Paleonews

Il blog dedicato ai Paleontologi !!!!

2008-11-26 – Tracce attuali di un Protista (Gromia sphaerica) mettono in discussione l’origine di quelle fossili

La scoperta di tracce lasciate da un piccolo organismo sul fondo dell’Oceano mette in dubbio l’origine di trace analoghe risalenti al Precambriano. Infatti molte tracce analoghe erano state attribuite ad organismi vermiformi (e quindi evolutivamente molto più complessi) mentre ora tale scoperta fa nascere nuovi interrogativi sull’interpretazione delle tracce fossili del precambriano e sulla diversificazione iniziale dei Bilateralia (wiki link: ENG, GER, ITA).

Tale scoperta ha inoltre spinto gli autori della ricerca a far partire il progetto “deep-sea palaeontology”, ossia la costruzione di una databas di trcce attuali da confrontare poi con quelle fossili

————————————————————

‘Grape’ is key to fossil puzzle

 

It moves slowly across the ocean floor
The protist is similar to a grape in size and shape

A single-celled ball about the size of a grape may provide an explanation for one of the mysteries of fossil history.

Writing in Current Biology, researchers say the creature leaves tracks on the seabed which mirror fossilised tracks left up to 1.8 billion years ago.

Many palaeontologists believe only multi-celled organisms could have made these tracks.

This has been difficult to confirm as no multi-cellular fossils of such an age have ever been found.

Covered in mud

The discovery was made by marine biologists monitoring the sea bed in the Bahamas. They noticed a great deal of tracks made by grape-shaped creatures called protists. Dr Mikhail “Misha” Matz from the University of Texas at Austin, US, led the research.

“We were looking for pretty animals that have eyes, are coloured, or glow in the dark; instead, the most interesting find was the organism that was blind, brainless, and completely covered in mud,” he said.

The researchers say the 3cm-wide, single-celled protists propel themselves using tiny protruding legs called pseudopodia. A number of openings all over the body act as mouths and outlets for waste.

The protists move very slowly, taking weeks or even months to make a track of a few centimetres. As the sea bed currents where they were observed are very slight, their tracks are not washed away.

Worm casts

The protist tracks look very similar to fossil tracks found in the pre-Cambrian era more than 500 million years ago.

Perhaps the most famous are the “worm casts” found in the Stirling Ranges in Western Australia. In 2002, these were dated to at least 1.2 billion years old.

The Stirling formation fossil is at least 1.2 billion years old
The worm casts may not have been made by worms after all

This dating presented a problem for palaeontologists; they assumed multi-cellular organisms with bilateral symmetry, where two halves of their bodies are approximate mirror images of each other, had to be responsible for such tracks, but there is no fossil evidence for the existence of such creatures until several million years later.

Fossil experts believe bilateral symmetry is what gave the organism the ability to make the tracks, with the impressions being produced when the organism moved its weight from one side to another.

Dr Matz believes protists provide an explanation of how the tracks could have been made without the need for organisms with bilateral symmetry.

“We used to think that it takes bilateral symmetry to move in one direction across the seafloor and thereby leave a track,” he explains.

“You had to have a belly and a backside and a front and back end. Now, we show that protists can leave traces of comparable complexity and with a very similar profile.”

Cambrian explosion

Bilateral symmetry appeared in the Cambrian era about about 542 million years ago, early creatures quickly diversified into all of the major animal groups of today. Quite how or why this rapid diversification, known as the Cambrian explosion, occurred is still one of the biggest questions in animal evolution.

Very few fossils exist of organisms that could be the pre-Cambrian ancestors of bilateral animals, and even those are highly controversial. Fossil traces, such as these tracks are the most accepted evidence of the existence of these proto-animals.

We now have to rethink the fossil record
Dr Mikhail ‘Misha’ Matz,University of Texas at Austin

Dr Matz says all tracks which predate the rapid evolution of life seen in the Cambrian explosion – could come from protists.

“Pretty much anything within the Precambrian fossil record can in principle be attributed to large protozoans, from the earliest traces and fossils of the Stirling formation,” he says.

The researchers say forms described as “globular or bulbous collapsible bodies” which were found fossilised together with the Stirling formation’s worm trails are probably the remains of creatures very similar to the protists they found at the bottom of the sea.

The protist is about the size of a grape
The tracks resemble those found in prehistoric times

Genetic analysis shows this moving protist from the Bahamas is broadly the same as a stationary type found in the Arabian sea.

The researchers are now beginning a project which they call “deep-sea palaeontology” to create a catalogue of tracks produced by a variety of present-day underwater animals for comparison with the fossil record.

Dr Matz says the giant protists’ bubble-like structure is probably one of the planet’s oldest body designs, and may have existed for 1.8 billion years.

“Our guys may be the ultimate living fossils of the macroscopic world,” he says.

Friday, 21 November 2008

source: http://news.bbc.co.uk/2/hi/science/nature/7739703.stm

Deep-sea protists may explain trace fossil evidence attributed to ancient animals

Public release date: 20-Nov-2008
[ Print Article | E-mail Article | Close Window ]

Contact: Cathleen Genova
cgenova@cell.com
617-397-2802
Cell Press

A new discovery challenges one of the strongest arguments in favor of the idea that animals with bilateral symmetry—those that, like us, have two halves that are roughly mirror images of each other—existed before their obvious appearance in the fossil record during the early Cambrian, some 542 million years ago. In the November 25th issue of Current Biology, a Cell Press publication, researchers report the first evidence that trace fossils interpreted by some as the tracks of ancient bilaterians could have instead been made by giant deep-sea protists, like those that can still be found at the seafloor to this day.

