Introducing Bajadasaurus pronuspinax.

Bajadasaurus reconstruction (Museo Municipal Ernesto Bachmann, Villa El Chocón, Neuquén).

Dicraeosauridae is a family of mid-sized sauropod dinosaurs characterized by a distinctive vertebral column with paired, long, neural spines. The group was first described in 1914 by Werner Janensch with the discovery of the nearly complete skeletons of Dicraeosaurus in the expeditions to the upper Jurassic beds of Tendaguru, Tanzania. Dicraeosauridae includes  Amargasaurus, Pilmatueia, Suuwassea, and Brachytrachelopan. Now, the description of Bajadasaurus pronuspinax gen. et sp. nov., from the Early Lower Cretaceous of Bajada Colorada Formation in Northern Patagonia, Argentina), shed new light on the function of its spines and the defense behavior in sauropod dinosaurs.

Bajadasaurus was discovered in 2013, by a team of paleontologists from CONICET, Fundación Félix de Azara, Universidad Maimónides, and Museo Paleontológico Ernesto Bachmann. The generic name derived from Bajada (Spanish, in reference to the locality Bajada Colorada) and saurus (Greek, lizard). The specific name derived from pronus (Latin, bent over forward) and spinax (Greek, spine), referring to the anteriorly pointed, curved, neural spines of the cervical vertebrae.

Skeletal elements of Bajadasaurus pronuspinax. From Gallina et al., 2019.

The holotype, MMCh-PV 75, includes a nearly complete skull (left maxilla, left lacrimal, both prefrontals, both frontals, both parietals, both postorbitals, both squamosals, left quadratojugal, both pterygoids, both quadrates, supraoccipital, exoccipital-opisthotic complex, basioccipital, basisphenoid, both prootics, both laterosphenoids, both orbitosphenoids, both dentaries, left surangular, both angulars, both splenials, left prearticular, left articular, isolated upper tooth row), both proatlases, atlantal neurapophyses, axis and the fifth cervical vertebra.

The skull of Bajadasaurus is gracile, with dorsally exposed orbits, dorsoventrally compressed occipital condyle, extremely narrow basipterygoid processes, elongate and slender anterior processes of the squamosals, medially extended post-temporal fenestrae, short lateral temporal fenestrae and a reduced dentition in the maxilla and dentary, that largely differs from other known taxa within Dicraeosauridae. But the most striking feature of Bajadasaurus is the presence of extremely long cervical neural spines that curve anteriorly. Amargasaurus exhibit the same development of cervical neural spine elongation as Bajadasaurus, but the spines of the former point backwards rather than forwards. Dicraeosaurus and Brachytrachelopan show anteriorly inclined neural spines in the cervical vertebrae, but the spines are much shorter than in Bajadasaurus.

A group of Bajadasaurus. Illustration: Jorge A. González.

The discovery of Amargasaurus cazaui in 1991, from the Early Cretaceous beds of La Amarga Formation of Northern Patagonia, renewed the discussion on the peculiar vertebral anatomy of these sauropod dinosaurs, including interpretations as a support structure for a thermoregulatory sail, a padded crest as a display and/or clattering structure, a dorsal hump, or as internal cores of dorsal horn. Those explanation, except the last one, require that these long and extremely gracile bone projections, now recognized in Bajadasaurus as well, can support enough physical stress to avoid fracturing. Bone is stronger and stiffer in passive situations, however, horns and other keratin-based materials are tougher and highly resistant to impact-related fractures. Therefore, the keratinous sheath in Amargasaurus and perhaps Bajadasaurus provides a better mechanical solution against a potential fracture.

 

References:

Gallina, Pablo A., Apesteguía, Sebastián, Canale, Juan I., Haluza, Alejandro (2019), A new long-spined dinosaur from Patagonia sheds light on sauropod defense system, Scientific Reports volume 9, Article number: 1392 DOI: https://doi.org/10.1038/s41598-018-37943-3

Janensch, W. Die Wirbelsäule der Gattung Dicraeosaurus. Palaeontographica Supplement 7, 37–133 (1929).

