Tuesday, March 31, 2009

Spring Break, Part 3

After scouring the coastal exposures of the Purisima Formation for every important fossil poking out of the cliffs, it was time to explore an exposure of the older Santa Margarita Sandstone that I had only visited once. It was much hotter and drier here.

Here local collector Chris Pirrone walks up an exposure of the 10-12 Ma Santa Margarita Sandstone, in the southern Santa Cruz Mountains.

The Santa Margarita Sandstone is locally famous for its abundant fossilized shark teeth that can easily be collected by screening through the sand. Also plentiful are very well preserved sand dollars (Astrodapsis spatiosus), which at this locality are sometimes concentrated into 3-4 meter thick coquinas. C. Pirrone displays a nice Astrodapsis in the above photo. These sand dollars usually occur in the 2-3" range, but at this locality, can measure up to around 5" in diamater.

But we weren't looking for shark teeth or sand dollars. Parts of the Santa Margarita Sandstone have in the past yielded partial and complete skeletons of sea cows and walruses, and cetacean and bird bones. When I took this photo, C. Pirrone (right) and A. Poust (left) had found something neat, they said, but wouldn't tell me what until I made it down the slope to see for myself.

What the heck was it? I was secretly hoping for a partial skull, and realistically thinking it would just be a bone fragment. Instead, I was greeted by the glint of blue enamel in the sunlight; it was a near complete, nearly perfect canine of the 'primitive' walrus, Imagotaria downsi. This is not a major discovery, by far; several complete and partial skulls of this same beast are known from elsewhere in the same formation. After looking through Repenning and Tedford (1977), this specimen appears to be a lower canine from a male I. downsi. Lower canines of many pinnipeds (probably Imagotaria as well) are more curved posteriorly than upper canines. Additionally, there is a large wear facet on the anteriomedial surface, from occlusion with the upper third incisor. More about this later.

Here is a photo of the top of the Santa Margarita Sandstone. It is conformably overlain by the Santa Cruz Mudstone, which you can see here. The orange colored portion near the top of the photo is a brecciated zone of the SC Mdst. The Santa Cruz Mudstone is locally 8-10 Ma (and as young as 6.8-6.0 Ma on the Point Reyes Peninsula).

To check out a shelf of sandstone with a lot of float on it, A. Poust daringly jumped down from above.

A. Poust found a chunk of bone; but it was only a highly abraded, blob-shaped chunk, probably originally a piece of mysticete whale bone. Too bad. Later on and higher up we found several more Imagotaria bones, including a juvenile humerus, and an adult thoracic vertebra. More on this later.


J. Velez-Juarbe said...

"partial and complete skeletons of sea cows"
I'm going to say Dusisiren jordani

Those look like great outcrops, thanks for sharing the pics!!

Robert Boessenecker said...

Hi J. Velez-Juarbe,

You'd be correct! The most complete known skeleton of Dusisiren jordani (and of any sirenian, if I remember correctly) was collected within a couple miles of this locality. UCMP 77037 is the cat #. Additionally, the type locality of "Halianassa vanderhoofi" (=D. jordani) is also nearby.

These outcrops are great, but the sand is so friable, it is impossible to climb - it is easy to go down, though.

Brian Lee Beatty said...

How is the wear facet oriented in the Imagotaria lower canine you found? I wonder, is canine interdigitating similar to that of other carnivorans?
Thanks for this post, it is excellent to see more of those formations!

Dan said...

I am a cubmaster with a cub scout pack in San Mateo, CA. This month we are focusing on dinosuars and I was wondering if you had any local contacts that could help with a field trip or exhibition of some sort. We don't need anything fancy. If you have any info please contact me at cubmaster@pack132.com

Robert Boessenecker said...

Hey Brian,

I'll post pictures of the canine soon, but the wear facet is oriented more or less longitudinally, and is located on the anteromedial surface of the canine. Looking at Lynx rufus and Canis latrans crania I have, I can't see any wear facet on the c2 from the I3. Then again, I don't have any ursid or procyonid (or any other 'fissipeds' for that matter).

Many pinnipeds ("imagotariine" and dusignathine odobenids, and otariids, for instance; I can't recall on early odobenids, 'enaliarctids', or phocids) have enlarged upper lateral incisors. In the case of otariids, it is the I3; I can't remember if it is the I3 or I2 in odobenids. In Leptonychotes, the I2 is the lateral incisor, and is very large and caniniform.

A similar condition occurs in the bizarre arctoid Kolponomos. I imagine that in most of these examples that enlarged, caniniform upper lateral incisors an adaptation for piscivory, except in Leptonychotes (for scraping breathing holes in ice instead) and Kolponomos (a mollusc 'scoop' instead).

In any event, I suspect the larger upper lateral incisor is responsible for pinnipeds having this wear facet, and 'fissipeds' not (or at least, less so).

This post is taking forever because my girlfriend's parrot is licking my ear.