Middle Jurassic montreme Krusatodon enters the LRT

Rarely are early mammal fossils complete enough
to understand their morphology from nose to toes. Newly described specimens of Middle Jurassic Krusatodon (Figs 1–3) from the Isle of Skye, Scotland, are complete enough.

From the Paciroli et al 2024 abstract
“We report exceptionally complete juvenile and adult specimens of the Middle Jurassic docodontan Krusatodon, providing anatomical data and insights into the life history of early diverging mammaliaforms. We used synchrotron X-ray micro-computed tomography imaging of cementum growth increments in the teeth^2,3,4 to provide evidence of pace of life in a Mesozoic mammaliaform. The adult was about 7 years and the juvenile 7 to 24 months of age at death and in the process of replacing its deciduous dentition with its final, adult generation. Our findings suggest that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals and the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.”

Figure 1. Krusatodon skull from Paciroli et al. 2024. On the diagram colors changed to DGS standards. ” data-image-caption=”

Figure 1. Krusatodon skull from Paciroli et al. 2024. On the diagram colors changed to DGS standards.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/krusatodon-skull588-1.jpg?w=265″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/krusatodon-skull588-1.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87777″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/krusatodon-skull588-1.jpg” alt=”Figure 1. Krusatodon skull from Paciroli et al. 2024. On the diagram colors changed to DGS standards.” width=”584″ height=”661″ />

Figure 1. Krusatodon skull from Paciroli et al. 2024. On the diagram colors changed to DGS standards.

Here
in the large reptile tree (LRT, 2323 taxa) Krusatodon (Sigogneau-Russell 2003) nested among basal monotremes (Fig 2) as the basalmost long-snouted monotreme leading to two extant monotremes, the echidna, Tachyglossus, and platypus, Ornithorhynchus.

Figure 2. Krusatodon and relatives among the basal monotremes to scale. ” data-image-caption=”

Figure 2. Krusatodon and relatives among the basal monotremes to scale.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/monotreme_skulls588-3.jpg?w=98″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/monotreme_skulls588-3.jpg?w=335″ tabindex=”0″ role=”button” class=”size-full wp-image-87776″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/monotreme_skulls588-3.jpg” alt=”Figure 2. Krusatodon and relatives among the basal monotremes to scale.” width=”584″ height=”1788″ />

Figure 2. Krusatodon and relatives among the basal monotremes to scale.

Figure 3. Krusatodon adult and juvenile, full scale on 72 dpi monitors. ” data-image-caption=”

Figure 3. Krusatodon adult and juvenile, full scale on 72 dpi monitors.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/krusatodon.skeleton200.jpg?w=92″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/krusatodon.skeleton200.jpg?w=273″ tabindex=”0″ role=”button” class=”size-full wp-image-87784″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/krusatodon.skeleton200.jpg” alt=”Figure 3. Krusatodon adult and juvenile, full scale on 72 dpi monitors.” width=”273″ height=”895″ />

Figure 3. Krusatodon adult and juvenile, full scale on 72 dpi monitors.

According to Wikipedia – Platypus,
“In captivity, platypuses have survived to 30 years of age, and wild specimens have been recaptured when 24 years old. Females are believed to become sexually mature in their second year, with breeding observed in animals over nine years old.”

So it’s no surprise that Krusatodon was a mother at 7 with a 2-year-old juvenile. Not sure if this monotreme data point was mentioned in the text, which remains behind a paywall. The authors do mention ‘size’ as an issue. Early monotremes were much smaller than extant taxa.

The definition of Mammaliaformes seems to be the issue here.
“the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium”

In the LRT, Morganucodon is a basal marsupial. So the LRT refutes the traditional definition of Mammaliaformes. Instead, LRT supports the traditional monotreme, marsupial and placental clades.

Krusatodon is not considered a mammal by the authors nor at its Wikipedia site. Rather, it is a ‘docodont’.

According to Wikipedia – Docodonta,
“Docodonta is an order of extinct Mesozoic mammaliaforms (advanced cynodonts closely related to true crown-group mammals). There is much uncertainty for how docodont teeth developed from their simpler ancestors.”

‘Docodonts’, like Krusatodon, are all monotremes in the LRT.

References
Panciroli E et al (7 co-authors) 2024. Jurassic fossil juvenile reveals prolonged life history in early mammals. Nature https://doi.org/10.1038/s41586-024-07733-1
Sigogneau-Russell D 2003. Docodonts from the British Mesozoic. Acta Palaeontologica Polonica, 48(3)

wiki/Mammaliaformes
wiki/Krusatodon
wiki/Docodonta

Publicity
amnh.org/explore/news-blogs/research-posts/mammal-growth

“The specimens were discovered decades apart in Scotland’s Isle of Skye. The adult was excavated in the 1970s, and the juvenile, the only juvenile Jurassic mammal skeleton known to science was discovered in 2016 by Roger Benson, the Museum’s Macaulay Curator in the Division of Paleontology, and colleagues.”

“Today, small mammals have much shorter lifespans, with some living as little as 12 months. They also typically mature quickly, losing their baby teeth and weaning within months of birth.”

theguardian.com/science/article/2024/jul/24/early-mammal-could-help-answer-one-of-biologys-biggest-question-say-experts