Salgado, L. & Bonaparte, J. F. Un nuevo saurópodo Dicraeosauridae, Amargasaurus cazaui gen et sp. nov., de la Formación La Amarga, Neocomiano de la provincia del Neuquén, Argentina. Ameghiniana 28, 333–346 (1991).

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An early juvenile enantiornithine specimen from the Early Cretaceous of Spain

The slab and counterslab of MPCM-LH-26189

Mesozoic remains of juvenile birds are rare. To date, the only records are from the Early Cretaceous of China and Spain, from the mid-Cretaceous of  Myanmar, and from the Late Cretaceous of Argentina and Mongolia. The most recent finding from the Early Cretaceous of Las Hoyas, Spain, provide an insight into the osteogenesis of the Enantiornithes, the most abundant clade of Mesozoic birds. Previous records of Enantiornithes from the Las Hoyas fossil site include: Eoalulavis hoyasi, Concornis lacustris, and Iberomesornis romerali.

The latest specimen, MPCM-LH-26189, a nearly complete and largely articulated skeleton (only the feet, most of its hands, and the tip of the tail are missing), is very small. The specimen died around the time of birth, a crucial moment to study the osteogenesis in birds. The skull, is partially crushed, and is large compared to the body size. The braincase is fractured. The frontals and the parietals form a uniformly curved cranial vault. The cerebrocast shows a very slight inflation, suggesting that the cerebral anatomy of MPCM-LH-26189 falls in between that of the Archaeopteryx, and the putative basal ornithurine Cerebavis, whose telencephalic expansion is close to most extant birds. The cervical series is composed of 9 vertebrae. There are 10  thoracic vertebrae, and the sacrum appears to be composed of 5–6 vertebrae. The prezygapophyses of the mid-thoracic vertebrae extend beyond the cranial articular surface. The thoracic ribs are joint to the thoracic vertebrae. The two coracoids, the furcula, and three sternal ossifications are preserved. The furcula is Y-shapped. Both humeri, ulnae, and radii are also preserved.

Reconstruction of MPCM-LH-26189 by Raúl Martín

The osteohistological analysis of the left humerus shows a dense pattern of longitudinal grooves. Those grooves correspond to primary cavities, which open onto the surface of the cortex in young and fast-growing bone. The shaft of the tibia and radius show very-thin cortices. In addition,  the primary nature of the vascularisation, the round shape of the osteocytes lacunae and the uneven peripheral margin of the medullary cavity (with no endosteal bone), strongly suggests that the bone was actively growing when the bird died.

Enantiornithines show a mosaic of characters, reflecting their intermediate phylogenetic position between the basal-pygostylians and modern bird. In this clade, the sternum adopts an elaborate morphology, and in adult Enantiornithes, no more than eight free caudal vertebrae precede the pygostyle. The differences observed in the ossification of the sternum and the number of free caudal vertebrae in MPCM-LH-26189, when it compared to other juvenile enantiornithines, reveal a clade-wide asynchrony in the sequence of ossification of the sternum and tail, suggesting that the developmental strategies of these basal birds may have been more diverse than previously thought.

References:

Fabien Knoll, et al., “A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds,” Nature Communications, volume 9, Article number: 937 (2018) doi:10.1038/s41467-018-03295-9

Chiappe, L. M., Ji, S. & Ji, Q. Juvenile birds from the Early Cretaceous of China: implications for enantiornithine ontogeny. Am. Mus. Novit. 3594, 1–46 (2007).

