are only semiaquatic, so it may be expected that swimming in this group would be relatively expensive compared to fully aquatic animals. Distribution of body mass bone tissue is strictly related to buoyancy control, characterizing the strategies of locomotion in vertebrates [28, 78, 81]. In the extant crocodile Crocodylus porosus, aquatic propulsion by paddling with limbs is energetically expensive and ineffective relative to axial propulsion by tail undulation [112–115]. It seems likely that the use of the appendages, which is observed in hatchling crocodiles and in medium-sized crocodiles at low speed only, is employed to stabilize the body in the water, particularly at low speeds, rather than to contribute substantially to propulsion . When Osteolaemus tetraspis, a species morphologically similar to S. anatoceps, is not able to touch the ground its body floats at a steep angle relative to the water surface, with the head remaining in a horizontal position . The limbs are held out nearly horizontally from the body with the fore and hindlimbs extended (Fig 6). If the buoyancy is disturbed, the animal controls its position in the water by small rowing movements of the limbs . This kind of resting posture has been already observed in A. mississipiensis, C. niloticus, C. johnstoni, C. porosus, Caiman crocodilus and Gavialis gangenticus . This strategy is broadly present in recent taxa and its compatible with the distribution of the bone pattern of S. anatoceps. These observations lead to an interpretation that Susisuchus, like modern crocodiles, controlled buoyancy and aquatic movements by using their limbs. Despite the morphological and histological similarities between Susisuchus and recent taxa, more sampling on other bones of this species are needed to validate this hypothesis.
(A) Resting in shallow water with both body and tail contacting the bottom. (B) In shallow water when it is not able to touch the ground the hind limbs and half of the tail helping to support the animal. (C) When resting in deep water the limbs are held out nearly horizontally from the body, with fore and hind limbs extended to controls its position in buoyancy.
Based on the parallel-fibered bone, vascular network, the high number of annual growth, the maintenance of the cortical bone in the ulna, and a highly remodeled rib with a thin cortex, Susisuchus anatoceps (MPSC R1136) is considered to be a late subadult individual. None of the sampled bones show evidence of EFS deposition, indicating that the growth asymptote was not reached at the time of its death. Taken together, the histological and morphological evidences presented here strongly suggest that Susisuchus anatoceps was a dwarf crocodylomorph and that the specimens collected to date do not represent juveniles. The distribution and number of cortical growth marksalso suggests that this species had a moderate growth rate. In the rib, the cortical tissue is thinner and remodeled, indicating that the axial skeleton was less dense than than the appendicular skeleton.
The distribution of compact and spongy tissues in the ulna suggests that Susisuchus could control its buoyancy and aquatic movements by using its limbs Similar to extant crocodylians. The presence of osteoclerotic bone favors a more aquatic lifestyle. However, more sampling of other bones from this species is needed to test this hypothesis.
The authors thank Dr Plácido Cidade Nuvens (Museu de Paleontologia da Universidade Regional do Cariri) for the access to specimen MPSC R1136, Dr. Cristiano A. Chagas (Centro Acadêmico de Vitória—Universidade Federal de Pernambuco) for imaging of the thin sections, as well as Matheus Barbosa and Aline M. Ghilard for helping with figures edition. We are in debt to Holly Woodward and Sarah Werning for the comments that greatly improved the manuscript. Funding was provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPqto JMS (CNPQ Proc.n°458164/2014-3), RAMB, RCLPA, FJL and RGF. Additional funding was provided by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES and the Fundação de Amparo à Pesquisa do Estado do Espírito Santo—FAPES. AAFS acknowledge the Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico—FUNCAP. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Conceived and designed the experiments: JMS RAMB RCLPA. Performed the experiments: JMS RAMB RCLPA FJL. Analyzed the data: JMS RAMB RCLPA FJL AAFS RGF AWAK. Contributed reagents/materials/analysis tools: JMS RAMB RCLPA. Wrote the paper: JMS RAMB RCLPA FJL AAFS RGF AWAK.
