Periodic Reporting for period 1 - EvoSaurAf (The evolution of long-necked dinosaurs of Africa before the Cenomanian-Turonian extinction)
Reporting period: 2022-06-01 to 2024-05-31
Sauropods were 4-legged plant eating dinosaurs with long necks and tails that thrived for more than 160 million years during the Jurassic and Cretaceous until their extinction at the K-Pg limit, 66 million years ago. By the beginning of Late Cretaceous (90 million years ago, Cenomanian stage) only two groups remained: Titanosauria and Rebbachisauridae. At the end of the Cretaceous before their final extinction, only titanosaurs remained. Both rebbachisaurids and titanosaurs evolved independently similar hyper-pneumaticity of their postcranial skeletons (hypothesized to be an adaptation to high temperatures) and adaptations to feed on ground level vegetation. However, why rebbachisaurids went extinct after the Cenomanian-Turonian boundary while titanosaurs thrived is unknown.
Little is known on why other dinosaurs diversified into many different groups during the Late Cretaceous while only one major group of sauropods survived, but this is poorly understood in particular for African sauropod dinosaurs. The scarce knowledge on Cenomanian African sauropods has prevented to know how terrestrial vertebrate faunas coped with sea surface temperature rose to 3 to 9 ºC higher than present-day tropical temperatures and sea levels on the rise, eventually forming shallow seas that completely flooded Europe and divided North America and Africa into separate landmasses. Fieldwork throughout the last 25 years in Niger led by Prof. Paul C. Sereno (University of Chicago) in cooperation with the Nigerien Institut de Recherche en Sciences Humaines (IRSH) has yielded abundant sauropod skeletons dating from Cenomanian beds.
The overall research aim of this project has been to significantly IMPROVE our understanding on the diversity of Cenomanian north African sauropod dinosaurs from unpublished fossils, how they were related to sauropods from other regions and whether there were major paleobiological differences between the two sauropod lineages, titanosaurs and rebbachisaurids.
The following hypotheses (H) will be evaluated by fulfilling the Objectives (Os):
H1: At least four sauropod species coexisted in the Farak Formation (Cenomanian), all most closely related to sauropods from other continents, revealing additional intercontinental biogeographic complexity during the Cretaceous.
O1: To describe the four new sauropod species, determining their unique macro and micro anatomical features, and to compare them to known Cretaceous sauropods from Africa, South America and elsewhere
O2: To score the new African specimens in updated phylogenetic datasets and assess their evolutionary relationships and probable biogeographic history
H2: The four sauropod species from the Farak Formation comprise differing ecomorphotypes (particular grouping of organisms by similar morphology, habitat and behavior regardless on whether they are evolutionarily related), with differing feeding habits (different feeding heights), locomotor capabilities, growth rates and postcranial pneumaticity.
O3: To assemble virtual skeletal mounts from CT-scanned bones for all four species from the Farak Formation and perform exhaustive biomechanical analyses on the virtual skeletons related to feeding function, locomotion and pneumatic apparatus.
H3: The sauropods from the Farak Formation represent subgroups that disappeared from northern Gondwana after the Cenomano-Turonian extinction and had disadvantageous ecomorphotypes to the high temperatures of this time period.
H4: Titanosaur sauropods evolved ecomorphotypes similar to those comprised by Rebbachisaurs only after these had become extinct after the Cenomanian-Turonian boundary.
O4: To integrate new knowledge generated in Os 1-3 into a paleogeographic and time-calibrated framework, evaluate the evolution and extinction of sauropods through the Cenomanian-Turonian boundary and compare the ecomorphology of sauropod faunas immediately before and after the boundary.
At the time of writing this report, O1, O2 and O3 are in highly advanced stages (specially with those fossils already in the lab), while O4 is still under progress and conclusions have not yet been drawn from it.
Little is known on why other dinosaurs diversified into many different groups during the Late Cretaceous while only one major group of sauropods survived, but this is poorly understood in particular for African sauropod dinosaurs. The scarce knowledge on Cenomanian African sauropods has prevented to know how terrestrial vertebrate faunas coped with sea surface temperature rose to 3 to 9 ºC higher than present-day tropical temperatures and sea levels on the rise, eventually forming shallow seas that completely flooded Europe and divided North America and Africa into separate landmasses. Fieldwork throughout the last 25 years in Niger led by Prof. Paul C. Sereno (University of Chicago) in cooperation with the Nigerien Institut de Recherche en Sciences Humaines (IRSH) has yielded abundant sauropod skeletons dating from Cenomanian beds.
