How far South did Brook Trout (Salvelinus fontinalis) Range in Georgia during Ice Ages?

Illustration of a Brook Trout from Cornell University.

No scientist has ever studied this topic, so my blog post is the first of a kind.  Scientists are limited due to a lack of data–no trout fossils have ever been found in the region.  There’s also a lack of interest, probably because I’m the only person in the world who daydreams about what it would be like to live in Georgia 37,000 years BP.  I want to know, if I traveled in a time machine to live then and there, whether or not brook trout would get caught in fish traps I would place in the Broad River.  I’ve concluded that it’s possible, maybe probable, that they did occur in the Broad River during stadials and even interstadials because summers were cooler then than they are today, an age within a full blown interglacial with hot summers.

Present day Georgia brook trout range map.  Map from the University of Georgia Museum of Natural History.

Today, brook trout are the only native species of trout in Georgia, and as the above map shows, their natural range is restricted to the extreme northeastern region of the state with a few outlying disjunct populations, suggesting formerly a  wider, more continous distribution.  Temperature is one obvious limiting factor in their present day range.  Brook trout suvive best in waters with temperatures between 55-65 degrees F.  They can withstand water temperatures between 32-72  F, however, they can only live in warm waters of up to 78 degrees F for a few hours.  Stocking trout in waters with temperatures greater than 70 degrees F generally fails in the long term.  The temperature of the Savannah River near Augusta, Georgia on June 13, 2011 was 73 degrees F.  (Modern reservoir temperatures are much warmer.)  Water temperatures this warm and warmer are common during summer months in southeastern rivers, making them unsuitable for trout.

Summer air temperatures in the southeast were much cooler during the last glacial maximum, and even during interstadials within the Ice Age, therefore water temperatures were as well.  Data from isotopic oxygen ratios of fossil foraminifera in the Atlantic Ocean suggest average annual summer temperatures were 9 degrees F cooler.  (See my May 15, 2011 blog entry for more detailed explanation of this.)  Taking the present day summer water temperature reading from the Savannah River near Augusta (73 degrees F) and subtracting it by 9 = 64 degrees F…within the brook trout’s comfort zone.  Potentially, trout could’ve lived as far south as what’s now Augusta.

Trout also prefer shallow fast moving water–fingerlings like water that’s just 16 inches deep, and adults prefer the water slightly deeper than that.  Trout stay in shallow water and avoid deeper water even if temperatures are cooler in the latter.  Much of the present day Savannah River, besides being too warm, is too deep, but during the LGM arid climate lowered the water table, creating more shallow fast moving streams for trout.  Suitable trout habitat likely existed in 2 notable streams that flow into the Savannah River: the confluence of the Broad River with the Savannah is only ~60 miles south of present day trout habitat, and the Little River’s confluence is only ~70 miles south.  The headwaters of the Broad River is just a scant few miles from present day trout habitat.  Both the Broad and the Little Rivers have the gravel and rocky bottoms brook trout prefer.  During cooler climate phases, there was no ecological reason brook trout couldn’t have expanded their range at least that far south.  The fossil record shows that many other species from northern climates extended their range farther south during the LGM including caribou, elk, woodchucks, bog lemmings, red backed voles, red squirrels, gray jays, pine siskins, wood turtles, and others.  So it’s not only possible but probable brook trout did too.  Unfortunately, finding Pleistocene fish bones in a region almost devoid of Pleistocene fossils is not likely, rendering my hypothesis difficult to prove.

A Review of Giants in the Storm by Mark Renz

There are so many fossil sites in Florida that academic experts don’t have the time and money to excavate the all.  Mark Renz, a professional fossil-hunting guide but not an academic expert, sought and gained permission to excavate 2 productive retention ponds in LaBelle, Florida.  Giants in the Storm is about his experiences spear-heading that effort.  His team found a truckload of fantastic fossils including lots of mammoth, mastodon, and horse material as well as specimens of llamas, flat-headed peccary, long-nosed peccary, white tail deer, Harlan’s ground sloth, saber-tooth, jaguar, bobcat, Armbruster’s wolf, alligator, and giant tortoise.  The site is unique because it dates to the middle Irvingtonian Land Mammal Age (~500,000 BP).  Though Florida has many fossil sites dating to the Rancholabrean Land Mammal Age (~300,000-~10,000 BP), and plenty from the late Pliocene/early Pleistocene (~2 million BP), the LaBelle fossil site is a rarity that helps fill in a gap in the fossil record, and it provides more specimens for comparison of the evolution of individual species.  The site is located in southwest Florida.  The types of mammals excavated here suggest the region was mostly grassland interspersed with riverine and lacustrine forests.