- 1. Kellner AWA, Pinheiro AEP, Campos DA. A New Sebecid from the Paleogene of Brazil and the Crocodyliform Radiation after the K–Pg Boundary. PLoS ONE 9(1): e81386. pmid:24454686
- 2. Riff D, Souza RG, Cidade GM, Martinelli AG, Souza Filho JP. Crocodilomorfos: A maior diversidade de répteis fósseis do Brasil. TERRÆ. 2012; 9:12–40
- 3. Campos DA, Oliveira GR, Figueiredo RG, Riff D, Azevedo SAK, Carvalho LB, et al. On a new peirosaurid crocodyliform from the Upper Cretaceous, Bauru Group, southeastern Brazil. An Acad Bras Cienc. 2011; 83: 317–327. pmid:21437389
- 4. Carvalho IS, Vasconcellos FM, Tavares SAS. Montealtosuchus arrudacamposi, a new peirosaurid crocodile (Mesoeucrocodylia) from the Late Cretaceous Adamantina Formation Brazil. Zootaxa. 2007; 1607: 35–46.
- 5. Carvalho IS, Teixeira VPA, Ferraz MLF, Ribeiro LCB, Martinelli AG, Macedo Neto F, et al. Campinasuchus dinizi gen. et sp. nov., a new Late Cretaceous baurusuchid (Crocodyliformes) from the Bauru Basin, Brazil. Zootaxa. 2011; 2871: 19–42.
- 6. Kellner AWA, Campos DA, Riff D, de Andrade MB. A new crocodylomorph (Sphagesauridae, Notosuchia) with horn-like tubercles from Brazil. Zool J Linn Soc-Lond. 2011a; 163: 57–65.
- 7. Kellner AWA, Figueiredo RG, Azevedo SAK, Campos DA. A new cretaceous notosuchian (Mesoeucrocodylia) with bizarre dentition from Brazil. Zool J Linn Soc-Lond. 2011b; 163: 109–115.
- 8. Marinho TS, Iori FV, Carvalho IS, Vasconcellos FM. Gondwanasuchus scabrosus gen. et sp. nov., a new terrestrial predatory crocodyliform (Mesoeucrocodylia: Baurusuchidae) from the Late Cretaceous Bauru Basin of Brazil. Cretaceus Research. 2013; 44: 104–111.
- 9. Pol D, Nascimento PM, Carvalho AB, Riccomini C, Pires-Domingues RA, Zaher H. A New Notosuchian from the Late Cretaceous of Brazil and the Phylogeny of Advanced Notosuchians. PloS One. 2014; 9(4): e93105. pmid:24695105
- 10. Kellner AWA. Ocorrência de um novo crocodiliano no Cretáceo Inferior da Bacia do Araripe, Nordeste do Brasil. An Acad Bras Cienc. 1987; 59(3): 219–232.
- 11. Cope ED. A contribution to the vertebrate paleontology of Brazil. Proc Am Philos Soc. 1886; 23: 1–20.
- 12. Roxo MO. On a new species of fossil Crocodilia from Brazil, Goniopholis paulistanus sp. n. An Acad Bras Cienc. 1936; 8: 33–34
- 13. Barbosa JA, Kellner AWA, Viana MSS. New dyrosaurid crocodylomorph and evidences for faunal turnover at the K–P transition in Brazil. Proc R Soc Lond B Biol Sci. 2008; 275: 1385–1391.
- 14. Buffetaut E, Taquet P. The giant crocodilian Sarcosuchus in the Early Cretaceous of Brazil and Niger. Paleontology. 1977; 20: 203–208.
- 15. Souza R, Riff D, Azevedo S, Kellner AWA. New material of Sarcosuchus hartti (Neosuchia: Pholidosauridae) from Bahia, Brazil. J Vert Paleo, Program and Abstracts. 2013; p. 158.