The overall research aim of this project has been to significantly IMPROVE our understanding on the diversity of Cenomanian north African sauropod dinosaurs from unpublished fossils, how they were related to sauropods from other regions and whether there were major paleobiological differences between the two sauropod lineages, titanosaurs and rebbachisaurids.
The following hypotheses (H) will be evaluated by fulfilling the Objectives (Os):
H1: At least four sauropod species coexisted in the Farak Formation (Cenomanian), all most closely related to sauropods from other continents, revealing additional intercontinental biogeographic complexity during the Cretaceous.
O1: To describe the four new sauropod species, determining their unique macro and micro anatomical features, and to compare them to known Cretaceous sauropods from Africa, South America and elsewhere
O2: To score the new African specimens in updated phylogenetic datasets and assess their evolutionary relationships and probable biogeographic history
H2: The four sauropod species from the Farak Formation comprise differing ecomorphotypes (particular grouping of organisms by similar morphology, habitat and behavior regardless on whether they are evolutionarily related), with differing feeding habits (different feeding heights), locomotor capabilities, growth rates and postcranial pneumaticity.
O3: To assemble virtual skeletal mounts from CT-scanned bones for all four species from the Farak Formation and perform exhaustive biomechanical analyses on the virtual skeletons related to feeding function, locomotion and pneumatic apparatus.
H3: The sauropods from the Farak Formation represent subgroups that disappeared from northern Gondwana after the Cenomano-Turonian extinction and had disadvantageous ecomorphotypes to the high temperatures of this time period.
H4: Titanosaur sauropods evolved ecomorphotypes similar to those comprised by Rebbachisaurs only after these had become extinct after the Cenomanian-Turonian boundary.
O4: To integrate new knowledge generated in Os 1-3 into a paleogeographic and time-calibrated framework, evaluate the evolution and extinction of sauropods through the Cenomanian-Turonian boundary and compare the ecomorphology of sauropod faunas immediately before and after the boundary.
At the time of writing this report, O1, O2 and O3 are in highly advanced stages (specially with those fossils already in the lab), while O4 is still under progress and conclusions have not yet been drawn from it.
Specimen collection: In late 2022 grant recipient participated and helped to coordinate a paleontological expedition organized by the University of Chicago (USA) and the Institut de Recherche en Sciences Humaines (Niger) to Niger. The goal of this expedition was both to collect several fossils located during prospections in 2018 and 2019 as well as to prospect areas that had never been explored by paleontologists.
Main achievements: The expedition worked the Earlier Jurassic and Lower Cretaceous beds of Niger, but more importantly and related to EvoSaurAF, the crew explored new Cenomanian outcrops Southeast of Agadez. This exploration turned out to be extremely successful, with the finding of several specimens of fish, turtles and theropod and sauropod dinosaurs. While the overall faunal composition is similar to what is known in other outcrops in Niger and North Africa, this new locality is unique in two ways: first, it was deposited in an inland environment evidenced by cross bedding of the sediments; and second it is the only site with a subequal representation of predatory and plant eating dinosaur remains (north African localities tend to preserve more predators, while northwestern Niger localities tend to have more herbivore remains). Among the sauropods, in addition to the two titanosaurs and one rebbachisaurid specimens spotted in 2019, two additional titanosaur partial skeletons, a partial rebbachisaurid skeleton and several isolated elements were found and collected.
Unfortunately, due to the current political situation in Niger after the expedition (an unexpected the coup d’etat in July 2023), most of the fossils remain in the country and return to study a great deal of them, so many observations on two of the Cenomanian sauropods are based upon field notes, pictures and 3D models gathered during fieldwork.
Specimen analysis: This has consisted in the following techniques:
Osteological comparative anatomy: the fossil skeletons have been characterized osteologically, figured 3D scanned (see below).
- Main achievements: Robust morphological evidence for 4 contemporaneous sauropod species.