Protists are a diverse group of predominantly microscopic organisms. They are commonly single-celled with a single nucleus, but they may attain larger size by having many nuclei or forming colonies of identical, unspecialized cells. In the new study, the team describes macroscopic groove-like traces produced by living giant protists, known as Gromia sphaerica, which look something like a grape in terms of shape and size. Those grooves bear a remarkable resemblance to the trace fossils from the Precambrian, including ones as much as 1.8 billion years old.

“Our paper gives the precedent of a protozoan that is motile, produces macroscopic traces, and has a large hydrostatically supported body,” said Mikhail Matz of the University of Texas at Austin. “With these possibilities demonstrated, pretty much anything within the Precambrian fossil record can in principle be attributed to large protozoans, from the earliest traces and fossils of the Stirling formation that are 1.8 billion years old to the weird Ediacaran biota with which the Precambrian culminated.”

This new “protozoan option” takes the edge off the most compelling evidence of primitive bilaterians in the Precambrian that is so important for what has been called the “ancient school,” he says. That line of thinking holds that the apparently explosive diversification of multicellular body plans during the Cambrian is an artifact of the fossil record; it suggests that bilaterians actually existed long before the Cambrian and evolved gradually over time. Others think instead that the Cambrian explosion really happened the way it appears that it did and that evolutionary mechanisms must therefore be sought to explain the rapid diversification.

“Previously one could say, ‘There were traces, therefore there must have been bilaterians,’ whereas now it is ‘There were traces, therefore there may have been bilaterians,’ which is, obviously, not nearly as strong a statement,” Matz said.

He calls the findings a “classic case of scientific serendipity.” They stumbled upon the giant protists while working on a project exploring the interaction between light and life in the ocean. “We were looking for pretty animals that have eyes, are colored, or glow in the dark,” Matz said. “Instead, the most interesting find was the organism that was blind, brainless, and completely covered in mud.”

Almost nothing is known about G. sphaerica, he added. His team is now deep sequencing the genes expressed in this giant protist and a few related protozoans to get a better idea about their evolutionary relationships to one another. They also plan to initiate a project on “deep-sea paleontology” to create a catalogue of traces produced by a variety of present-day animals. “There is surprisingly little data on this, so paleontologists have to resort to speculations a lot when interpreting fossil traces,” Matz said.

 ###

 The researchers include Mikhail V. Matz, University of Texas at Austin, Austin, TX; Tamara M. Frank, Harbor Branch Oceanographic Institute at Florida Atlantic University, Fort Pierce, FL; N. Justin Marshall, The University of Queensland, Brisbane, Queensland, Australia; Edith A. Widder, Ocean Research and Conservation Association, Fort Pierce, FL; and Sonke Johnsen, Duke University, Durham, NC, USA.

source: http://www.eurekalert.org/pub_releases/2008-11/cp-dpm111308.php

——————————————————————————-

scientific article:

Giant Deep-Sea Protist Produces Bilaterian-like Traces

Mikhail V. Matz, Tamara M. Frank, N. Justin Marshall, Edith A. Widder, Sönke Johnsen
Current Biology – 20 November 2008
Summary | Full Text | PDF (846 kb)

Abstract:

One of the strongest paleontological arguments in favor of the origin of bilaterally symmetrical animals (Bilateria) prior to their obvious and explosive appearance in the fossil record in the early Cambrian, 542 million years ago, is the occurrence of trace fossils shaped like elongated sinuous grooves or furrows in the Precambrian. Being restricted to the seafloor surface, these traces are relatively rare and of limited diversity, and they do not show any evidence of the use of hard appendages. They are commonly attributed to the activity of the early nonskeletonized bilaterians or, alternatively, large cnidarians such as sea anemones or sea pens. Here we describe macroscopic groove-like traces produced by a living giant protist and show that these traces bear a remarkable resemblance to the Precambrian trace fossils, including those as old as 1.8 billion years. This is the first evidence that organisms other than multicellular animals can produce such traces, and it prompts re-evaluation of the significance of Precambrian trace fossils as evidence of the early diversification of Bilateria. Our observations also render indirect support to the highly controversial interpretation of the enigmatic Ediacaran biota of the late Precambrian as giant protists.

novembre 26, 2008 - Posted by | America Central, Articolo sc. di riferimento, Attuale, Italiano (riassunto), P - Evoluzione, P - Impronte, P - Ritrovamenti fossili, Paleontology / Paleontologia, Precambriano | , , , , , , , , , , , ,

Non c'è ancora nessun commento.

Lascia un commento

Inserisci i tuoi dati qui sotto o clicca su un'icona per effettuare l'accesso:

Logo WordPress.com

Stai commentando usando il tuo account WordPress.com. Chiudi sessione / Modifica )

Foto Twitter

Stai commentando usando il tuo account Twitter. Chiudi sessione / Modifica )

Foto di Facebook

Stai commentando usando il tuo account Facebook. Chiudi sessione / Modifica )

Google+ photo

Stai commentando usando il tuo account Google+. Chiudi sessione / Modifica )

Connessione a %s...

%d blogger cliccano Mi Piace per questo: