Answer to the last post

Ok, so while I slowly prepare a more lengthy post. Here’s the answer from last post. The skull I showed last time is that of a species of Metaxytherium, exactly which one, well, I’ll talk about that in the future. That skull is from Early Oligocene deposits, therefore it is several million years older than other reported species of Metaxytherium.

Here’s the skull in lateral and dorsal views. (Notice that part of the rostrum is missing.)

It is somewhat similar to M. crataegense (=M. calvertense [Aranda-Manteca et al., 1994]) from the Early Miocene of the Western Atlantic and Caribbean, as well as to other species of Metaxytherium, but, it also displays plesiomorphic conditions not seen in those other species.

Another possibility would be Caribosiren turneri (drawing above modified from Reinhart, 1959), from the Early Oligocene of Puerto Rico. But, unlike Caribosiren the rostrum of this critter doesn’t seem to have been as extremely downturned; also it belonged to an animal at least 15% larger than Caribosiren. Other Oligocene halitheriines such as Halitherium schinzii from the European Early Oligocene, have different morphology of the parietal as well as much larger nasals; Eosiren imenti, known from the Early Oligocene of Egypt (Domning et al., 1994), is even more primitive than Halitherium schinzii, as it still has permanents premolars and canines, among other characteristics.

So, as far as I can tell, it seems to be a primitive Metaxytherium. There are other skulls from Early or Late Oligocene of the western Atlantic, which are very similar to this one, possibly the same species, that’s one thing I have to figure out as part of my thesis. If you’re going to the SVP meeting this year, you might learn more about this critter.

Aranda-Manteca, F. J., D. P. Domning and L. G. Barnes. 1994. A new Middle Miocene sirenian of the genus Metaxytherium from Baja California: relationships and paleobiogeographic implications. Proceedings of the San Diego Society of Natural History 29:191-204.

Domning, D. P., P. D. Gingerich, E. L. Simons and F. A. Ankel-Simons. 1994. A new Early Oligocene dugongid (Mammalia, Sirenia) from Fayum Province, Egypt. Contributions from the Museum of Paleontology, the University of Michigan 29(4):89-108.

Reinhart, R. H. 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1-146.

A river runs through an Oligocene sea: parte III

So, keeping up with stuff I’ve collected at the Río G locality (for previous entries about this locality go here and here), I bring you some of the sirenian material that I collected last January. It is not much, but it adds to stuff I’ve collected previously (as you’ll see).

In tropical regions, a lot of times, good outcrops are along riverbanks. Puerto Rico is no exception and that is why the Rio G locality is so good, the exposure is kept “fresh” because of the nearly constant river erosion. Of course the drawback is that fossils are also lost if nobody visits these type of localities at least once a month or after big rainstorms (at least that’s what I used to do).

The picture above shows one such example. I spotted this bone fragment on the wall, very close to water level (a little more than half a meter). And, as you can see in the inset, the surface facing away from the rock looks freshly broken. I did collect the fossil, and unsuccessfully looked for additional fragments nearby.

Above is the picture of the fossil, in dorsal view (anterior towards the top), and the interpretative drawing. As it turns out the fossil was part of a sirenian skull. What was left of the fossil, is the anterior part of the frontals (Fr) and the nasals (N), the supraorbital processes of the frontals are missing. The convex frontal roof and shallow nasal incisure (the concave area between the frontals) are some characters that identify this fossil as belonging to a halitheriine dugongid. In fact, it is very similar to the same part of a much more complete skull that I collected from that locality several years ago (see below).

The figure above shows the more complete skull. A and B show a close-up dorsal view (anterior towards the top) of the area that was preserved in the fragmentary fossil. C is a dorsal view of the skull (anterior to the right) with the outline of the enlarged area in A. As you can see the frontal and its relationship with the nasals look much the same as the fragmentary fossil. It also displays a shallow nasal incisure at the posterior end of the mesorostral fossa (MRF) and convex frontal roof. In the more complete skull the premaxilla (Pmx) partially cover the nasals and the supraorbital processes (SOP) are preserved. (In A and C it is missing the left nasal process of the premaxilla which is loose and needs to be re-attached, but I was able to put it in the drawing).