Phylogenetic analysis: 3 main data matrices (with many updates by subsequent authors) are used at present to explore titanosaur relationships. Since their root taxa, taxa selection and character selection, formulation and scoring is different, their conflicting results are non comparable. Assessing the formulation of homologous characters in different datasets has enabled to perform a comparative cladistics analysis of the datasets using a recent option of the phylogenetic software TNT.
- Main achievements: a comparative analysis that diagnoses the issues with each matrix, shows what specific taxa and character scorings are favoring different tree topologies and allows to craft a hybrid matrix to evaluate titanosaur relationships further.
Histological analysis: so far, only 2 of the 4 sauropod taxa have been sampled. Limb bones from the articulated skeletons from a rebbachisaurid and a titanosaur were sectioned (two long bones per specimen), embedded in resin, polished and observed with magnification to observe the bone microstructure.
- Main achievements: To show that coeval titanosaur and rebbachisaurid had radically different growth strategies in the same environment.
Digitization and modelization: This has consisted in the following techniques:
- Surface scanning of bones: SFM photogrammetry has been used to produce 3D models of the surface of all fossil bones by taking hundreds of overlapping high resolution pictures of each individual bone during and after preparation.
Tomographic bone scanning: using X-rays to obtain tomographic sections of selected fossils to show their internal anatomy were used to produce 3D models of those internal structures, specially focused on learning the configuration of their postcranial pneumatic apparatus with detail.
- Main achievements: This has resulted in the most complete virtual collection of African dinosaur fossils at present, and in the most detailed survey of the postcranial pneumaticity of a sauropod dinosaur, with estimations of air space / bone proportion in 3D (previous attempts used selected tomographic sections to estimate).
Main achievements: The expedition worked the Earlier Jurassic and Lower Cretaceous beds of Niger, but more importantly and related to EvoSaurAF, the crew explored new Cenomanian outcrops Southeast of Agadez. This exploration turned out to be extremely successful, with the finding of several specimens of fish, turtles and theropod and sauropod dinosaurs. While the overall faunal composition is similar to what is known in other outcrops in Niger and North Africa, this new locality is unique in two ways: first, it was deposited in an inland environment evidenced by cross bedding of the sediments; and second it is the only site with a subequal representation of predatory and plant eating dinosaur remains (north African localities tend to preserve more predators, while northwestern Niger localities tend to have more herbivore remains). Among the sauropods, in addition to the two titanosaurs and one rebbachisaurid specimens spotted in 2019, two additional titanosaur partial skeletons, a partial rebbachisaurid skeleton and several isolated elements were found and collected.
Unfortunately, due to the current political situation in Niger after the expedition (an unexpected the coup d’etat in July 2023), most of the fossils remain in the country and return to study a great deal of them, so many observations on two of the Cenomanian sauropods are based upon field notes, pictures and 3D models gathered during fieldwork.
Specimen analysis: This has consisted in the following techniques:
Osteological comparative anatomy: the fossil skeletons have been characterized osteologically, figured 3D scanned (see below).
- Main achievements: Robust morphological evidence for 4 contemporaneous sauropod species.
Phylogenetic analysis: 3 main data matrices (with many updates by subsequent authors) are used at present to explore titanosaur relationships. Since their root taxa, taxa selection and character selection, formulation and scoring is different, their conflicting results are non comparable. Assessing the formulation of homologous characters in different datasets has enabled to perform a comparative cladistics analysis of the datasets using a recent option of the phylogenetic software TNT.
- Main achievements: a comparative analysis that diagnoses the issues with each matrix, shows what specific taxa and character scorings are favoring different tree topologies and allows to craft a hybrid matrix to evaluate titanosaur relationships further.
Histological analysis: so far, only 2 of the 4 sauropod taxa have been sampled. Limb bones from the articulated skeletons from a rebbachisaurid and a titanosaur were sectioned (two long bones per specimen), embedded in resin, polished and observed with magnification to observe the bone microstructure.
- Main achievements: To show that coeval titanosaur and rebbachisaurid had radically different growth strategies in the same environment.