Like the other Mark Renz book I reviewed, Fossiling in Florida, the best thing about this book are the copious photographs of fossils that are useful for amateur  and serious fossil collectors.  Some of my favorites include parts of the anatomy that don’t automatically come to mind such as a mammoth’s calcaneum (heel bone) and an hyoid (tongue bone) from either mammoth or mastodon.  Of course, there were the usual proboscidean teeth, and spectacular fossils–a relatively complete mammoth skull and a 9 foot long, 300 pound tusk found in place but broken into 4 pieces.

Mr. Renz imagined a single dramatic event, a storm of great magnitude causing a flood, as an explanation for why these fossils concentrated here.  It gave him an excuse for a dramatic title for this book, but he concedes his title is a misnomer.  After consulting with Richard Hulbert, curator of the University of Florida Natural History Museum, he agrees with the conclusion that all the fossil specimens were accumulated here gradually over a long period of time.

Fossiling in Florida by Mark Renz

The best chapter in Fossiling in Florida: A Guide for Diggers and Divers by Mark Renz is the one called Prehistoric Portraits–33 pages of nice black and white photographs of fossils that a fossil hunter would likely find in Florida and other southern states.  This makes the book a handy indispensible reference.  Its arrival in my mail box was timely–next week I’m going on an actual fossil-hunting expedition to the low country of South Carolina.  I found a site there readily accessible to the public where late Rancholabrean fossils (over 30 species) have turned up, mixed in with Pliocene-aged marine fossils and shark’s teeth.  In next week’s blog entry I’ll have lots of interesting photographs, hopefully of some specimens I discover myself.

Now back to the book.

Mr. Renz snorkles the backwaters of Florida’s alluvial fossiliferous deposits, feeling his way through sediment that is much richer in prehistoric treasure than that of most other states in North America.  His accounts of avoiding alligators and speeding boats, while searching for fossils, are some of the most entertaining parts of the book.  His wife’s sketches also add to the charm of this work.

One of the reasons I bought the book was because I thought it was self-published, and I wanted to see how another non-academic, self-published author tackled a similar subject to that of my book.  I didn’t know the University of Florida Press published this book.  I found his website–www.paleopress.net/paleo5.htm.  He has two other books: Megalodon: Hunting the Hunter and Giants in the Storm.  Those must be the ones he self-published.  I’ll have to check those out too.  The cover for the latter looks outstanding.

Anyway, stay tuned for next week’s blog entry.  I’m really looking forward to the upcoming rare opportunity to find some fossils.

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This week, I’ve been obsessed with a new paper I came across that was published last fall.  (I’ve only read the abstract.) Two scientists did a thorough study of Panthera leo atrox skulls.  They determined that the North American lion was more like a giant jaguar, or a completely different species altogether than a lion.  The skull does resemble that of a lion, but the lower jaw was more like that of a jaguar.  They theorize that when the glacier cut Beringia and Eurasia off from the North America that the large Panthera cat south of the glacier evolved into two species–Panthera atrox and Panthera onca.  The species of big cats from the Panthera genus in America then consisted of a giant jaguar and a large jaguar.  Panthera onca augusta (the Pleistocene subspecies jaguar)  is considered large compared to modern jaguars, but Panthera atrox was gigantic weighing on average 25% larger than modern African lions.

I think this study makes sense.  If specimens of an extinct cat are consistently that much larger than living representatives of the presumed species, than the chances are good it was a different species.  Moreover, atrox had a larger brain capacity.  This is evidence it may have  hunted in prides like African lions, but we’ll never know for sure.  Based on where most of its skeletel material has been found, it seems to have preferred open country.  The large size of the males would have made it difficult for this species to hunt singly.  At the very least, they must have hunted in pairs.

I’m going to try to get my hands on this paper, so I can give a more detailed discussion in a future blog entry.