I’m pretty certain that the fragmentary fossil belongs to the same species as the more complete specimen; it was a pity that part of it was lost previous to it being found. If you’re well acquainted with extinct sirenians, you can probably guess what genus this skull belongs to. So, go ahead and make a guess!

Chevron weirdness in a sirenian

Following up on a comment I made over at Updates from the Vertebrate Paleontology Lab, I here bring you pictures of an unusual sirenian chevron from the Late Oligocene of Puerto Rico.

These fossils were collected as part of a partial articulated postcranium which I mentioned here. Chevrons, also known as hemal arches, are (normally) V-shaped* bones, consisting of two rami that meet ventrally, hence the shape. They protect blood vessels.

*More like Y-shaped due to the length of the symphysis in some specimens.

In the picture above you can see left lateral views of two of the chevrons I collected with that specimen. The chevron on the left seems to represent fused chevron 2 + 3 (missing part of its ventral edge), and the other one is chevron 4, which is normal.

Here is a posterior view, again, the one on the left likely represents fused chevron 2 + 3, and on the right is chevron 4, which is missing the right ramus.

To me this seems to have been a developmental anomaly rather than occurring due to an injury. Apparently, this is a first, at least for Sirenia! As I mentioned above these are part of an articulated partial postcranium, belonging to a new dugongine taxon from the Late Oligocene of northern Puerto Rico (you can see the skull here).

So, leave a comment, let me know what you think about this unusual bone!

A river runs through an Oligocene sea: part II

This is the first post of the year and, hopefully, more will follow. Anyways, today I bring you a brief overview of fieldwork that I did back in January while I was in Puerto Rico. I had written in a previous occasion about this locality (you can read part I here) and unlike back then, I did find some vertebrate fossils.

The picture above shows one of the most productive outcrops along this locality. As you can see there are about four distinct units. I and III are paleosols (ancient soil horizons) whereas II seems to be shallow marine/brackish and IV shallow marine deposits. Unit II has yielded good fossils in the past, including a sirenian skull and associated axial skeleton, croc teeth, rodent teeth and some nurse shark teeth (gynglymostomatids) (update: you can read about newer discoveries at this site and other nearby ones here, here, here, here, and here). These beds are part of the San Sebastian Formation of early Oligocene age, which in the past have yielded other interesting fossils such as the sirenian Caribosiren turneri (Reinhart, 1959) and the gharial Aktiogavialis puertoricencis (Vélez-Juarbe et al. 2007), among others.

Like I mentioned in the intro, this time around I did find some cool stuff!

In the photo above you can see a closeup of unit II showing some of the fossils as I found them and before I started digging. The red circles are for turtle shell fragments while the green is a sirenian rib. Yes, I know, they are somewhat difficult to see but click on the picture and look carefully, you’ll see them.

The sirenian rib was isolated and easy to collect; it is fairly normal to find isolated sirenian ribs in the San Sebastian Fm and other Oligocene and Miocene localities in Puerto Rico. In contrast, while digging around the turtle shell fragments shown in the picture above, I kept stumbling upon more fragments, until it was apparent that this represented a partially disarticulated turtle shell. Not only that, but there was also an associated left pelvis (shown in the picture below), not bad!!

One of the first posts on this blog was an overview of what's known of the fossil side-neck turtles from Puerto Rico. In it I mentioned that some pelomedusid (a more technical name for side-necks) material from the San Sebastian Fm. had been described by Wood (1972) as an unknown taxon. In fact, Wood (1972) not only described an incomplete shell and plastron but also an associated pelvis. Maybe this material I collected represents additional specimens of that unknown taxon. However, comparison with the description as well as with other turtle fossils from the overlying Lares Limestone will have to wait until after the specimens are prepared (you can get a glimpse of the preparation process here). Below you can see the jackets with the specimens inside.

Preliminarily, I am quite certain that this San Sebastian turtle is a pelomedusoid just by the morphology of the pelvis. Also, there was one other fossil collected that day (is in the jacket in the far left), which was both interesting and frustrating, but I'll leave that for next time!

Recommended Literature

Reinhart, R. H. 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1-146.

Vélez-Juarbe, J., C. A. Brochu, and H. Santos. 2007. A gharial from the Oligocene of Puerto Rico: transoceanic dispersal in the history of a non-marine reptile. Proceedings of the Royal Society B 274:1245-1254.