Digitization and modelization: This has consisted in the following techniques:
- Surface scanning of bones: SFM photogrammetry has been used to produce 3D models of the surface of all fossil bones by taking hundreds of overlapping high resolution pictures of each individual bone during and after preparation.
Tomographic bone scanning: using X-rays to obtain tomographic sections of selected fossils to show their internal anatomy were used to produce 3D models of those internal structures, specially focused on learning the configuration of their postcranial pneumatic apparatus with detail.
- Main achievements: This has resulted in the most complete virtual collection of African dinosaur fossils at present, and in the most detailed survey of the postcranial pneumaticity of a sauropod dinosaur, with estimations of air space / bone proportion in 3D (previous attempts used selected tomographic sections to estimate).
Specimen collection 3D mapping techniques: In addition to finding the fossil remains, structure from motion (STM) photogrammetric mapping techniques for these paleontological quarries were developed and tested in order to document their extraction to the most minute detail possible, including daily digitizations on certain quarries. This mapping was done choosing a minimum of 3 stable points marked with stakes that would remain unmoved during the excavation. Early in the morning or late in the evening (when lighting is better), pictures were taken all over the quarry, registering all exposed bones and their spatial relation to the stakes. These pictures were be processed in the computer and aligned to get a record on progress of the excavation, with a stack of 3D models showing the different stages.
These 3D models of the quarries have been supplemented later in the lab with digitization of jackets during the preparation process to achieve a more detailed model and to map elements that might not have been exposed during the excavation stage. Finally, the final prepared bones were also digitized either using STM photogrammetry or tomographic techniques (CT, microCT) as part of other stages of research. However, this 3D models can also be aligned with the excavation and preparation 3D model stacks and get an extremely faithful representation of the disposition of the different bones. This new mapping protocol not only will help future researchers in verifying the ID of bones within a quarry, but also to conduct taphonomic studies on the biostratinomy phase with unprecedented detail and the ability to replicate results and observations.
As a sidenote, many observations done in taxa discovered in the 2022 expedition have only been possible from 3D models of the quarries.
SPECIMEN ANALYSIS:
1) DIVERSITY OF THE CENOMANIAN SAUROPOD FAUNA FROM NIGER: Anatomical comparisons show strong evidence for 4 different contemporaneous sauropods in the Cenomanian of Niger, here named A to D:
- Taxon A: A small sized Rebbachisaur: This taxon is known from at least two relatively well preserved specimens found right next to each other with their associated elements in anatomical order (in unbroken cervical-dorsal-sacral-caudal order, but with limited articulation of the elements). It is roughly the same size of the older nigerien taxon Nigersaurus taquetii but preliminary phylogenetic analyses retrieve it closer to South American taxa such as Katepensaurus. This small rebbachisaurid has short neural spines in its posterior dorsal and anterior and middle caudal vertebrae, with neural spines twice as long as their centra. Cervical vertebrae have lateral pleurocoels in the centra divided by a thin, plate-like bony septum and anteroposteriorly short neural spines. Dorsal vertebrae lack hyposphene and hypantra with the exception of the last dorsal vertebra, which as a laminar hyposphene but no hypantrum. The iliac blade is short dorsoventrally, with the pubic pedicle making an angle close to 90º with the prepubic process.
- Taxon B: A medium sized Rebbachisaur: This taxon is known from a partial articulated skeleton, a partial associated skeleton and isolated remains. It is about 40% larger than the other rebbachisaurid (Taxon A) and has extremely elongated neural spines in its anterior caudal vertebrae, 4 to 5 times taller than their centra. Cervical vertebrae are a little bit more elongate, with its pleurocoel divided by a thick strut like lamina. Middle caudal vertebrae have neural spines taller than the centrum as well. The ischial blade is taller than in the other rebbachisaurid, and the pubic process makes a strongly acute angle with the prepubic process. As this specimen has not been prepared yet most observations on its anatomy are based off field observations, but its differences with the other rebbachisaurid are clear indications of a different taxon as Taxon A was a sexually mature individual (see Histology below). The data on this taxon is too scarce at this point to perform a phylogenetic analysis.
