The authors have declared that no competing interests exist.
Conceived and designed the experiments: AS HZ. Performed the experiments: AS HZ. Analyzed the data: AS HZ. Contributed reagents/materials/analysis tools: AS HZ FN CM RC. Wrote the paper: AS HZ FN CM.
Macrostomatan snakes, one of the most diverse extant clades of squamates, display an impressive arsenal of cranial features to consume a vast array of preys. In the absence of indisputable fossil representatives of this clade with well-preserved skulls, the mode and timing of these extraordinary morphological novelties remain obscure. Here, we report the discovery of
In recent years, the discovery of new and nearly complete fossil specimens as well as the reanalysis of previously known materials has dramatically improved our knowledge about the evolution of snakes
Macrostomata constitutes the most diverse group of snakes today, including nearly all of the extant species
Here we report a new fossil snake from Paleocene beds of Bolivia that emerges in our analysis as the sister-group of the clade formed by bolyeriids, tropidophiids, and caenophidians. The new fossil preserves the most complete and oldest macrostomatan skull found so far, filling an important gap in the evolutionary history of this relevant clade of snakes. It further provides a relevant calibration point to discuss the evolutionary timing of advanced terms of macrostomatan snakes.
All necessary permits were obtained for the described study from Comité de Preservación del Patrimonio Departamental (Cochabamba department, Bolivia), which complied with all relevant regulations.
The character-taxon matrix used in the phylogenetic analysis is mainly based on a published phylogenetic analysis
We analysed our dataset using TNT
Additionally, the morphological dataset was analyzed with the 13 extant terminal taxa constrained with a backbone formed by the topology derived from the molecular analysis performed by Wiens and colleagues
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “
Serpentes Linnaeus 1758.
Alethinophidia Nopcsa 1923.
Macrostomata Müller 1831.
The generic name derives from the Aymara word “
MHNC 13323 (Museo de Historia Natural de Cochabamba “Alcides D´orbigny, Cochabamba, Bolivia), an articulated incomplete skull consisting of a left vomer, incomplete left septomaxilla, left maxilla, left ectopterygoid, left palatine, the anterior tip of the left pterygoid, left postorbital, both frontals, parietal, and parasphenoid rostrum (
Photographs and half-tone drawings in (A) left lateral, (B) right lateral, (C) dorsal and (D) ventral views. Dotted areas indicate matrix. chp, choanal process; ec, ectopterygoid; fr, frontal; ip, interchoanal process; mx, maxilla; mxp, maxillary process; op, optic foramen; p, parietal; pf, prefrontal; pl, palatine; plp, palatine process; po, postorbital; ps, parasphenoid; pt, pterygoid; sm, septomaxilla; v, vomer.
Tiupampa locality, Mizque province of the department of Cochabamba, Bolivia. Medium-grained sandstones of the middle levels of Santa Lucía Formation, Early Paleocene (Danian
A small, derived macrostomatan snake that can be distinguished from all other members of Serpentes by the following combination of characters: an elongated vomer with a reduced contribution to the vomeronasal fenestra; one foramen piercing the cavity housing Jacobson’s organ; maxilla with 21 tooth positions and the posterior most tooth separated by a diastema from the others; ectopterygoid with a small medial process and a ventral articular surface with the pterygoid; a broad choanal process of the palatine; optic fenestra formed by both frontal and parietal.
Estimation of the ontogenetic stage of a fossil snake skull is problematic because there are few studies on postnatal ontogeny in snakes. Those that do exist all focus on allometric variations in skull elements
At a first glance, the tiny size (
Maxilla length | 4,6 |
Ectopterygoid length | 2,2 |
Palatine length | 3,1 |
Frontal length | 1,75 |
Postorbital height | 0,9 |
Measurements are in mm;
refers to estimated value.
Based on personal observations, ontogenetic transformations in the postorbital bone may be useful in distinguishing ontogenetic stages in macrostomatan snakes. Like those of several adult macrostomatan skulls examined (see
The type specimen consists of a small articulated skull, with some elements (e.g. snout bones) barely displaced. Its anatomy reveals booid traits in combination with some apomorphic features present in tropidophiids and caenophidian snakes.
The preserved snout bones of
(A) frontal view of the partial skull; (B) dorsolateral view of the left orbit; (C) ventral view of the palatal region; (D) scanning electron microscope image of the rear maxillary region. chp, choanal process; dot, ductus for olfactory tract; ec, ectopterygoid; fr, frontal; ip, interchoanal process; mfr, medial frontal flange; mx, maxilla; mxp, maxillary process; op, optic foramen; p, parietal; pf, prefrontal; pl, palatine; plp, palatine process; po, postorbital; ps, parasphenoid; pt, pterygoid; sm, septomaxilla; v, vomer.
The maxilla of
(A) the boid
The preserved left prefrontal of
The frontals are slender bones that show complete (fused) interolfactory pillars (
The tree obtained from the phylogenetic analysis of parsimony (
Temporally calibrated cladogram of the most parsimonious tree obtained in this analysis. Thick gray lines indicate stratigraphic range of known taxa (dashed area indicates that these records are based on vertebral remains). Dashed lines represent ghost lineages implied by the stratigraphic distribution of fossils with respect to the phylogenetic relationships shown here (note the exceptionally abundant ghost lineages for Macrostomata). Ages of first appearance for taxa used in the calibrated phylogeny are given in electronic supplementary material. Al, Albian; Ce, Cenomanian; Tu, Turonian; Co, Coniacian; Sa, Santonian; Cam, Campanian; Ma, Maastrichtian; Pal, Paleocene; Eoc, Eocene; Oli, Oligocene; Mi, Miocene; Pl-Ple, Plio-Pleistocene.
As in more recent morphological analyses
Our additional constrained analysis conducted to test the effect of a molecular topology in the phylogenetic dataset resulted in two most parsimonious topologies. The strict consensus tree (
With the exception of a few differences, the maxillary morphology of
Another feature of
It is widely assumed that the most important evolutionary innovation of macrostomatan snakes is the increase of gape size to swallow large items of food
Recent discoveries of relevant fossil specimens in Mesozoic and Caenozoic strata have elucidated the central role of southern landmasses in the origin of snakes
Numerous snake materials assigned to different groups of macrostomatans have been found in Cretaceous and Paleocene deposits around the world
In light of these comments about the nature of the early fossil record of Macrostomata,
(DOC)
(NEX)
We thank Darrell Frost, Ivan Ineich, Julián Faivovich, Jorge Williams and Francisco L. Franco for loan of specimens. Gratitude is also due to Manuel Sosa for the skillful drawings and Camilo Arredondo for the photographs. Jack Conrad and an anonymous reviewer gave thorough and constructive criticisms. We are grateful to Diego Pol for his advice on the use of the T.N.T. program package. Finally, we are grateful to Anjan Bhullar for the revision of English grammar. The phylogenetic analysis was performed with the program TNT that is freely available through the Willi Hennig Society.