Wood, R. C. 1972. A fossil pelomedusid turtle from Puerto Rico. Breviora 392:1-13.

This post was updated April 28, 2020

Nuevos sirénidos del Eoceno

Este año ha visto la publicación de dos artículos describiendo sirénidos del Eoceno (55.8-33.9 millones de años atrás [ma]). En adición a esos, otras dos especies más fueron descritos del Mioceno, los cuales mencioné en una entrada pasada. Creo que no está mal, cuatro nuevas especies de sirénidos fósiles en un año, especialmente para un grupo que hoy día solo cuenta con cuatro especies (tres de manatí y el dugong). El conocimiento sobre la diversidad de sirénidos en el pasado sigue aumentando a muy buen ritmo.

Durante el Eoceno existían al menos tres de las cuatro familias de sirénidos que se conocen, Prorastomidae (que incluyen los miembros más primitivos), Protosirenidae y Dugongidae. El registro fósil de los Trichechidae (la cuarta familia, que es la que incluye al manatí) es más escaso y hasta donde sabemos, estos se originaron durante el Oligoceno Tardío (28.4-23 ma). Los nuevos fósiles del Eoceno representan a los Prorastomidae y los Dugongidae.

Un protosirénido de India

En una entrada pasada mencioné que India es uno de los lugares más prolíficos en términos de hallazgos de fósiles de sirénidos. No debe sorprendernos que otra especie adicional ha sido descrita. Ashokia antiqua Bajpai et al. 2009 representa un nuevo género y especie de protosirénido del Eoceno Medio temprano (Lutetian: 48.6-40.4 ma). Esta nueva especie se distingue por una combinación de caracteres primitivos y derivados, morfológicamente se acerca más a otro protosirénido de Libya, aún sin describir (Bajpai et al., 2009). Ashokia se distingue de otros prorastómidos al tener una cresta sigmoidal prominente, la apertura auditiva externa más ancha, el puente zigomatico-orbital elevado, el borde del exoccipital más delgado y poseer un proceso zigomático del hueso temporal que disminuye en grosor gradualmente en dirección al rostro.

En la figura arriba Ashokia antiqua en vista lateral (foto e ilustración hechas por mi).

Este no es el primer prorastómido que se conoce de India. Bajpai et al. (2006) refirieron un cráneo incompleto al género Protosiren sp., este, al igual que Ashokia, provienen de la Formación Harudi. Otros sirénido del Eoceno indio son Eotheroides babiae y Eosiren sp. también de la misma formación, estos sin embargo son dugónguidos (Bajpai et al. 2006). Los protosirénidos se distinguen de todas otras especies conocidas de sirénidos por ciertas características craneales y por tener las epífisis* cartilaginosas, incluso en los adultos (Sickenberg, 1934; Zalmout et al., 2003; Bajpai et al. 2006). Aún no se han reportado protosirénidos en rocas del Oligoceno, por lo que parece que este grupo fue uno exclusivo del Eoceno.

*Los extremos de los huesos.

Un dugónguido de Madagascar

Poco se conoce de los animales que habitaron Madagascar en el pasado. No fue hasta en años recientes que se comenzó a descubrir fósiles de los antiguos pobladores de esta isla, especialmente durante el Mesozoico y Cenozoico. Uno de los descubrimientos más recientes es el cráneo de un dugóngido proveniente de estratas del Eoceno Medio. Estos han sido descritos por Samonds et al. (2009) quienes han identificado el fósil como una especie nueva del género Eotheroides. E. lambondrano Samonds et al. 2009 pertenece a un género de dugónguidos que están se han encontrado en rocas del Eoceno de Egipto e India (Domning, 1996; Bajpai et al., 2006), esta nueva especie es la única dentro de ese género que posee el rostro completo, permitiéndonos conocer su morfología en mayor detalle. Único entre otras especies de Eotheroides por la morfología distinta de los huesos nasales, tener procesos supraorbitales bien desarrollados y el puente zigomatico-orbital de la maxilla corto, también se distingue de otras especies de dugonguidos al poseer una fórmula dental primitiva.

En la figura arriba Eotheroides lambondrano en vista lateral, escala = 4 cm (compuesto de fig. 3A y 4A de Samonds et al., 2009).