- Taxon C: A large and slender titanosaur: Several skeletons and isolated specimens are referable to this taxon, one of them remarkably complete with an almost complete axial sequence in articulation, limb bones, girdles and ribs. It has a remarkably long neck that makes up half of the length of the dinosaur. Its cervical vertebrae have a relatively low neural spine next to some titanosaurs (e.g. Longkosaurs), but not as low as non-titanosaur somphospondylian Euhelopids. The caudal vertebrae are a fraction the size of the presacral vertebrae, specially the cervicals, with the complete length of the tail less than half that of the neck. The anterior caudal neural spines are mediolaterally narrow with parallel sides in antero-posterior view and lack lamination, except for a very faintly developed prespinal lamina at its base. Middle caudal vertebrae are procoelous while posterior caudal vertebrae are platycoelous. None of the caudal vertebrae are pneumatic. Caudal vertebrae have relatively tall pedicels up to the distal caudals. The limb bones of this taxon are quite gracile, with gracility indices of 7.0 for the humerus, 6.7 for the femur and 11.9 for the fibula. Phylogenetic analyses of this taxon place it at the base of Eutitanosauria.
- Taxon D: A giant and robust titanosaur: This taxon is known from 4 partial skeletons and several isolated or associated remains. The most complete of those skeletons has a femur greater than 190 cm, in the size realm of giant titanosaurs like Dreadnoughtus but a smaller than the largest titanosaurs like Patagotitan or Argentinosaurus. Its posterior dorsal vertebrae have moderately short spines slightly inclined posteriorly. Anterior caudal neural spines are wide mediolaterally and distally expanded, with a little bit more developed prespinal laminae also distally expanded. Anteriormost caudal vertebrae have internal pneumaticity. Middle caudal vertebrae are platycoelous and solid, with short prezygapophyses barely reaching the anterior rim of the centrum. The posterior limb bones are far more robust, the 190 cm long femur having a gracility index of 4.4. The manus is also extremely robust, with stout metacarpals. Like the other titanosaur, this taxon lies at the base of Eutitanosauria.
The morphological evidence strongly shows 4 different taxa coexisted in the Cenomanian in what today is Niger, compatible with Hypothesis 1.
2) ECOMORPHOTYPES OF THE CENOMANIAN SAUROPODS FROM NIGER: The body plan of the four Cenomanian sauropods of the Farak formation is radically different in size and shape.
Taxon A is one of the smallest sauropods from Niger (Cenomanian and otherwise), with a femur less than half that of Taxon D. Moreover, it has a humerus to femur ratio of 0.65 and a relatively short neck. Teeth from recovered from the quarry are pencil-like as in Nigersaurus but lack the asymmetrical enamel distribution. Overall, the feeding capabilities of this taxon are those of a low browser, with a maximum feeding height of less than 3 meters. While it could feed at ground level, the ground-level feeding cranial adaptions of Nigersaurus could not be evaluated.
Taxon B is known from relatively scarcer remains than the other Cenomanian sauropods, but comparative measurements of the pelvic girdle and caudal vertebra centra show Taxon B is 40% larger than Taxon A. Coupled with slightly more elongate cervical vertebrae, it’s likely that the feeding envelope of Taxon B exceeded 3 meters in height, although probably a large part of this envelope overlapped with Taxon A.
Taxon C is moderately large, with a 130 cm femur and a humerus to femur ratio of 0.8. With a shoulder height of only 250 cm, its extremely elongated neck allowed it to reach a height of almost 9 meters osteologically neutral pose. This shows its feeding envelope exceeded those of taxa A and B in height with a small overlap (on the bottom half of Taxon C feeding envelope). This, together with more robust teeth suggest it fed on different vegetation than Taxa A and B.
Taxon D is the largest sauropod from Niger (Cenomanian and otherwise). While only a fragment of anterior cervical vertebra was retrieved, assuming it had proportions similar to non Eutitanosaur somphospondylians, it probably had an elongate neck with at least 14 cervical vertebrae. Even if the neck was not as elongate relative to the trunk as in taxon C, its greater shoulder height and longer neck (in absolute terms) likely conferred it a slightly to greater feeding height than taxon C.