 

 

Darwin and the flowering plant evolution in South America.

pollen

Retimonocolpites sp. (Adapted from Llorens and Loinaze, 2015)

Charles Darwin’s fascination and frustration with the evolutionary events associated with the origin and early radiation of flowering plants are legendary. In a letter to Oswald Heer, a famous Swiss botanist, and paleontologist, Darwin wrote: “the sudden appearance of so many Dicotyledons in the Upper Chalk appears to me a most perplexing phenomenon to all who believe in any form of evolution, especially to those who believe in extremely gradual evolution, to which view I know that you are strongly opposed”. Heer discussed about the early angiosperm fossil record with Darwin, in a letter dated 1 March 1875: “if we say that the Dicotyledons begin with the Upper Cretaceous, we must still concede that this section of the vegetable kingdom, which forms the bulk of modern vegetation, appears relatively late and that, in geological terms, it underwent a substantial transformation within a brief period of time.” 

Darwin’s defense of a gradualist perspective led him to suggest that prior to the Cretaceous record of flowering plants, angiosperms had slowly evolved and diversified on a remote landmass. On 22 July 1879, in a letter to Joseph Dalton Hooker, Darwin refers to the early evolution of flowering plants as an “abominable mystery”. Nearing the end of his life, he wrote to Hooker another letter about a lost fossil record in the earliest phases of angiosperm diversification:  “Nothing is more extraordinary in the history of the Vegetable Kingdom, as it seems to me, than the apparently very sudden or abrupt development of the higher plants. I have sometimes speculated whether there did not exist somewhere during long ages an extremely isolated continent, perhaps near the South Pole.”

Letter from Charles Darwin to Joseph Dalton Hooker, written 22 July 1879 (provenance: Cambridge University Library DAR 95: 485–488)

Letter from Charles Darwin to Joseph Dalton Hooker, written 22 July 1879 (provenance: Cambridge University Library DAR 95: 485–488)

While the oldest records of the different groups of angiosperms are still in discussion, the outcrops of the Baquero Group, located in Argentinean Patagonia, contain one of the richest and most diverse Early Cretaceous floras in the Southern Hemisphere. The unit comprises three formations: Anfiteatro de Tico, Bajo Tigre and Punta del Barco. The first reports of angiosperm remains for the Anfiteatro de Tico Formation were made in 1967. The dominant types are Clavatipollenites, and Retimonocolpites.

Pollen grains  could enter into the fossil record by falling directly into swamps or lakes, or being washed into them or into the rivers and seas. The ones which are not buried in reducing sediments will tend to become oxidized and be destroyed. They reflects the ecology of their parent plants and their habitats and provide a continuous record of their evolutionary history. Gymnosperms pollen often is saccate (grains with two or three air sacs attached to the central body), while Angiosperm pollen shows more variation and covers a multitude of combinations of features: they could be  in groups of four (tetrads),  in pairs (dyads),  or single (monads). The individual grains can be inaperturate, or have one or more pores, or slit-like apertures or colpi (monocolpate, tricolpate).

pollen

Clavatipollenites sp. SEM (Adapted from Archangelsky 2013)

Clavatipollenites pollen grains are interpreted as related to the modern family Chloranthaceae. The genus was established by Couper for dispersed monosulcate pollen grains recovered from the Early Cretaceous of Britain. Currently, the genus has a very broad definition. The genus Retimonocolpites include elongated to subcircular semitectate, columellate and microreticulate pollen grains with well defined monocolpate aperture (Llorens and Loinaze, 2015). The new species Jusinghipollisticoensis sp. nov. represents one of the oldest records of trichotomosulcate, and extends the geographical distribution of Early Cretaceous trichotomosulcate pollen grains to southern South America.

The data also indicates strong similarities between the Baquero Group assemblages and other coeval units from Argentina, Australia and United States.

References:

M. Llorens, V.S. Perez Loinaze, Late Aptian angiosperm pollen grains from Patagonia: Earliest steps in flowering plant evolution at middle latitudes in southern South America, Cretaceous Research 57 (2016) 66-78

Archangelsky, S.,et al. (2009). Early angiosperm diversification: evidence from southern South America. Cretaceous Research, 30, 1073-1082.

Doyle, J. A., & Endress, P. K. (2014). Integrating Early Cretaceous fossils into the phylogeny of living Angiosperms: ANITA. Lines and relatives of Chloranthaceae. International Journal of Plant Sciences, 175, 555-560.