El largo del cráneo de E. lambondrano es de 270 mm (~10 pulg.), haciendo de este uno de los dugones de menor dimensión que se conocen. Posiblemente se acercaba en tamaño a las especies de dugónguino Nanosiren los cuales tenian dimensiones similares y cual largo total de cuerpo se estima en alrededor de 2 metros (~6’6”) (Domning & Aguilera, 2008), lo cual es considerado pequeño para un sirénido adulto. Asi que según se ha escrito en otros lugares (como en este artículo de National Geographic) puede que la nueva especie de Madagascar sea el dugónguido más pequeño que conocemos.

La importancia de estos nuevos fósiles resta en que demuestran cuan diverso eran los sirénidos durante el Eoceno. El nuevo protosirénido añade otra especie a un interesante grupo de sirénidos, que incluso se ha postulado como el grupo que dio origen a los Trichechidae. El hallazgo de Eotheroides lambondrano en Madagascar nos muestra cuán amplia era la distribución de este género y de los dugónguidos durante el Eoceno; en adición nos puede ayudar a entender la evolución de estos en la región del Tethys. Y por supuesto, aquellos de nosotros que están al tanto de lo que ocurren en el mundo de la paleosirenología saben que aún quedan más por ser descritos, asi que pendientes!

Otras entradas sobre sirénidos

Prep work: update II and a note on sirenian periotics

Prep work: update

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

What’s wrong with the hands of Steller’s sea cow

Bajpai, S., D. P. Domning, D. P. Das & V. P. Mishra. 2009. A new middle Eocene sirenian (Mammalia, Protosirenidae) from India. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 252/3:257-267.

Bajpai, S., J. G. M. Thewissen, V. V. Kapur, B. N. Tewari & A. Sahni. 2006. Eocene and Oligocene sirenians (Mammalia) from Kachchh, India. Journal of Vertebrate Paleontology 26(2):400-410.

Domning, D. P. 1996. Bibliography and index of the Sirenia and Desmostylia. Smithsonian Contributions to Paleobiology 80:1-611.

Domning, D. P. & O. A. Aguilera. 2008. Fossil Sirenia of the West Atlantic and Caribbean region. VIII. Nanosiren garciae, gen. et sp. nov. and Nanosiren sanchezi, sp. nov. Journal of Vertebrate Paleontology 28(2):479-500.

Samonds, K. E., I. S. Zalmout, M. Irwin, D. W. Krause, R. R. Rogers & L. L. Raharivony. 2009. Eotheroides lambondrano, new Middle Eocene seacow (Mammalia, Sirenia) from the Mahajanga basin, northwestern Madagascar. Journal of Vertebrate Paleontology 29(4):1233-1243.

Prep work: update II and a note on sirenian periotics

This has been a long hiatus! I’ve been really busy doing some more prep work on the Puerto Rican Dioplotherium and the Yucatán skull (another new species of dugongine). On top of that I’ve been preparing a couple of manuscripts describing some sirenian remains from PR, which I hope to submit sometime next year.

The subject of this post is to show you more of the Puerto Rican Dioplotherium, which was featured on the previous post. I have done additional prep work on the left squamosal, which was detached from the skull.

The composite picture above shows the skull as it was back in 2006 (top picture) and an outline of the enlarged area below. All that was visible of the sqamosal were the zygomatic arch, post-tympanic process and the mastoid part of the periotic (bottom picture). It was exciting knowing that part of the ear bones were preserved, even if it was only the periotic. Fortunately I got more than I bargained for.

In this figure we see the squamosal, now free of matrix, in lateral (A) and medial (B) views. Notice that the tympanic bone was preserved as well as the periotic. And there is more!

Additional removal of matrix revealed the three auditory ossicles, in articulation! The picture below shows a posteroventral view into the middle ear (anterior is to the right, medial towards the top of the picture). This is really neat as these bones are easily lost in most fossils (they are, apparently, missing on the right side of the skull).

A little on sirenian periotics

The periotic can be divided into three parts tegmen tympani, pars mastoidea and pars petrosa. The latter can be subdivided into pars canalicularis and pars cochlearis (see picture above) (Robineau, 1969). In the pars cochlearis, the structure labeled perilymphatic foramen, is uniquely found in (most) sirenians, (most) proboscideans and Arsinoitherium (hinting at their tethytherian affinity?). The homologous structure in other mammals consists of two openings known as the fenestra cochleae (rotunda) and aqueductus cochleae (Court, 1994).

