Lower Hamstead Mbr. Early Oligocene Micro-Vertebrates

[TXV24]

Hi, 

 

I've recently fully processed some matrix from the Lower Hamstead Mbr. that I collected back in November, and I thought I'd share some of my finds in a similar way to my Bembridge Marls Mbr. material. The matrix originates from a 'shelly' horizon in the Lower Hamstead Mbr. and was collected from fallen blocks at the base of a low cliff exposure at Bouldnor Cliff. The Lower Hamstead Mbr. overlays the late Eocene Bembridge Marls and dates from the very earliest Oligocene epoch, approximately 33.75 - 33.5 million years ago.

 

To put the finds into an environmental context the Lower Hamstead Mbr. was deposited during a period of rapid global cooling and drop in sea levels associated with the onset of antarctic glaciation (Oi-1). The cooling and eustatic change had begun in the late Eocene, with the palaeo-environments of the Bembridge Marls becoming increasingly terrestrial towards the Eocene/Oligocene boundary. By the Lower Hamstead Member the southern Hampshire Basin was a low lying coastal plain with extensive wetlands, lakes, ponds and sluggish rivers flowing south east towards the early channel (at this time the channel was more a large embayment with only occasional connection to the North Sea). The dense sub-tropical forests of the late Eocene had disappeared and the landscape was dominated by open woodlands of pine, sequoia, and oak. The environment was much cooler and annual rainfall had significantly dropped since the Eocene, although temperatures would begin to rise again further into the rupelian and Hamstead Mbrs. The basin was surrounded by areas of chalk upland (still existing today) with forests of sequoia and broadleaf species. 

 

This dramatic climate change is likely what triggered the Grande Coupure, in which endemic Eocene mammals like the palaeotheres disappeared and were replaced with Asian groups such as carnivorans, rhinocerotids, anthracotheres, and a variety of other artiodactyls.  The mammals of the dense tropical Eocene forests simply couldn't adapt fast enough to the new open environments of the Oligocene and ultimately failed to compete against the better adapted migrants.  By the Upper Hamstead Member the mammals on the Hampshire Basin coastal plain are almost entirely of Asian origin. Therefore the micro-vertebrates lived in an environment of large scale climatic and ecological change, which I think adds another level of interest to collecting from this member of the Bouldnor Fm. 

 

The material I've collected so far is a lot more varied than the Bembridge Marls, but overall is less abundant. So far it's produced at least 3 fish taxa, 2 mammals, and an indeterminate piece of jaw which may be reptilian or mammal. 

 

1. A skull element from a Bowfin (Amia sp.), these fish are very common in most horizons of the Bouldnor Fm. 

 

2. A vertebra from a Bowfin (Amia sp.)

 

3. A damaged lateral scute from a Sturgeon (Acipenser sp.) showing the transition to a freshwater environment

 

4. An indeterminate piece of a tiny jaw, may be crocodilian although I'm not sure. 

 

5. The nicest find of the lot, a lower incisor from the theridomyid rodent Isoptychus (ID'd by Jerry hooker from the NHM). These rodents looked similar to modern kangaroo rats, hopping along the ground on large rear legs. Bite marks on Isoptychus bones collected from Thorness Bay suggest that they were common prey for the bear-dog Cynodictis. 

 

6. Finally 2 images of an unidentified mammal tooth. I'm unsure as to whether this is part of the tooth or the entire crown, but it doesn't appear to be from a rodent. 

 

Hope you all enjoyed the finds, 

 

Theo 

 

 

 

 

[Tidgy's Dad]

I did indeed. 

Very nicely written article, again.

And lovely finds and photos. 

[Darktooth]

Great report, complete with good information and pics! Thanks for sharing.

[ynot]

Very nice report and finds.

Thanks for sharing.

[jpc]

fun stuff, Theo.  The last tooth is tough to see, but you are right, it is not a rodent.  Jerry hooker Id'ed the rodent incisor; can he look at this one for you?  he is THE guy to ask on your island.

[doushantuo]

MECO continues to be studied intensively.

 

Nice report,Kudos

You have probably already read the articles below,but I'll post them regardless,because they're not without merit

mec1-s2.0-S0012821X16306100-main.pdf

mecrep43386.pdf

[doushantuo]

Dunno if you know

this being:

[TXV24]

@doushantuo Thanks for including those resources, I've read similar papers before but never those two so they'll make for interesting reading. Also I've never come across that rodent paper before either so that'll be really useful! Thank you!

 

@jpc Yeah I might give him a try and see what his opinion is, from the looks of things I'd tentatively say it's perhaps a heavily worn premolar but it'll be interesting to see what he says.

[jpc]

now that I look at it again, I would say worn premolar is probably what it is. If so, it is pretty complex as premolars go.  I am not sure what we are looking at in the last photo.  Can you post a side on view?  From both sides.

[TXV24]

@jpc Sure, here's a couple of side views of the tooth. It's definitely an odd little tooth, doesn't look like the usual anthracothere premolars especially in the wear pattern.