3) PNEUMATIC APPARATUS FROM THE CENOMANIAN SAUROPODS FROM NIGER: Both Rebbachisaur taxa (A and B) show the same type of pneumaticity, with large camerae inside their vertebrae supported by plate or strut like laminae, more similar to a beam in cross-section. On the other hand, both titanosaurs have camellate (or somphospondylous) vertebral tissue, with a pneumatic foramen that invades the vertebra and creates hundreds of smaller chambers inside the centrum and neural arch, more similar to an irregular honeycomb in cross section. Some differences between Taxon D and Taxon C could be observed, since the latter has completely solid anterior caudal vertebrae (arch and centrum) while the former had camellate tissue in anteriormost caudal vertebrae.
Presacral vertebrae were pneumatized in both rebbachisaurids and titanosaurs, but the sacra are remarkably apneumatic in both rebbachisaurids (while the neural spines were still built by plate like laminae and very light, the remainder of neural arches had smaller fossae and the centra completely solid. The anterior caudal vertebrae, however, have pneumatic neural arches and sometimes fossae in the centra.
4) HISTOLOGICAL MICROANATOMY FROM THE CENOMANIAN SAUROPODS FROM NIGER: The previously reported differential bone microanatomy seen in other titanosaurs is present in both thin sections from Taxon C examined. This consists in extremely remodeled bone throught the whole section but few generations of osteons (at most 3 in some spots) and without an external fundamental system (EFS), suggesting a relatively young adult with a histological ontogenetic stage (HOS) of 11. The rebbachisaurid Taxon A shows the same histological remodeling seen in other non-titanosaurs and some titanosaurs with secondary osteons appearing in a more spotty way but with at least two generations of secondary osteons, an HOS of 8-9 (adult but slightly younger than Taxon C). This shows for the first time that coeval titanosaur and rebbachisaurids grew their skeletal system in slightly different fashion. Also, it shows that Taxon A and Taxon C are well past their sexual maturity stage, which likely rules out them being ontomorphs of Taxa B and D respectively.
The preliminary skeletal reconstructions of the Cenomanian sauropods from the Farak Formation show feeding envelopes with large non overlapping regions that suggest differentiated ecological niches. The pneumatic apparatus of rebbachisaurs had a slightly different distribution on the skeleton from those of the titanosaurs. While the presacral vertebrae were thoroughly pneumatic in both groups, Rebbachisaurs show pneumatic anterior caudal neural arches and some centra, while only Taxon D had anterior pneumatic caudal vertebrae. The sacral vertebrae are less pneumatized in Rebbachisaurs while both titanosaurs have greatly pneumatized caudal vertebrae. The histological analysis shows very different growth strategies in the same environment, with the Titanosaur suffering a more aggressive remodeling on its bone microstructure, suggesting a higher metabolism. All this evidence shows coeval rebbachisaurids and titanosaurs had different feeding and growth strategies compatible with Hypothesis 2.
These 3D models of the quarries have been supplemented later in the lab with digitization of jackets during the preparation process to achieve a more detailed model and to map elements that might not have been exposed during the excavation stage. Finally, the final prepared bones were also digitized either using STM photogrammetry or tomographic techniques (CT, microCT) as part of other stages of research. However, this 3D models can also be aligned with the excavation and preparation 3D model stacks and get an extremely faithful representation of the disposition of the different bones. This new mapping protocol not only will help future researchers in verifying the ID of bones within a quarry, but also to conduct taphonomic studies on the biostratinomy phase with unprecedented detail and the ability to replicate results and observations.
As a sidenote, many observations done in taxa discovered in the 2022 expedition have only been possible from 3D models of the quarries.
SPECIMEN ANALYSIS:
1) DIVERSITY OF THE CENOMANIAN SAUROPOD FAUNA FROM NIGER: Anatomical comparisons show strong evidence for 4 different contemporaneous sauropods in the Cenomanian of Niger, here named A to D:
- Taxon A: A small sized Rebbachisaur: This taxon is known from at least two relatively well preserved specimens found right next to each other with their associated elements in anatomical order (in unbroken cervical-dorsal-sacral-caudal order, but with limited articulation of the elements). It is roughly the same size of the older nigerien taxon Nigersaurus taquetii but preliminary phylogenetic analyses retrieve it closer to South American taxa such as Katepensaurus. This small rebbachisaurid has short neural spines in its posterior dorsal and anterior and middle caudal vertebrae, with neural spines twice as long as their centra. Cervical vertebrae have lateral pleurocoels in the centra divided by a thin, plate-like bony septum and anteroposteriorly short neural spines. Dorsal vertebrae lack hyposphene and hypantra with the exception of the last dorsal vertebra, which as a laminar hyposphene but no hypantrum. The iliac blade is short dorsoventrally, with the pubic pedicle making an angle close to 90º with the prepubic process.