The occurrence of a perilymphatic foramen in some tethytheres (I’m not sure what is the condition in desmostylians) seems to indicate that it might be a unique derived character of the group. Nonetheless, when we look at the fossil record, primitive proboscideans (Phosphatherium escuilliei) and sirenians (Prorastomus sirenoides) do have fenestra cochleae and aqueductus cochleae (Gheerbrant et al., 2005; Court, 1990; Savage et al., 1994). Meaning that this condition is homoplasic in tethytheres (Court, 1994, Gheerbrant et al. 2005). Whether resulting from multiple origins or multiple reversals, I still think it is an interesting characteristic that is found in at least some tethytheres.

Previous post about sirenians:

Prep work: update

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

What's wrong with the hands of Steller's sea cow

Court, N. 1990. Perotic anatomy of Arsinoitherium (Mammalia, Embrithopoda) and its phylogenetic implications. Journal of Vertebrate Paleontology 10(2):170-182.

Court, N. 1994. The periotic of Moeritherium (Mammalia, Proboscidea): homology or homoplasy in the ear region of Tethytheria McKenna, 1975? Zoological Journal of the Linnean Society 112:13-28.

Gheerbrant, E., J. Sudre, P. Tassy, M. Amaghzaz, B. Bouya and M. Iarochène. 2005. Nouvelles données sur Phosphatherium escuilliei (Mammalia, Proboscidea) de ‘Éocène inférieur du Maroc, apports à la phylogénie des Proboscidea et des ongulés lophodontes. Geodiversitas 27(2):239-333.

Robineau, D. 1969. Morphologie externe du complexe osseux temporal chez les sireniens. Mémoires du Muséum National d’Histoire Naturelle, Série A, Zoologie 60(1)-1-32.

Savage, R. J. G., D. P. Domning and J. G. M. Thewissen. 1994. Fossil Sirenia of the west Atlantic and Caribbean region. V. The most primitive known sirenian, Prorastomus sirenoides Owen, 1855. Journal of Vertebrate Paleontology 14(3):427-449.

Prep Work: Update

Wow! It’s been a while since I posted something. Working with some of the material collected back in August when I went to Puerto Rico to do fieldwork with my advisor has kept me busy. In addition, I had no computer for a while, just after posting the previous post, my computer’s hard drive died! Luckily, not much was lost.

Back in June I wrote about some prep work I had been doing on a sirenian skull from Puerto Rico. Well I am glad to say that four years after I collected said skull (in 2005), it is nearly done! So, here are some pictures, from the time it was collected until now.

In the picture above (from 2005) I am in the outcrop with my hand next to where the fossil is. This is a Late Oligocene limestone unit from northern Puerto Rico.

Here is a dorsal and right lateral view of how the fossil looked around 2006 (and actually it looked like that for the last 2 years). You might notice that on the top picture there is a bone floating in the matrix next to the braincase, this is the left squamosal, which is disarticulated.

Here is how the fossil looks like now (2009), with most of the matrix gone and the left squamosal removed. Beautiful, don't you think?!

If you know something about sirenians, you might have noticed that this is a dugongine (large tusks [broken, unfortunately], thickened supraorbital process of frontal, etc.). It is actually quite similar to Dioplotherium manigaulti from the Early Miocene of South Carolina and Florida (Cope, 1883; Domning, 1989). Nonetheless, the Puerto Rican skull is older, Late Oligocene, and it also has some primitive characters that sets it apart from D. manigaulti. This skull along with another one from the same locality make up an important part of my thesis. Fortunately, some postcranial material that was collected this summer, from the same outcrop and same unit, is referable to this taxon. This material also displays differences from other known sirenian postcrania. Pretty cool stuff!!

Previous post about sirenians:

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

What's wrong with the hands of Steller's sea cow

Cope, E. D. 1883. On a new extinct genus of Sirenia from South Carolina. Proceedings, Academy of Natural Sciences of Philadelphia 1883:52-54.

Domning, D. P. 1989. Fossil Sirenia of the West Atlantic and Caribbean region. II. Dioplotherium manigualti Cope, 1883. Journal of Vertebrate Paleontology 9:415-428.