- 16. Salisbury SW, Frey E, Martill DM, Buchy MC. A new crocodilian from the Lower Cretaceous Crato Formation of northeastern Brazil. Palaeont Abt A. 2003; 270: 3–47.
- 17. Frey E, Salisbury SW. Crocodilians of the Crato Formation: evidence for enigmatic species. In: Martill DM, Bechly G, Loveridge RF, editors. The Crato fossil beds of Brazil: window into an ancient world. New York: Cambridge University Press. 2003; 463–476.
- 18. Figueiredo RG, Kellner AWA. A new crocodylomorph specimen from the Araripe Basin (Crato Member, Santana Formation), northeastern Brazil. Paläontol Z. 2009; 83: 323–331.
- 19. Figueiredo RG, Moreira JKR, Saraiva AAF, Kellner AWA. Description of a new specimen of Susisuchus anatoceps (Crocodylomorpha: Mesoeucrocodylia) from the Crato Formation (Santana Group) with comments on Neosuchia. Zool J Linn Soc-Lond. 2011; 163: 273–288.
- 20. Fortier DC, Schultz CL. A new neosuchian Crocodylomorph (Crocodyliformes, Mesoeucrocodylia) from the Early Cretaceous of North-East Brazil. Palaeontology. 2008; 52: 991–1007.
- 21. Salisbury SW, Frey E, Martill DM, Buchy MC. A new crocodilian from the Lower Cretaceous Crato Formation of northeastern Brazil. Palaeont Abt A. 2003; 270: 3–47.
- 22. Pol D, Turner AH, Norell MA. Morphology of the Late Cretaceous crocodylomorph Shamosuchus djadochtaensis and a discussion of neosuchian phylogeny as related to the origin of Eusuchia. B Am Mus Nat Hist. 2009; 324:1–103
- 23. Turner AH, Pritchard AC. The monophyly of Susisuchidae (Crocodyliformes) and its phylogenetic placement in Neosuchia. PeerJ. 2015; 3:e759. pmid:25699208
- 24. Sayão JM. Histovariability in bones of two pterodactyloid pterosaurs from the Santana Formation, Araripe Basin, Brazil: preliminary results. Geol Soc Spec Publ. 2003; 217: 335–342.
- 25. Hua S, de Buffrénil V. Bone histology as a clue in the interpretation of functional adaptations in the Thalattosuchia (Reptilia, Crocodylia). J Vert Paleontol. 1996; 16: 703–717.
- 26. Padian K, Horner JR, de Ricqlés A. Growth in small dinosaurs and pterosaurs: the evolution of archosaurian growth strategies. J Vert Paleontol. 2004; 24: 555–571.
- 27. Chinsamy A, Codorniú L, Chiappe L. Palaeobiological implications of the bone histology of Pterodaustro guinazui. Anat Rec. 2009; 292(9): 1462–77.
- 28. Andrade RCLP Sayão JM. Paleohistology and Lifestyle Inferences of a Dyrosaurid (Archosauria: Crocodylomorpha) from Paraíba Basin (Northeastern Brazil). PLoS ONE. 2014; 9(7): e102189. pmid:25032965
- 29. Buffetaut E, de Buffrénil V, de Ricqlès A, Spinar ZV. Remarques anatomiques et paléohistologiques sur Dyrosaurus phosphaticus, crocodilien mesosuchien des Phosphates yprésiens de Tunisie. Annls paléont. 1982; 68: 327–341.
- 30. Assine ML. Bacia do Araripe. Boletim de Geociências. Petrobras, Rio de Janeiro. 2007; 15(2): 371–389.
- 31. Mabesoone JM, Viana MSS, Lima Filho MF. Sedimentary fill of the Araripe–Potiguar depression (NE Brazil). Abstracts, 14th Intern. Sedim. Cong., Recife-Brazil. Universidade Federal de Pernambuco, Pernambuco, Brazil. 1994; p. 46–47.