- Taxon B: A medium sized Rebbachisaur: This taxon is known from a partial articulated skeleton, a partial associated skeleton and isolated remains. It is about 40% larger than the other rebbachisaurid (Taxon A) and has extremely elongated neural spines in its anterior caudal vertebrae, 4 to 5 times taller than their centra. Cervical vertebrae are a little bit more elongate, with its pleurocoel divided by a thick strut like lamina. Middle caudal vertebrae have neural spines taller than the centrum as well. The ischial blade is taller than in the other rebbachisaurid, and the pubic process makes a strongly acute angle with the prepubic process. As this specimen has not been prepared yet most observations on its anatomy are based off field observations, but its differences with the other rebbachisaurid are clear indications of a different taxon as Taxon A was a sexually mature individual (see Histology below). The data on this taxon is too scarce at this point to perform a phylogenetic analysis.
- Taxon C: A large and slender titanosaur: Several skeletons and isolated specimens are referable to this taxon, one of them remarkably complete with an almost complete axial sequence in articulation, limb bones, girdles and ribs. It has a remarkably long neck that makes up half of the length of the dinosaur. Its cervical vertebrae have a relatively low neural spine next to some titanosaurs (e.g. Longkosaurs), but not as low as non-titanosaur somphospondylian Euhelopids. The caudal vertebrae are a fraction the size of the presacral vertebrae, specially the cervicals, with the complete length of the tail less than half that of the neck. The anterior caudal neural spines are mediolaterally narrow with parallel sides in antero-posterior view and lack lamination, except for a very faintly developed prespinal lamina at its base. Middle caudal vertebrae are procoelous while posterior caudal vertebrae are platycoelous. None of the caudal vertebrae are pneumatic. Caudal vertebrae have relatively tall pedicels up to the distal caudals. The limb bones of this taxon are quite gracile, with gracility indices of 7.0 for the humerus, 6.7 for the femur and 11.9 for the fibula. Phylogenetic analyses of this taxon place it at the base of Eutitanosauria.
- Taxon D: A giant and robust titanosaur: This taxon is known from 4 partial skeletons and several isolated or associated remains. The most complete of those skeletons has a femur greater than 190 cm, in the size realm of giant titanosaurs like Dreadnoughtus but a smaller than the largest titanosaurs like Patagotitan or Argentinosaurus. Its posterior dorsal vertebrae have moderately short spines slightly inclined posteriorly. Anterior caudal neural spines are wide mediolaterally and distally expanded, with a little bit more developed prespinal laminae also distally expanded. Anteriormost caudal vertebrae have internal pneumaticity. Middle caudal vertebrae are platycoelous and solid, with short prezygapophyses barely reaching the anterior rim of the centrum. The posterior limb bones are far more robust, the 190 cm long femur having a gracility index of 4.4. The manus is also extremely robust, with stout metacarpals. Like the other titanosaur, this taxon lies at the base of Eutitanosauria.
The morphological evidence strongly shows 4 different taxa coexisted in the Cenomanian in what today is Niger, compatible with Hypothesis 1.
2) ECOMORPHOTYPES OF THE CENOMANIAN SAUROPODS FROM NIGER: The body plan of the four Cenomanian sauropods of the Farak formation is radically different in size and shape.
Taxon A is one of the smallest sauropods from Niger (Cenomanian and otherwise), with a femur less than half that of Taxon D. Moreover, it has a humerus to femur ratio of 0.65 and a relatively short neck. Teeth from recovered from the quarry are pencil-like as in Nigersaurus but lack the asymmetrical enamel distribution. Overall, the feeding capabilities of this taxon are those of a low browser, with a maximum feeding height of less than 3 meters. While it could feed at ground level, the ground-level feeding cranial adaptions of Nigersaurus could not be evaluated.
