[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: The Very Very Latest Paper From 2006!!!



After writing this http://dml.cmnh.org/2007Feb/msg00158.html, I wrote this:

----- Original Message -----
From: "David Marjanovic" <david.marjanovic@gmx.at>
To: "DML" <dinosaur@usc.edu>
Sent: Wednesday, February 14, 2007 1:43 AM
Subject: Re: The Very Very Latest Paper From 2006!!!

----- Original Message -----
From: "Jura" <pristichampsus@yahoo.com>
Sent: Tuesday, February 13, 2007 8:22 PM

Do they offer any explanation for why these LAGs are
present?

They only offer a speculation (climate, migrations...); I'll elaborate on this tomorrow, after I'll have read Sander's paper from 2000 about sauropod long bone histology.

Now you see why I have to reject all those polite e-mails that offer me 40 % of 48 megabucks if I can just answer the question "BUT CAN YOU BE TRUSTED?." in the affirmative.


Here's a collection of quotes from Sander & Andrássy 2006 (author names in small capitals in the original):

p. 144:
"Compared to dinosaurs, the bone histology of large mammals has received very little attention by paleontologists. This is surprising, beccause a thorough understanding of mammalian bone histology is crucial to the paleobiological interpretation of the bone histology of extinct tetrapods such as dinosaurs and non-mammalian synapsids. Although birds and crocodiles are phylogenetically closer to dinosaurs than mammals, the bone histology of large modern mammals probably is most informative for the interpretation of dinosaurian bone histology. Crocodile bone histology differs from dinosaur bone histology too much because of the much lower growth rates of crocodiles. Except for ratites such as ostriches and moas, birds are poorly suited for comparison because of their much smaller body size and their adaptation to flight rather than to a terrestrial lifestyle, despite being surviving dinosaurs. Large herbivorous mammals over 1 t body mass ("megaherbivores") empirically offer the best basis of comparison for many aspects of dinosaurian biology despite the convergent acquisition of many dinosaurian characters such as large body size, high growth rates, and a fully upright stance."


LAGs are observed in "most dinosaurs [...]. The most notable exception is the sauropod dinosaurs. They have only in a minority of specimens well developed LAGs in their long bones".

"While CHINSAMY & HILLENIUS (2004 and references therein) argue that regularly spaced LAGs in dinosaur bones indicate a metabolic rate below that of modern mammals, which are believed to generally lack LAGs, PADIAN & HORNER (2004 and references therein) take a phylogenetic perspective by pointing out that LAGs in dinosaurs may be a poor indicator of metabolic status because they are plesiomorphic. According to PADIAN & HORNER (2004), LAGs thus simply may be inherited from some ectothermic ancestor of the dinosaurian lineage and have no particular meaning in terms of thermoregulation. In addition, PADIAN & HORNER note that LAGs have been occasionally observed in large mammals such as elk (*Cervus elaphus* [...]), seals (*Phocoena phocoena* [...]), and polar bears (*Ursus maritimus* [...]). Elk in particular are of interest because unlike bears, they do not hibernate. Temperate zone small mammals also commonly show LAGs [...], as do large birds such as moas".

p. 145:
"Our study grew out of the fortuitous observation of LAGs in fracture surfaces of Pleistocene fossil bones. We expanded this observation by studying fossil bone because samples of Pleistocene large mammals are much easier to obtain in museum collections than those of Recent mammals. Fragmentary but identifiable material is usually not found in recent osteological collections, but only complete skeletons to which sampling access is more restricted."


p. 156:
"A surprising result of this study is that the different large mammal taxa can be discerned based on their long bone histology, i.e. that there are taxonomic differences in histology at the family level and above."


p. 157:
"Although indications of growth marks were discovered in all taxa, the ungulates (cervids, bovids, and *Equus*) differ from the mammoth and the rhino [...which is not an ungulate anymore?] in that their cycles and LAGs are more clearly expressed. [...]
The result of this study, that large mammal taxa can be distinguished based on their bone histology, is in agreement with a recent study on sauropod long bones (SANDER 2000), in which individual taxa also could be distinguished based on bone histology.


8.2. The frequency and causes of growth marks in the sample

Growth marks, specifically lines of arrested growth, were detected in the great majority of all specimens studied in thin section. This leads to the question whether LAGs are a general but previously overlooked feature of all large Pleistocene mammals, or possibly of all large mammals in general. We do not have the data to provide the answer because we preseleccted the material for macroscopic evidence of cyclical growth [...] [ = concentric rings visible in broken bones, or spalling of weathered bone layers]. We did not sample those bones that lacked this evidence (but may have LAGs), and these were the great majority [...]. On the other hand, not all samples that showed macroscopic evidence for LAGs did, in fact, show them in their histology. Especially in *Mammuthus*, the presence of typical LAGs has not been established, and additional work is necessary. The regular spacing of the LAGs in the ungulates observed in this study [see above for what "ungulates" means] suggests a potential use in individual aging of Late [sic] Pleistocene mammals (skeletochronology).
However, we would like to make a few general points about the frequency of LAG occurrence. It is clear that LAGs are much more common than previously assumed, in frequency of occurrence in a specific taxon as well as in frequency across taxa. It is very likely that the presence and prominence of LAG[s] vary among bones of the skeleton, as is well known from recent vertebrates [...] and from dinosaurs [...].
Because most of our sample derives from animals living during glacial conditions, this could also have affected the frequency of growth marks. The comparison of Pleistocene mammals with extant representatives of the same or very similar species offers great potential to address the issue of climate dependence of LAG occurrence in large mammals. It is tempting to link the LAGs to particular climatic conditions in Central [sic] Europe during the glacial periods. This included strong seasonality which possibly induced long seasonal migrations in the herbivores studied here. However, we refrain from speculating further in the virtual absence of modern comparative data on LAGs and their origin in large mammals."