- 32. Beurlen K. Geologia da Chapada do Araripe. An Acad Bras Cienc. 1962; 34: 365–370.
- 33. Beurlen K. As condições ecológicas e faciológicas da Formação Santana na Chapada do Araripe (Nordeste do Brasil). An Acad Bras Cienc. 1972; 43: 411–415.
- 34. Mabesoone JM, Tinoco IM. Palaeoecology of the Aptian Santana Formation (northeastern Brazil). Palaeogeogr Palaeocl Palaeoecol. 1973; 14: 97–118.
- 35. Brito Neves BBA. Bacia do Araripe no contexto geotectônico regional. In: Simpósio sobre a Bacia do Araripe e Bacias Interiores do Nordeste, 1, Crato. Atas. Crato: DNPM/SBP/SBG. 1990; pp 21–33.
- 36. Ponte FC, Appi CJ. Proposta de revisão da coluna litoestratigráfica da Bacia do Araripe. Congresso Brasileiro de Geologia, 36, Natal. 1990; pp 211–226.
- 37. Ponte FC, Ponte Filho FC. Estrutura geológica e evolução tectônica da Bacia do Araripe. Departamento Nacional da Produção Mineral. 1996.
- 38. Neumann VH, Cabrera L. Una nueva propuesta estratigrafica para la tectonosecuencia post-rifte de la cuenca de Araripe, noreste de Brasil. In: BOLETIM DO 5° SIMPÓSIO SOBRE O CRETÁCEO DO BRASIL, São Paulo. 1999; pp 279–285.
- 39. Viana MSS, Neumann VHL. Membro Crato da Formação Santana, Chapada do Araripe, CE. Riquíssimo registro de fauna e flora do Cretáceo. In: Schobbenhaus C Editor. Sítios Geológicos e Paleontológicos do Brasil. Brasília: DNPM/CPRM/ SIGEP. 2002; pp 113–120.
- 40. Valença LMM, Neumann VH, Mabesoone JM. An overview on Calloviane Cenomanian intracratonic basins of northeast Brazil: onshore stratigraphic record of the opening of the southern Atlantic. Geol Acta. 2003; 1: 261–275.
- 41. Martill DM. The age of the Cretaceous Santana Formation fossil Konservat Lagerstätte of northeast Brazil: a historical review and an appraisal of the biochronostratigraphic utility of its paleobiota. Cretaceous Res. 2007a; 28: 895–920.
- 42. Kellner AWA, Campos DA, Sayão JM, Saraiva AAF, Rodrigues T, Oliveira G, et al. The largest flying reptile from Gondwana: a new specimen of Tropeognathus cf. T. mesembrinus Wellnhofer, 1987 (Pterodactyloidea, Anhangueridae) and other large pterosaurs from the Romualdo Formation, Lower Cretaceous, Brazil. An Acad Bras Cienc. 2013; 85(1): 113–135. pmid:23538956
- 43. Crane PR, Maisey JG. Fossil plants. In: Maisey JG, editor. Santana fossils: an illustrated atlas. T.F.H. Publications. 1991; pp 414–419.
- 44. Neumann VH, Borrego AG, Cabrera L, Dino R. Organic matter composition and distribution through the Aptian-Albian lacustrine sequences of the Araripe Basin, northeastern Brazil. Int J Coal Geol. 2003; 54: 21–40.
- 45. Mohr BAR, Bernardes-de-Oliveira MEC. Endressinia brasiliana, a Magnolialean angiosperm from the Lower Cretaceous Crato Formation (Brazil). Int J Plant Sci. 2004; 165: 1121–1133.
- 46. Mohr BAR, Bernardes-de-Oliveira MEC, Barale G, Ouaja M. Palaeogeographic distribution and ecology of Klitzschophyllites, and early Cretaceous angiosperm in Southern Laurasia and Northern Gondwana. Cretaceous Res. 2006; 27: 464–472.
