Sánchez-Villagra and Forasiepi Zoological Letters (2017) 3:1 DOI 10.1186/s40851-017-0062-y REVIEW Open Access On the development of the chondrocranium and the histological anatomy of the head in perinatal stages of marsupial mammals Marcelo R. Sánchez-Villagra1* and Analía M. Forasiepi2 Abstract An overview of the literature on the chondrocranium of marsupial mammals reveals a relative conservatism in shape and structures. We document the histological cranial anatomy of individuals representing Monodelphis domestica, Dromiciops gliroides, Perameles sp. and Macropus eugenii. The marsupial chondrocranium is generally characterized by the great breadth of the lamina basalis, absence of pila metoptica and large otic capsules. Its most anterior portion (cupula nasi anterior) is robust, and anterior to it there are well-developed tactile sensory structures, functionally important in the neonate. Investigations of ossification centers at and aroundthenasalseptumareneededtotracethepresenceof certain bones (e.g., mesethmoid, parasphenoid) across marsupial taxa. In many adult marsupials, the tympanic floor is formed by at least three bones: alisphenoid (alisphenoid tympanic process), ectotympanic and petrosal (rostral and caudal tympanic processes); the squamosal also contributes in some diprotodontians. The presence of an entotympanic in marsupials has not been convincingly demonstrated. The tubal element surrounding the auditory tube in most marsupials is fibrous connective tissue rather than cartilage; the latter is the case in most placentals recorded to date. However, we detected fibrocartilage in a late juvenile of Dromiciops, and a similar tissue has been reported for Tarsipes. Contradictory reports on the presence of the tegmen tympani can be found in the literature. We describe a small tegmen tympani in Macropus. Several heterochronic shifts in the timing of development of the chondocranium and associated structures (e.g., nerves, muscles) and in the ossification sequence have been interpreted as largely being influenced by functional requirements related to the altriciality of the newborn marsupial during early postnatal life. Comparative studies of chondocranial development of mammals can benefit from a solid phylogenetic framework, research on non-classical model organisms, and integration with imaging and sectional data derived from computer-tomography. Keywords: Ontogeny, Skull, Tegmen tympani, Auditory bulla, Entotympanic, Tubal element, Dromiciops, Monodelphis, Macropus, Perameles dimensional, non-invasive imaging made possible by “…in morphology general principles are founded on computed tomography (hereafter, CT scanning) [2–4]. matters of quite intricate detail and require detail for Scanning offers a means to acquire high-quality informa- their illustration.” tion from areas of the skull that would otherwise be unavailable except via invasive techniques. De Beer [1], page xxix. Studies based on CT scanning involve the documenta- tion of sections, analogous to histological work. In this regard, CT scanning is a very helpful guide, permitting Background quick identification of structures and the reconstruction The study of comparative anatomy has been dramatically of soft-tissues without the effort needed to produce well- enhanced in the last decades by the availability of three- procesed and stained serial sections [5]. High-resolution * Correspondence: [email protected] CT scanning, together with new methods of staining, 1Paläontologisches Institut und Museum der Universität Zürich, Karl Schmid have been used to generate three-dimensional images of Strasse 4, Zürich 8006, Switzerland organs and soft tissues, which could potentially have a Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Sánchez-Villagra and Forasiepi Zoological Letters (2017) 3:1 Page 2 of 33 significant impact on studies of organogenesis and anat- newborn marsupial must be able to withstand the mech- omy [6, 7]. However, the precise level of anatomical anical strain of sucking at the teat, from which it is sus- resolution which may be attained by histological serial pended within the marsupium [40–43]. Heterochronic sectioning cannot yet be matched by CT scanning. shifts have been detected towards the extreme precocity Thanks to color differentiation, serial sections can pro- of ossification of the bones of the snout (e.g. premaxilla, vide detailed information about cartilage, membranes, maxilla, palatine, dentary), particularly the palate [1] and relationships of blood vessels and nerves, and different the back of the skull (e.g., exoccipital) [44] to support ac- tissues and organs, in some cases leading to the discov- tive suckling. With respect to the chondrocranium, com- ery of cladistically diagnostic features [8–12]. Histo- parisons with monotremes, which do not suckle at birth, logical serial sections can also provide information on are needed [12, 45] as to establish the polarity of many the nature of the tissues, giving insights into their origin, traits and test some of the claims on a novel functional development and distribution of mechanical strain [13]. significance for several marsupial traits. The objective of this contribution is to provide a refer- The chondrocranium consists of numerous individual ence on the sectional anatomy of the developing marsu- chondrification centers that appear at different times and pial head, summarize major features of the anatomy of fuse, forming a single structure that forms the earliest the marsupial chondrocranium, and discuss some con- phase of the developing fetal skull. Most of the chondro- troversial issues concerning the ethmoidal, orbitotem- cranium is replaced by ossifications; however, some parts poral and basicranial regions. We document species that become resorbed in later ontogenetic stages while others represent several major groups of extant marsupials, in- persist into the adult stage (e.g., nasal cartilages). The skull cluding Didelphimorphia, Microbiotheria, Peramelemor- has different ossification modes; it is completed via direct phia and Diprotodontia (Fig. 1). (intramembranous) and indirect (perichondral and endo- chondral) osteogenesis. These processes operating in the Marsupial evolution, cranial anatomy and development formation of the different kinds of bones and their hist- Marsupialia is the crown group that includes the common ology, ontogenetic and phylogenetic history, have been dis- ancestor of all extant marsupials and their descendants. cussed comprehensively by Hall [13] and Padian and Metatheria is the most inclusive group of mammals more Lamm [46]. Intramembranous osteogenesis, also called closely related to opossums and their fossil relatives than periosteal or subperiosteal osteogenesis [13], represents the to placentals [14]. Metatheria encompasses stem marsu- conversion of a fibrous mesenchymal precursor directly pials going back to perhaps Jurassic times [15]. into bone, the result of which is usually called “dermal” Traditionally, marsupials, in particular opossums (didel- bone. Perichondral and endochondral osteogenesis require phids), have been taken as models to investigate early an intermediate cartilaginous precursor before bone depos- mammalian evolution [16–19]. Several plesiomorphic ition. This sequence produces endochondral bones, also features retained in the musculoskeletal system resulted in called “replacement” bones. Appositional bone (“Zuwachs- the interpretation of opossums as models for understand- knochen”) does not have a cartilaginous precursor [47] and ing the therian last common ancestor. However, this does thus resembles dermal bone, although it grows by appos- not imply that their anatomy is lacking in complexity [20]. ition on a cartilaginous core. Monodelphis is a didelphid of growing importance in biomedical research, and one that has been used in evolu- Developing head anatomy—the chondrocranium tionary studies of mammalian development [21–23]. Much of the literature on cranial marsupial develop- Dromiciops is a key taxon to understand issues within ment is from the first part of the 20th century. marsupial phylogeny, representing a member of an other- What makes it somewhat difficult to access is not so wise Australasian clade (Fig. 1). much the fact that most of it was published in Several aspects of the adult cranial anatomy of German, but rather that it is highly descriptive in marsupials have been treated since the late 19th century nature, without what we would understand today as (e.g., [9, 17, 24–37]). These works are based mostly on the a clear phylogenetic underpinning, and with a pleth- macroscopic examination of macerated skulls, including ora of terms that tend to describe single or only a that of Wible [30] on the skull of Monodelphis brevicau- few stages. In what follows, our descriptions suffer data, which clarifies terminological issues and establishes from these two limitations by necessity.