Upper Hamstead Mbr. Oligocene Micro-vertebrates

[TXV24]

Hi, 

 

I recently finished processing 4kg of matrix from a horizon in the Upper Hamstead Mbr. of the Bouldnor Fm. from Bouldnor Cliff and thought I'd share the results! The White Band is definitely the most diverse vertebrate fauna I've collected so far in my short time screen washing, with at least 2/3 genera of fish, 2 genera of reptiles,  and 2 genera of mammals, it also has some interesting taphonomy. 

 

The White Band refers to a thick Polymesoda shell bed in the Upper Hamstead Mbr. and dates to approximately 33 million years bp during the Rupelian. The Upper Hamstead Member is the youngest strata in the entire paleogene sequence of the Hampshire Basin (Late Palaeocene to Early Oligocene). The horizon was deposited in a shallow freshwater lacustrine environment on the low-lying Solent Group coastal plain of the southern Hampshire Basin. By the time the White Band was deposited average annual temperatures in the region were beginning to warm up again after the sudden and rapid cooling that marked the Eocene-Oligocene transition. Global sea levels were also beginning to rise. The Grande Coupure, the large scale turnover of European mammalian faunas had been and gone, and the endemic Eocene groups such as Palaeotheres, Omomyid primates, and anoplotheres had long vanished. The lake/pond system that deposited the White Band was home to aquatic plants such as Stratiotes and was fringed by patches of open woodlands of Sequoia, Pine, and broadleaf.

 

With the post-grande coupure fauna now established the landscape was home to anthracotheres, hornless rhinos, hyaenodonts, bear-dogs, entelodonts, primitive ruminants, choeropotamids, and a myriad of smaller mammals including bats, adapid primates, rodents, insectivores, marsupials, and the otter-like pantolestids. Not to mention the alligators, birds, and freshwater turtles. 

 

 

1. Worn fragment of Emys carapace 

   

 

2. Possible fragment of crocodilian osteoderm? 

 

 

3. Fragment of Bowfin skull bone

 

 

4. Isoptychus sp. cheek tooth. Theridomyid rodents like Isoptychus are the most commonly found micro-mammal throughout the entire Solent Group. This molar has been heavily worn which may suggest an older individual. Theridomyids were bipedal and foraged along the ground and in low trees. They also seem to have fed on the seeds of marginal aquatic plants such as Stratiotes, which may be the reason this individual was in the vicinity of the pond/lake. The Theridomyids were one of only a few Eocene mammal groups to survive the Grande Coupure and seemed to have survived fairly unscathed in terms of diversity etc. showing what hardy and adaptable rodents they probably were. 

 

5. Fragment of M₃ from a talpid, most likely Myxomygale sp. (just 1.5mm long!). Talpids (or as we call them today, Moles) were newcomers to Europe with the Grande-Coupure, arriving from Asia. Belonging to the tribe Urotrichini (Shrew-Moles) which are only found in North America and Asia today, Myxomygale may have spent most of the day underground in burrows before emerging at night to feed on invertebrates. Modern Shrew-Moles prefer moist habitats such as swampy areas, a habitat which was abundant on the low-lying coastal plain of the Oligocene Isle Of Wight. 

 

The taphonomy of the White Band is also interesting. Some specimens i.e. the Emys fragment and osteoderm are highly 'polished' and worn, suggesting transport prior to deposition. Whereas others such as the mammals and most fish material I've recovered are unworn and 'fresh' looking. I'm not sure what conditions could have caused this, and if anyone has any suggestions I'd be really interested. My take is that the mammals and fish were likely living in the immediate area, in and around the lake/pond, whereas the polished material is from animals living some distance away brought to the pond/lake by floods or streams etc. although I'm no trained geologist or palaeontologist. 

 

Thanks for reading, 

 

Theo

[ynot]

Hey hi Theo,

Nice write up and neat fossils.

Thanks for sharing.

Looking forward to more.

[Fossildude19]

Thanks for providing an educating read!

Neat stuff you are finding.  

[Auspex]

Fascinating! Thank you!
The only other agent for specimen wear besides water transport that I can think of is digestion (though I think that usually results in detectable micro-pitting).

 

EDIT: The wear and polish exhibited on your specimens reminds me very much of the 'wave polishing' so familiar to me on small Calvert Cliffs sharks teeth (though that wear is post-depositional for the most part).

[doushantuo]

Nicely written,extremely nice pix too.Love the murid 

some familiar names below( PDF about 1,8 Mb):

Vasileiad07oumammarodtaphagepopulstudJournalofTaphonomy.pdf

 

[TXV24]

@ynot @Fossildude19 No worries, and thank you!

 

@Auspex Thanks for the suggestion, this was definitely pre-depositional wear as these pieces were all in-situ in the matrix blocks so it's interesting you mention the parallels to wave wear which backs up that they were exposed to a higher energy environment than just the calm pools. 

 

@doushantuo Thank you, and thanks for the PDF, I've been looking for a accessible copy of that paper for a while to try and see if I can determine the age of some of my micro mammals so it'll definitely come in handy. 

[Tidgy's Dad]

Excellent work and a fascinating read, as always.

Thank you for sharing. 

[jpc]

NIce work... and great little mammal teeth.  

[doushantuo]

[Miocene_Mason]

Cool stuff! And quite informative as well!