- 47. Lima FJ, Saraiva AAF, Sayão JM. Revisão da Paleoflora das Formações Missão Velha, Crato e Romualdo, Bacia do Araripe, Nordeste do Brasil. Estudos Geológicos. 2012; 22: 99–115.
- 48. Lima FJ, Saraiva AAF, da Silva MAP, Bantim RAM, Sayão JM. A new angiosperm from the Crato Formation (Araripe Basin, Brazil) and comments on the Early Cretaceous Monocotyledons. An Acad Bras Cienc. 2014; 86(4): 1657–1672. pmid:25590706
- 49. Martins-Neto RG. Primeiro registro de Trichoptera (Insecta) na Formação Santana (Cretáceo Inferior), Bacia do Araripe, nordeste do Brasil, com descrição de sete novos táxons. In: Simpósio sobre a Bacia do Araripe e Bacias Interiores do Nordeste, 1 e 2, Crato, 1990/1997. Boletim. 2001; pp 212–226.
- 50. Berthou PY, Depeche F, Colin JP, Filgueira JBM, Teles MSL. New data on the ostracods from Crato lithologic units (lower member of the Santana Formation, Latest Aptian-Lower Albian) of the Araripe Basin (Northeastern Brazil). Acta Geol Leopol. 1994; 39(2): 539–554.
- 51. Carvalho IS, Viana MSS. Os conchostráceos da Bacia do Araripe. An Acad Bras Cienc. 1993; 65(2): 181–188.
- 52. Santos RS. Uma redescrição de Dastilbe elongatus, com algumas considerações sobre o gênero Dastilbe. Divisão de Geologia e Mineralogia, Notas preliminares e Estudos. 1947; 42: 1–7.
- 53. Castro-Leal ME, Brito PM. The ichthyodectiform Cladocyclus gardneri (Actinopterygii: Teleostei) from the Crato and Santana formations, Lower Cretaceous of Araripe Basin, north-eastern Brazil. Ann Palaontol. 2004; 90: 103–113.
- 54. Kellner AWA, Campos DA. Primeiro registro de Amphibia (Anuro) no Cretáceo Inferior da Bacia do Araripe, Nordeste do Brasil. An Acad Bras Cienc. 1986; 58 (4): 610.
- 55. Báez AM, Moura GJB, Batten RG. Anurans from the Lower Cretaceous Crato Formation of northeastern Brazil: implications for the early divergence of neobatrachians. Cretaceous Res 2009; 30: 829–846.
- 56. Frey E, Martill DM. A new pterosaur from the Crato Formation (Lower Cretaceous, Aptian) of Brazil. N Jb Geol Paläont Abh. 1994; 194: 379–412.
- 57. Sayão JM, Kellner AWA. Description of a pterosaur rostrum from the Crato Member, Santana Formation (Aptian-Albian) northeastern Brazil. Bol Mus Nac. 2000; 54: 1–8.
- 58. Sayão JM, Kellner AWA. Novo esqueleto parcial de pterossauro (Pterodactyloidea, Tapejaridae) do Membro Crato (Aptiano), Formação Santana, Bacia do Araripe, nordeste do Brasil. Estud Geol. 2006; 16: 16–40.
- 59. Frey E, Martill DM, Buchy MC. A new crested ornithocheirid from the Lower Cretaceous of northeastern Brazil and the unusual death of an unusual pterosaur. In: Buffetaut E, Mazin JM Editors, Evolution and Palaeobiology of Pterosaurs. London: Geol Soc, Spec Pub. 2003; 217: pp 55–63. https://doi.org/10.1144/GSL.SP.2003.217.01.05
- 60. Kellner AWA, Campos DA. Short note on the ingroup relationships of the Tapejaridae (Pterosauria, Pterodactyloidea). Bol Museu Nacional. 2007; 75: 1–14.
- 61. Witton MP. A new azhdarchoid pterosaur from the Crato Formation (Lower Cretaceous, Aptian?) of Brazil. Palaeontology. 2008; 51(6): 1289–1300.
- 62. Bantim RAM, Saraiva AAF, Oliveira GR, Sayão JM. A new toothed pterosaur (Pterodactyloidea: Anhangueridae) from the Early Cretaceous Romualdo Formation, NE Brazil. Zootaxa. 2014; 3869 (3): 201–223.
- 63. Sayão JM, Saraiva AAF, Uejima AMK. New evidence of feathers in the Crato Formation supporting a reappraisal on the presence of aves. An Acad Bras Cienc. 2011; 83(1): 197–210. pmid:21437381
- 64. Martill DM, Frey E. A new pterosaur Lagerstätten in NE Brazil (Crato Formation, Aptian, Lower Cretaceous), Preliminary observations. Oryctos. 1998; 1: 79–85.
- 65. Kellner AWA, Campos DA. Vertebrate paleontology in Brazil–a review. Episodes. 1999; 22(3): 238–251.
- 66. Martill DM. The Crato fossil beds of Brazil, window to an ancient world, Cambridge: University Press. 2007a
- 67. Martill DM. The geology of the Crato Formation. In: Martill DM Editor, The Crato fossil beds of Brazil, window to an ancient world, Cambridge: University Press. 2007b; pp 8–24.
- 68. Da Silva JH, Freire PTC, Abagaro BTO, Silva JAF, Saraiva GD, Lima FJ, et al. Spectroscopic studies of wood fossils from the Crato Formation, Cretaceous Period. Spectrochim Acta A Mol Biomol Spectrosc. 2013; 115: 324–329. pmid:23856041
- 69. Kellner AWA. Comments on the Pteranodontidae (Pterosauria,Pterodactyloidea) with the description of two new species. Anais da Academia Brasileira de Ciências. 2010; 82: 1063–1084. pmid:21152777
- 70. Vila Nova BC, Saraiva AAF, Moreira JKR, Sayão JM. Controlled excavations in the Romualdo Formation lagerstätte (Araripe Basin, Brazil) and pterosaur diversity: remarks based on new findings. Palaios. 2011; 26: 173–179.
- 71. Francillon-Vieillot HJ, Arntzen W, Geraudie J. Age, growth and longevity of sympatric Triturus cristatus, Triturus marmoratus and their hybrids (Amphibia, Urodela): A skeletochronological comparison. J. Herpetol. 1990; 24: 13–22.
- 72. Lamm ET. Bone Histology of Fossil Tetrapods. In: Padian K, Lamm ET, editors. Preparation and Sectioning of Specimens. University of California Press. 2013; pp 55–160.
- 73. Chinsamy A, Raath MA. Preparation of fossil bone for histological examination. Palaeontol Afr. 1992; 29: 39–44.
- 74. Horner JR, de Ricqlès AJ, Padian K. Variation in dinosaur skeletochronology indicators: implications for age assessment and physiology. Paleobiol. 1999; 25: 295–304.
- 75. Padian K. Why Study the Bone Microstructure of Fossil Tetrapods? Chapter 1. In: Padian K. and Lamm ET editors, Bone Histology of Fossil Tetrapods: Advancing Methods, Analysis, and Interpretation. University of California Press, Berkeley. 2013; pp 1–11.
- 76. Scheyer TM, Desojo JB, Cerda IA. Bone Histology of Phytosaur, Aetosaur, and Other Archosauriform Osteoderms (Eureptilia, archosauromorpha). The Anatomical Record. 2014; 297: 240–260. pmid:24376217
- 77. Hill RV. Osteoderms of Simosuchus clarki (Crocodyliformes: Notosuchia) from the late cretaceous of Madagascar. J Vert Pal. 2010; 6: 154–176.
- 78. Hutton JM. Age determination of living Nile crocodiles from the cortical stratification of bone. Copeia. 1986; 1986 (2): 332–341.
- 79. Games I. The Feeding Ecology of Two Nile Crocodile Populations in the Zambezi Valley. Unpubl. Ph.D. diss., University of Zimbabwe, Harare, Zimbabwe. 1990
- 80. Tucker AD. Validation of skeletochronology to determine age of freshwater crocodiles (Crocodylus johnstoni). Aust J Mar Fresh Res. 1997; 48: 343–351.
- 81. Woodward HN, Horner JR, Farlow JO. Osteohistological evidence for determinate growth in the American Alligator. J Herpetol. 2011; 45(3): 339–342. 2011.
- 82. Woodward HN, Horner JR, Farlow JO. Quantification of intraskeletal histovariability in Alligator mississippiensis and implications for vertebrate osteohistology. PeerJ. 2014; 2:e422. pmid:24949239
- 83. de Buffrènil V. Morphogenesis of bone ornamentation in extant and extinct crocodilians. Zoomorphology. 1982; 99:155–166.
- 84. de Ricqlés A. On bone histology of fossil and living reptiles, with comments on its functional and evolutionary significance. In: Bellairs ADA, Cox CB, editors. Morphology and biology of reptiles. New York: Academic Press Inc. 1976; 123–150.
- 85. Amprino R. La structure du tissu osseux envisagée comme expression de différences dans la vitesse de l’accroissement. Arch Bio. 1947; 58: 315–330.
- 86. de Margerie E, Cubo J, Castanet J. Bone typology and growth rate: testing and quantifying ‘Amprino's rule’ in the mallard (Anas platyrhynchos). C. R. Biologies. 2002; 325: 221–230. pmid:12017770
- 87. Houssaye A. Bone histology of aquatic reptiles: what does it tell us about secondary adaptation to an aquatic life. Biol J Linn Soc. 2013; 108: 3–21.
- 88. Enlow DH, Brown SO. A comparative histological study of fossil and recent bone tissues. Part I. The Texas Journal of Science. 1956; 8: 405–445.
- 89. Ray S, Botha J, Chinsamy A. Bone histology and growth patterns of some nonmammalian therapsids. J Vert Paleo. 2004; 24: 634–648.
- 90. Kohler M, Marin-Moratalla N, Jordana X, Aanes R. Seasonal bone growth and physiology in the endotherms shed light on dinosaur physiology. Nature. 2012; 487: 358–361. pmid:22763443
- 91. Castanet J, Francillon-Vieillot H, Meunier PJ, de Ricqlés A. Bone and individual aging. In: Hall BK, editor, Bone. London: CRC Press. 1993; pp 245–283.
- 92. de Buffrenil V. Preliminary data on the structure of growth marks among living and fossil crocodilians. Bulletin de la Socit Zoologique de France.1980; 105: 355–361.
- 93. Woodward AR, More CT. Alligator age determination, Final Report, Florida Game and Freshwater Fish Comission. Tallahassee. 1992.
- 94. Eaton MJ. Dwarf Crocodile Osteolaemus tetraspis. In: Manolis SC, Stevenson C, editors. Crocodiles. Status Survey and Conservation Action Plan. Third Edition. 2010; pp 127–132.
- 95. Ross JP. Crocodiles: status survey and conservation action plan. 2° ed. Gland: The World Conservation Union. 1998.
- 96. Magnusson WE. Paleosuchus trigonatus. Catalogue of American Amphibians and Reptiles. 1992.
- 97. Padian K, Stein K. Evolution of growth rates and their implications. In: Padian K, Lamm ET, editors. Bone Histology of Fossil Tetrapods: Advancing Methods, Analysis, and Interpretation. University of California Press, Berkeley. 2013; pp 253–264.
- 98. Lehman TM, Woodward HN. Modeling growth rates for sauropod dinosaurs. Paleobiology. 2008; 34: 264–281.