"8.3. Significance for deductions about physiology

[...] It is noteworthy that the largest mammals studied, the mammoth [...], as well as the largest dinosaurs, the sauropods [...][,] show the least development of lines of arrested growth.
Future work on mammals will need to include a more controlled approach to sampling to detect the frequency of LAG occurrence in cold-climate vs. warm-climate living [sic] and Pleistocene mammals. The reliability of macroscopic indicators for LAGs also needs to be tested by comprehensive sampling. A possible explanation for the apparent rarity of such indicators in the fossil population from the Rhine-Herne ship channel is that most long bones have undergone Haversian replacement in their cortex which would have obliterated LAGs [...]. In other words, the absence of macroscopic indicators for LAGs may simply indicate the prevalence of Haversian bone in mammals compared to dinosaurs and not their rarity in primary bone. Long bones of large mammals differ in their histology from those of dinosaurs in generally being much more remodelled".


p. 158:
"This difference may not necessarily be an indicator of a different thermal physiology of mammals and dinosaurs but of different life history patterns. At least the largest dinosaurs, the sauropods, appear to differ from almost all large living mammals in their continued fast growth after sexual maturity (SANDER 2000). The continued fast apposition of bone throughout most of the life of the animal thus may have outpaced the front of Haversian remodeling moving peripherally. Only after growth had stopped, could Haversian remodeling entirely replace the primary bone, as seen in the largest sauropod long bones (SANDER 2000). In mammals, growth stops relatively early in the life of the animal, but remodeling continues, resulting in complete replacement of primary bone by secondary bone even in middle-aged individuals."


"[C]ontinued fast growth after sexual maturity"? This got me wondering. So I read

P. Martin Sander: Longbone histology of the Tendaguru sauropods: implications for growth and biology, Paleobiology 26(3), 466 -- 488 (2000)

This paper finds (fig. 8) that growth slowed down three times in a sauropod's life. The first such event is interpreted as the transition from "hatchling" to "juvenile", the second as sexual maturity, and the third, after which only lamellar-zonal bone is added to the cortex in thin, poorly vascularized layers separated by LAGs, as the effective end of growth. In Sander's own words (p. 481):

"The sequence of three distinctive types of fibrolamellar bone followed by lamellar-zonal bone suggests that the growth curve derived from the outward changes in cortical bone tissue will be characterized by three consecutive decreases in growth rate, the last of which is most dramatic. This sequence can be observed in a growth series or, ideally, in the cortex of a single, large bone. [Indeed Sander found one such *Tornieria* femur.] Growth rate apparently did not decrease gradually, because the bone types are distinct and the transitions between them are rather abrupt [...].
The next question to ask is which events in the life history of the animal can the three transitions between the four tissue types and the attendant drops in growth rate be correlated with? [sic] The first transition must have happened during preadult ontogeny, simply because of the small absolute size of the bones in which it occurs. Probably it is correlated with an event such as leaving the nest or termination of parental care. The interpretation of the second transition, occurring at roughly between 40% and 80% maximum size (depending on [the] taxon, see below) is crucial for the reconstruction of life history. Conceivably, it could represent an event during juvenile ontogeny, but it is unclear what this event could be because juveniles normally show a uniformly high or only gradually decreasing growth rate. More likely, the second transition represents sexual maturity. A growth pattern with sexual maturity attained at well below maximum size is normal in living reptiles and is also comparable with that observed in the largest [extant] terrestrial mammal, the African elephant, which shows considerable adult growth in males (Jarman 1983).
If the second transition does not represent sexual maturity, it would have had to occur at the third transition, close to maximum size, suggesting a determinate growth pattern not observed in living reptiles. Viewing the second transition as an expression of sexual maturation is thus the most parsimonious interpretation, providing justification for the terminology of 'hatchling', 'juvenile', and 'adult' fibrolamellar bone."


Of course the _argumentum ad reptilia_ is a logical fallacy; instead, what should be done is phylogenetic bracketing, and here the results are ambiguous because, unlike crocodiles, birds do have determinate growth that stops AFAIK even before sexual maturity. But what about the elephants? I gather I should find out if the museum library here has the Biological Reviews of the Cambridge Philosophical Society (where Jarman 1983 was published).

In the meantime, are there any suggestions for an event between "juvenile" and "subadult" stages where a slowdown of growth rates would make sense? Leaving the undergrowth and joining a subadult herd? I can't come up with anything testable, but I don't quite see what the point of determinate growth is if it extends beyond sexual maturity. Last I read "the terminations of growth and life almost coincide" in elephants, but they seemingly did not in *Brachiosaurus brancai*, *Tornieria*, *Janenschia*, and both species of *Dicraeosaurus*... Well, I'm confused.