Taxon B is known from relatively scarcer remains than the other Cenomanian sauropods, but comparative measurements of the pelvic girdle and caudal vertebra centra show Taxon B is 40% larger than Taxon A. Coupled with slightly more elongate cervical vertebrae, it’s likely that the feeding envelope of Taxon B exceeded 3 meters in height, although probably a large part of this envelope overlapped with Taxon A.
Taxon C is moderately large, with a 130 cm femur and a humerus to femur ratio of 0.8. With a shoulder height of only 250 cm, its extremely elongated neck allowed it to reach a height of almost 9 meters osteologically neutral pose. This shows its feeding envelope exceeded those of taxa A and B in height with a small overlap (on the bottom half of Taxon C feeding envelope). This, together with more robust teeth suggest it fed on different vegetation than Taxa A and B.
Taxon D is the largest sauropod from Niger (Cenomanian and otherwise). While only a fragment of anterior cervical vertebra was retrieved, assuming it had proportions similar to non Eutitanosaur somphospondylians, it probably had an elongate neck with at least 14 cervical vertebrae. Even if the neck was not as elongate relative to the trunk as in taxon C, its greater shoulder height and longer neck (in absolute terms) likely conferred it a slightly to greater feeding height than taxon C.
3) PNEUMATIC APPARATUS FROM THE CENOMANIAN SAUROPODS FROM NIGER: Both Rebbachisaur taxa (A and B) show the same type of pneumaticity, with large camerae inside their vertebrae supported by plate or strut like laminae, more similar to a beam in cross-section. On the other hand, both titanosaurs have camellate (or somphospondylous) vertebral tissue, with a pneumatic foramen that invades the vertebra and creates hundreds of smaller chambers inside the centrum and neural arch, more similar to an irregular honeycomb in cross section. Some differences between Taxon D and Taxon C could be observed, since the latter has completely solid anterior caudal vertebrae (arch and centrum) while the former had camellate tissue in anteriormost caudal vertebrae.
Presacral vertebrae were pneumatized in both rebbachisaurids and titanosaurs, but the sacra are remarkably apneumatic in both rebbachisaurids (while the neural spines were still built by plate like laminae and very light, the remainder of neural arches had smaller fossae and the centra completely solid. The anterior caudal vertebrae, however, have pneumatic neural arches and sometimes fossae in the centra.
4) HISTOLOGICAL MICROANATOMY FROM THE CENOMANIAN SAUROPODS FROM NIGER: The previously reported differential bone microanatomy seen in other titanosaurs is present in both thin sections from Taxon C examined. This consists in extremely remodeled bone throught the whole section but few generations of osteons (at most 3 in some spots) and without an external fundamental system (EFS), suggesting a relatively young adult with a histological ontogenetic stage (HOS) of 11. The rebbachisaurid Taxon A shows the same histological remodeling seen in other non-titanosaurs and some titanosaurs with secondary osteons appearing in a more spotty way but with at least two generations of secondary osteons, an HOS of 8-9 (adult but slightly younger than Taxon C). This shows for the first time that coeval titanosaur and rebbachisaurids grew their skeletal system in slightly different fashion. Also, it shows that Taxon A and Taxon C are well past their sexual maturity stage, which likely rules out them being ontomorphs of Taxa B and D respectively.
The preliminary skeletal reconstructions of the Cenomanian sauropods from the Farak Formation show feeding envelopes with large non overlapping regions that suggest differentiated ecological niches. The pneumatic apparatus of rebbachisaurs had a slightly different distribution on the skeleton from those of the titanosaurs. While the presacral vertebrae were thoroughly pneumatic in both groups, Rebbachisaurs show pneumatic anterior caudal neural arches and some centra, while only Taxon D had anterior pneumatic caudal vertebrae. The sacral vertebrae are less pneumatized in Rebbachisaurs while both titanosaurs have greatly pneumatized caudal vertebrae. The histological analysis shows very different growth strategies in the same environment, with the Titanosaur suffering a more aggressive remodeling on its bone microstructure, suggesting a higher metabolism. All this evidence shows coeval rebbachisaurids and titanosaurs had different feeding and growth strategies compatible with Hypothesis 2.