- 99. Fernandez JM. Caracterizacion morfometrica del enanismo en cocodrilos. Paleolusitana. 2009; 1: 287–295.
- 100. Tennant JP, Mannion PD. Revision of the Late Jurassic crocodyliform Alligatorellus, and evidence for allopatric speciation driving high diversity in Western European atoposaurids. Peer J. 2014; 599.
- 101. de Ricqlès A, de Buffrénil V. Sur la presence de pachyostéosclérosechez la rhytine de Steller [Rhytina (Hydrodamalis) gigas], sirénienrécent éteint. Ann Sci Nat Zool. 1995; 16: 47–53.
- 102. Chinsamy A, Rich T, Vickers-Rich P. Polar dinosaur bone histology. J Vert Paleo. 1998; 18: 385–390.
- 103. de Margerie E, Sanchez S, Cubo J, Castanet J. Torsional resistance as a principal component of the structural design of long bones: comparative multivariate evidence in birds. Anat Rec A Discov Mol Cell Evol Biol. 2005; 282: 49–66. pmid:15584036
- 104. Kriloff A, Germain D, Canoville P, Vincent P, Sache M, Laurin M. Evolution of bone microanatomy of the tetrapod tibia and its use in palaeobiological inference. J. Evol. Biol. 2008; 21: 807–826. pmid:18312321
- 105. Wall WP. The correlation between high limb-bone density and aquatic habits in recent mammals. J Paleontol. 1983; 57: 197–207.
- 106. Houssaye A. ‘Pachyostosis” in aquatic amniotes: a review. Integrative Zool. 2009; 4: 325–340.
- 107. Amson E, de Muizon C, Laurin M, Argot C, de Buffrénil V. Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru. Proc. Biol. Sci. 2014;
- 108. Ibrahim N, Sereno PC, Sasso CD, Maganuco S, Fabbri M, Martill DM, et al. Semiaquatic adaptations in a giant predatory dinosaur. Science. 2014; 345 (6204): 1613–1616. pmid:25213375
- 109. Fish FE. Kinematics of undulatory swimming in the American alligator. Copeia. 1984; 839–845.
- 110. Salisbury SW, Frey E. The kinematics of aquatic locomotion in Osteolaemus tetraspis Cope. In: Grigg GCF, Seebacher F, Franklin CE, editors. Crocodilian Biology and Evolution. Surrey Beatty & Sons. 2000; pp. 165–179.
- 111. De Ricqlès A, de Buffrénil V. Bone Histology, heterochronies and the return of tetrapods to life in water: where are we? In: Mazin JM, de Buffrénil V, editors. Secondary Adaptation of Tetrapods to Life in Water. Verlag Dr. Friedrich Pfeil. 2001; pp 289–310.
- 112. Adams L, Isaza R, Greiner E. Fatal pentastomiasis in captive African dwarf crocodile hatchings (Osteolaemus tetraspis). J Zoo Wildlife Med. 2001; 32: 500 502.
- 113. Baudinette RV, Gill P. The energetics of ‘flying’ and ‘paddling’ in water: locomotion in penguins and ducks. J comp Physiol. 1985; 155: 373–380.
- 114. Elsworth PG, Seebacher F, Franklin CE. Sustained swimming performance in crocodiles (Crocodylus porosus): effects of body size and temperature. J Herpetol. 2003; 37: 363–368.
- 115. Seebacher F, Guderley H, Elsey RM, Trosclair PL. Seasonal acclimation of muscle metabolic enzymes in a reptile (Alligator mississippiensis). J. Exp. Biol. 2003; 206 (7): 1193–1200.
- 116. Walker JA, Westneat MW. Mechanical performance of aquatic rowing and flying. Proc R Soc Lond B. 2000; 267: 1875–1881.
- 117. Brock J. Krokodile. Kleine Datz-Bücher. 1965; 22:48pp.